From f88611c6d06ce1d2b7fe0e0ca440ff0fbbd45da8 Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:39:09 -0400
Subject: [PATCH 01/16] t/t4216-log-bloom.sh: harden
 `test_bloom_filters_not_used()`

The existing implementation of test_bloom_filters_not_used() asserts
that the Bloom filter sub-system has not been initialized at all, by
checking for the absence of any data from it from trace2.

In the following commit, it will become possible to load Bloom filters
without using them (e.g., because the `commitGraph.changedPathVersion`
introduced later in this series is incompatible with the hash version
with which the commit-graph's Bloom filters were written).

When this is the case, it's possible to initialize the Bloom filter
sub-system, while still not using any Bloom filters. When this is the
case, check that the data dump from the Bloom sub-system is all zeros,
indicating that no filters were used.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 t/t4216-log-bloom.sh | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/t/t4216-log-bloom.sh b/t/t4216-log-bloom.sh
index 2ba0324a69..b7baf49d62 100755
--- a/t/t4216-log-bloom.sh
+++ b/t/t4216-log-bloom.sh
@@ -82,7 +82,19 @@ test_bloom_filters_used () {
 test_bloom_filters_not_used () {
 	log_args=$1
 	setup "$log_args" &&
-	! grep -q "statistics:{\"filter_not_present\":" "$TRASH_DIRECTORY/trace.perf" &&
+
+	if grep -q "statistics:{\"filter_not_present\":" "$TRASH_DIRECTORY/trace.perf"
+	then
+		# if the Bloom filter system is initialized, ensure that no
+		# filters were used
+		data="statistics:{"
+		data="$data\"filter_not_present\":0,"
+		data="$data\"maybe\":0,"
+		data="$data\"definitely_not\":0,"
+		data="$data\"false_positive\":0}"
+
+		grep -q "$data" "$TRASH_DIRECTORY/trace.perf"
+	fi &&
 	test_cmp log_wo_bloom log_w_bloom
 }
 

From 1343c893138e63e35b38324387ea790aba77a49f Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:39:13 -0400
Subject: [PATCH 02/16] revision.c: consult Bloom filters for root commits
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

The commit-graph stores changed-path Bloom filters which represent the
set of paths included in a tree-level diff between a commit's root tree
and that of its parent.

When a commit has no parents, the tree-diff is computed against that
commit's root tree and the empty tree. In other words, every path in
that commit's tree is stored in the Bloom filter (since they all appear
in the diff).

Consult these filters during pathspec-limited traversals in the function
`rev_same_tree_as_empty()`. Doing so yields a performance improvement
where we can avoid enumerating the full set of paths in a parentless
commit's root tree when we know that the path(s) of interest were not
listed in that commit's changed-path Bloom filter.

Suggested-by: SZEDER Gábor <szeder.dev@gmail.com>
Original-patch-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 revision.c           | 26 ++++++++++++++++++++++----
 t/t4216-log-bloom.sh |  8 ++++++--
 2 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/revision.c b/revision.c
index 2424c9bd67..0e6f7d02b6 100644
--- a/revision.c
+++ b/revision.c
@@ -833,17 +833,28 @@ static int rev_compare_tree(struct rev_info *revs,
 	return tree_difference;
 }
 
-static int rev_same_tree_as_empty(struct rev_info *revs, struct commit *commit)
+static int rev_same_tree_as_empty(struct rev_info *revs, struct commit *commit,
+				  int nth_parent)
 {
 	struct tree *t1 = repo_get_commit_tree(the_repository, commit);
+	int bloom_ret = -1;
 
 	if (!t1)
 		return 0;
 
+	if (!nth_parent && revs->bloom_keys_nr) {
+		bloom_ret = check_maybe_different_in_bloom_filter(revs, commit);
+		if (!bloom_ret)
+			return 1;
+	}
+
 	tree_difference = REV_TREE_SAME;
 	revs->pruning.flags.has_changes = 0;
 	diff_tree_oid(NULL, &t1->object.oid, "", &revs->pruning);
 
+	if (bloom_ret == 1 && tree_difference == REV_TREE_SAME)
+		count_bloom_filter_false_positive++;
+
 	return tree_difference == REV_TREE_SAME;
 }
 
@@ -881,7 +892,7 @@ static int compact_treesame(struct rev_info *revs, struct commit *commit, unsign
 		if (nth_parent != 0)
 			die("compact_treesame %u", nth_parent);
 		old_same = !!(commit->object.flags & TREESAME);
-		if (rev_same_tree_as_empty(revs, commit))
+		if (rev_same_tree_as_empty(revs, commit, nth_parent))
 			commit->object.flags |= TREESAME;
 		else
 			commit->object.flags &= ~TREESAME;
@@ -977,7 +988,14 @@ static void try_to_simplify_commit(struct rev_info *revs, struct commit *commit)
 		return;
 
 	if (!commit->parents) {
-		if (rev_same_tree_as_empty(revs, commit))
+		/*
+		 * Pretend as if we are comparing ourselves to the
+		 * (non-existent) first parent of this commit object. Even
+		 * though no such parent exists, its changed-path Bloom filter
+		 * (if one exists) is relative to the empty tree, using Bloom
+		 * filters is allowed here.
+		 */
+		if (rev_same_tree_as_empty(revs, commit, 0))
 			commit->object.flags |= TREESAME;
 		return;
 	}
@@ -1058,7 +1076,7 @@ static void try_to_simplify_commit(struct rev_info *revs, struct commit *commit)
 
 		case REV_TREE_NEW:
 			if (revs->remove_empty_trees &&
-			    rev_same_tree_as_empty(revs, p)) {
+			    rev_same_tree_as_empty(revs, p, nth_parent)) {
 				/* We are adding all the specified
 				 * paths from this parent, so the
 				 * history beyond this parent is not
diff --git a/t/t4216-log-bloom.sh b/t/t4216-log-bloom.sh
index b7baf49d62..cc6ebc8140 100755
--- a/t/t4216-log-bloom.sh
+++ b/t/t4216-log-bloom.sh
@@ -88,7 +88,11 @@ test_bloom_filters_not_used () {
 		# if the Bloom filter system is initialized, ensure that no
 		# filters were used
 		data="statistics:{"
-		data="$data\"filter_not_present\":0,"
+		# unusable filters (e.g., those computed with a
+		# different value of commitGraph.changedPathsVersion)
+		# are counted in the filter_not_present bucket, so any
+		# value is OK there.
+		data="$data\"filter_not_present\":[0-9][0-9]*,"
 		data="$data\"maybe\":0,"
 		data="$data\"definitely_not\":0,"
 		data="$data\"false_positive\":0}"
@@ -175,7 +179,7 @@ test_expect_success 'setup - add commit-graph to the chain with Bloom filters' '
 
 test_bloom_filters_used_when_some_filters_are_missing () {
 	log_args=$1
-	bloom_trace_prefix="statistics:{\"filter_not_present\":3,\"maybe\":6,\"definitely_not\":9"
+	bloom_trace_prefix="statistics:{\"filter_not_present\":3,\"maybe\":6,\"definitely_not\":10"
 	setup "$log_args" &&
 	grep -q "$bloom_trace_prefix" "$TRASH_DIRECTORY/trace.perf" &&
 	test_cmp log_wo_bloom log_w_bloom

From cf73936ddf4ba56b34ea03cce1657efa5fa3decc Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:39:16 -0400
Subject: [PATCH 03/16] commit-graph: ensure Bloom filters are read with
 consistent settings
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

The changed-path Bloom filter mechanism is parameterized by a couple of
variables, notably the number of bits per hash (typically "m" in Bloom
filter literature) and the number of hashes themselves (typically "k").

It is critically important that filters are read with the Bloom filter
settings that they were written with. Failing to do so would mean that
each query is liable to compute different fingerprints, meaning that the
filter itself could return a false negative. This goes against a basic
assumption of using Bloom filters (that they may return false positives,
but never false negatives) and can lead to incorrect results.

We have some existing logic to carry forward existing Bloom filter
settings from one layer to the next. In `write_commit_graph()`, we have
something like:

    if (!(flags & COMMIT_GRAPH_NO_WRITE_BLOOM_FILTERS)) {
        struct commit_graph *g = ctx->r->objects->commit_graph;

        /* We have changed-paths already. Keep them in the next graph */
        if (g && g->chunk_bloom_data) {
            ctx->changed_paths = 1;
            ctx->bloom_settings = g->bloom_filter_settings;
        }
    }

, which drags forward Bloom filter settings across adjacent layers.

This doesn't quite address all cases, however, since it is possible for
intermediate layers to contain no Bloom filters at all. For example,
suppose we have two layers in a commit-graph chain, say, {G1, G2}. If G1
contains Bloom filters, but G2 doesn't, a new G3 (whose base graph is
G2) may be written with arbitrary Bloom filter settings, because we only
check the immediately adjacent layer's settings for compatibility.

This behavior has existed since the introduction of changed-path Bloom
filters. But in practice, this is not such a big deal, since the only
way up until this point to modify the Bloom filter settings at write
time is with the undocumented environment variables:

  - GIT_TEST_BLOOM_SETTINGS_BITS_PER_ENTRY
  - GIT_TEST_BLOOM_SETTINGS_NUM_HASHES
  - GIT_TEST_BLOOM_SETTINGS_MAX_CHANGED_PATHS

(it is still possible to tweak MAX_CHANGED_PATHS between layers, but
this does not affect reads, so is allowed to differ across multiple
graph layers).

But in future commits, we will introduce another parameter to change the
hash algorithm used to compute Bloom fingerprints itself. This will be
exposed via a configuration setting, making this foot-gun easier to use.

To prevent this potential issue, validate that all layers of a split
commit-graph have compatible settings with the newest layer which
contains Bloom filters.

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Original-test-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 commit-graph.c       | 25 ++++++++++++++++
 t/t4216-log-bloom.sh | 68 +++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 92 insertions(+), 1 deletion(-)

diff --git a/commit-graph.c b/commit-graph.c
index bba316913c..00113b0f62 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -543,6 +543,30 @@ static int validate_mixed_generation_chain(struct commit_graph *g)
 	return 0;
 }
 
+static void validate_mixed_bloom_settings(struct commit_graph *g)
+{
+	struct bloom_filter_settings *settings = NULL;
+	for (; g; g = g->base_graph) {
+		if (!g->bloom_filter_settings)
+			continue;
+		if (!settings) {
+			settings = g->bloom_filter_settings;
+			continue;
+		}
+
+		if (g->bloom_filter_settings->bits_per_entry != settings->bits_per_entry ||
+		    g->bloom_filter_settings->num_hashes != settings->num_hashes) {
+			g->chunk_bloom_indexes = NULL;
+			g->chunk_bloom_data = NULL;
+			FREE_AND_NULL(g->bloom_filter_settings);
+
+			warning(_("disabling Bloom filters for commit-graph "
+				  "layer '%s' due to incompatible settings"),
+				oid_to_hex(&g->oid));
+		}
+	}
+}
+
 static int add_graph_to_chain(struct commit_graph *g,
 			      struct commit_graph *chain,
 			      struct object_id *oids,
@@ -666,6 +690,7 @@ struct commit_graph *load_commit_graph_chain_fd_st(struct repository *r,
 	}
 
 	validate_mixed_generation_chain(graph_chain);
+	validate_mixed_bloom_settings(graph_chain);
 
 	free(oids);
 	fclose(fp);
diff --git a/t/t4216-log-bloom.sh b/t/t4216-log-bloom.sh
index cc6ebc8140..c1977961d0 100755
--- a/t/t4216-log-bloom.sh
+++ b/t/t4216-log-bloom.sh
@@ -421,8 +421,74 @@ test_expect_success 'Bloom generation backfills empty commits' '
 	)
 '
 
+graph=.git/objects/info/commit-graph
+graphdir=.git/objects/info/commit-graphs
+chain=$graphdir/commit-graph-chain
+
+test_expect_success 'setup for mixed Bloom setting tests' '
+	repo=mixed-bloom-settings &&
+
+	git init $repo &&
+	for i in one two three
+	do
+		test_commit -C $repo $i file || return 1
+	done
+'
+
+test_expect_success 'ensure Bloom filters with incompatible settings are ignored' '
+	# Compute Bloom filters with "unusual" settings.
+	git -C $repo rev-parse one >in &&
+	GIT_TEST_BLOOM_SETTINGS_NUM_HASHES=3 git -C $repo commit-graph write \
+		--stdin-commits --changed-paths --split <in &&
+	layer=$(head -n 1 $repo/$chain) &&
+
+	# A commit-graph layer without Bloom filters "hides" the layers
+	# below ...
+	git -C $repo rev-parse two >in &&
+	git -C $repo commit-graph write --stdin-commits --no-changed-paths \
+		--split=no-merge <in &&
+
+	# Another commit-graph layer that has Bloom filters, but with
+	# standard settings, and is thus incompatible with the base
+	# layer written above.
+	git -C $repo rev-parse HEAD >in &&
+	git -C $repo commit-graph write --stdin-commits --changed-paths \
+		--split=no-merge <in &&
+
+	test_line_count = 3 $repo/$chain &&
+
+	# Ensure that incompatible Bloom filters are ignored.
+	git -C $repo -c core.commitGraph=false log --oneline --no-decorate -- file \
+		>expect 2>err &&
+	git -C $repo log --oneline --no-decorate -- file >actual 2>err &&
+	test_cmp expect actual &&
+	grep "disabling Bloom filters for commit-graph layer .$layer." err
+'
+
+test_expect_success 'merge graph layers with incompatible Bloom settings' '
+	# Ensure that incompatible Bloom filters are ignored when
+	# merging existing layers.
+	>trace2.txt &&
+	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \
+		git -C $repo commit-graph write --reachable --changed-paths 2>err &&
+	grep "disabling Bloom filters for commit-graph layer .$layer." err &&
+	grep "{\"hash_version\":1,\"num_hashes\":7,\"bits_per_entry\":10,\"max_changed_paths\":512" trace2.txt &&
+
+	test_path_is_file $repo/$graph &&
+	test_dir_is_empty $repo/$graphdir &&
+
+	git -C $repo -c core.commitGraph=false log --oneline --no-decorate -- \
+		file >expect &&
+	trace_out="$(pwd)/trace.perf" &&
+	GIT_TRACE2_PERF="$trace_out" \
+		git -C $repo log --oneline --no-decorate -- file >actual 2>err &&
+
+	test_cmp expect actual &&
+	grep "statistics:{\"filter_not_present\":0," trace.perf &&
+	test_must_be_empty err
+'
+
 corrupt_graph () {
-	graph=.git/objects/info/commit-graph &&
 	test_when_finished "rm -rf $graph" &&
 	git commit-graph write --reachable --changed-paths &&
 	corrupt_chunk_file $graph "$@"

From 23e91c0ca3f1f700884812990bbfa48d09039b77 Mon Sep 17 00:00:00 2001
From: Jonathan Tan <jonathantanmy@google.com>
Date: Tue, 25 Jun 2024 13:39:34 -0400
Subject: [PATCH 04/16] gitformat-commit-graph: describe version 2 of BDAT

The code change to Git to support version 2 will be done in subsequent
commits.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 Documentation/gitformat-commit-graph.txt | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/Documentation/gitformat-commit-graph.txt b/Documentation/gitformat-commit-graph.txt
index 31cad585e2..3e906e8030 100644
--- a/Documentation/gitformat-commit-graph.txt
+++ b/Documentation/gitformat-commit-graph.txt
@@ -142,13 +142,16 @@ All multi-byte numbers are in network byte order.
 
 ==== Bloom Filter Data (ID: {'B', 'D', 'A', 'T'}) [Optional]
     * It starts with header consisting of three unsigned 32-bit integers:
-      - Version of the hash algorithm being used. We currently only support
-	value 1 which corresponds to the 32-bit version of the murmur3 hash
+      - Version of the hash algorithm being used. We currently support
+	value 2 which corresponds to the 32-bit version of the murmur3 hash
 	implemented exactly as described in
 	https://en.wikipedia.org/wiki/MurmurHash#Algorithm and the double
 	hashing technique using seed values 0x293ae76f and 0x7e646e2 as
 	described in https://doi.org/10.1007/978-3-540-30494-4_26 "Bloom Filters
-	in Probabilistic Verification"
+	in Probabilistic Verification". Version 1 Bloom filters have a bug that appears
+	when char is signed and the repository has path names that have characters >=
+	0x80; Git supports reading and writing them, but this ability will be removed
+	in a future version of Git.
       - The number of times a path is hashed and hence the number of bit positions
 	      that cumulatively determine whether a file is present in the commit.
       - The minimum number of bits 'b' per entry in the Bloom filter. If the filter

From 460b15699dcadae5a642fbf67276cbeb85ea035c Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:39:37 -0400
Subject: [PATCH 05/16] t/helper/test-read-graph.c: extract `dump_graph_info()`

Prepare for the 'read-graph' test helper to perform other tasks besides
dumping high-level information about the commit-graph by extracting its
main routine into a separate function.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 t/helper/test-read-graph.c | 31 ++++++++++++++++++-------------
 1 file changed, 18 insertions(+), 13 deletions(-)

diff --git a/t/helper/test-read-graph.c b/t/helper/test-read-graph.c
index 8c7a83f578..3375392f6c 100644
--- a/t/helper/test-read-graph.c
+++ b/t/helper/test-read-graph.c
@@ -5,20 +5,8 @@
 #include "bloom.h"
 #include "setup.h"
 
-int cmd__read_graph(int argc UNUSED, const char **argv UNUSED)
+static void dump_graph_info(struct commit_graph *graph)
 {
-	struct commit_graph *graph = NULL;
-	struct object_directory *odb;
-
-	setup_git_directory();
-	odb = the_repository->objects->odb;
-
-	prepare_repo_settings(the_repository);
-
-	graph = read_commit_graph_one(the_repository, odb);
-	if (!graph)
-		return 1;
-
 	printf("header: %08x %d %d %d %d\n",
 		ntohl(*(uint32_t*)graph->data),
 		*(unsigned char*)(graph->data + 4),
@@ -57,6 +45,23 @@ int cmd__read_graph(int argc UNUSED, const char **argv UNUSED)
 	if (graph->topo_levels)
 		printf(" topo_levels");
 	printf("\n");
+}
+
+int cmd__read_graph(int argc UNUSED, const char **argv UNUSED)
+{
+	struct commit_graph *graph = NULL;
+	struct object_directory *odb;
+
+	setup_git_directory();
+	odb = the_repository->objects->odb;
+
+	prepare_repo_settings(the_repository);
+
+	graph = read_commit_graph_one(the_repository, odb);
+	if (!graph)
+		return 1;
+
+	dump_graph_info(graph);
 
 	UNLEAK(graph);
 

From a09858d43d818cfd23a9408a41460301741fa251 Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:39:41 -0400
Subject: [PATCH 06/16] bloom.h: make `load_bloom_filter_from_graph()` public

Prepare for a future commit to use the load_bloom_filter_from_graph()
function directly to load specific Bloom filters out of the commit-graph
for manual inspection (to be used during tests).

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 bloom.c | 6 +++---
 bloom.h | 5 +++++
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/bloom.c b/bloom.c
index e529f7605c..401999ed3c 100644
--- a/bloom.c
+++ b/bloom.c
@@ -48,9 +48,9 @@ static int check_bloom_offset(struct commit_graph *g, uint32_t pos,
 	return -1;
 }
 
-static int load_bloom_filter_from_graph(struct commit_graph *g,
-					struct bloom_filter *filter,
-					uint32_t graph_pos)
+int load_bloom_filter_from_graph(struct commit_graph *g,
+				 struct bloom_filter *filter,
+				 uint32_t graph_pos)
 {
 	uint32_t lex_pos, start_index, end_index;
 
diff --git a/bloom.h b/bloom.h
index adde6dfe21..1e4f612d2c 100644
--- a/bloom.h
+++ b/bloom.h
@@ -3,6 +3,7 @@
 
 struct commit;
 struct repository;
+struct commit_graph;
 
 struct bloom_filter_settings {
 	/*
@@ -68,6 +69,10 @@ struct bloom_key {
 	uint32_t *hashes;
 };
 
+int load_bloom_filter_from_graph(struct commit_graph *g,
+				 struct bloom_filter *filter,
+				 uint32_t graph_pos);
+
 /*
  * Calculate the murmur3 32-bit hash value for the given data
  * using the given seed.

From 57982b8f2af9563cb54aafbf46e21f2b744ab59e Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:39:44 -0400
Subject: [PATCH 07/16] t/helper/test-read-graph: implement `bloom-filters`
 mode

Implement a mode of the "read-graph" test helper to dump out the
hexadecimal contents of the Bloom filter(s) contained in a commit-graph.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 t/helper/test-read-graph.c | 42 +++++++++++++++++++++++++++++++++-----
 1 file changed, 37 insertions(+), 5 deletions(-)

diff --git a/t/helper/test-read-graph.c b/t/helper/test-read-graph.c
index 3375392f6c..899b5f41cc 100644
--- a/t/helper/test-read-graph.c
+++ b/t/helper/test-read-graph.c
@@ -47,10 +47,32 @@ static void dump_graph_info(struct commit_graph *graph)
 	printf("\n");
 }
 
-int cmd__read_graph(int argc UNUSED, const char **argv UNUSED)
+static void dump_graph_bloom_filters(struct commit_graph *graph)
+{
+	uint32_t i;
+
+	for (i = 0; i < graph->num_commits + graph->num_commits_in_base; i++) {
+		struct bloom_filter filter = { 0 };
+		size_t j;
+
+		if (load_bloom_filter_from_graph(graph, &filter, i) < 0) {
+			fprintf(stderr, "missing Bloom filter for graph "
+				"position %"PRIu32"\n", i);
+			continue;
+		}
+
+		for (j = 0; j < filter.len; j++)
+			printf("%02x", filter.data[j]);
+		if (filter.len)
+			printf("\n");
+	}
+}
+
+int cmd__read_graph(int argc, const char **argv)
 {
 	struct commit_graph *graph = NULL;
 	struct object_directory *odb;
+	int ret = 0;
 
 	setup_git_directory();
 	odb = the_repository->objects->odb;
@@ -58,12 +80,22 @@ int cmd__read_graph(int argc UNUSED, const char **argv UNUSED)
 	prepare_repo_settings(the_repository);
 
 	graph = read_commit_graph_one(the_repository, odb);
-	if (!graph)
-		return 1;
+	if (!graph) {
+		ret = 1;
+		goto done;
+	}
 
-	dump_graph_info(graph);
+	if (argc <= 1)
+		dump_graph_info(graph);
+	else if (!strcmp(argv[1], "bloom-filters"))
+		dump_graph_bloom_filters(graph);
+	else {
+		fprintf(stderr, "unknown sub-command: '%s'\n", argv[1]);
+		ret = 1;
+	}
 
+done:
 	UNLEAK(graph);
 
-	return 0;
+	return ret;
 }

From 08b6ae38c6024e05407fd1d1703d7e176102241b Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:39:47 -0400
Subject: [PATCH 08/16] t4216: test changed path filters with high bit paths

Subsequent commits will teach Git another version of changed path
filter that has different behavior with paths that contain at least
one character with its high bit set, so test the existing behavior as
a baseline.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 t/t4216-log-bloom.sh | 51 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/t/t4216-log-bloom.sh b/t/t4216-log-bloom.sh
index c1977961d0..49d1113171 100755
--- a/t/t4216-log-bloom.sh
+++ b/t/t4216-log-bloom.sh
@@ -488,6 +488,57 @@ test_expect_success 'merge graph layers with incompatible Bloom settings' '
 	test_must_be_empty err
 '
 
+get_first_changed_path_filter () {
+	test-tool read-graph bloom-filters >filters.dat &&
+	head -n 1 filters.dat
+}
+
+# chosen to be the same under all Unicode normalization forms
+CENT=$(printf "\302\242")
+
+test_expect_success 'set up repo with high bit path, version 1 changed-path' '
+	git init highbit1 &&
+	test_commit -C highbit1 c1 "$CENT" &&
+	git -C highbit1 commit-graph write --reachable --changed-paths
+'
+
+test_expect_success 'setup check value of version 1 changed-path' '
+	(
+		cd highbit1 &&
+		echo "52a9" >expect &&
+		get_first_changed_path_filter >actual
+	)
+'
+
+# expect will not match actual if char is unsigned by default. Write the test
+# in this way, so that a user running this test script can still see if the two
+# files match. (It will appear as an ordinary success if they match, and a skip
+# if not.)
+if test_cmp highbit1/expect highbit1/actual
+then
+	test_set_prereq SIGNED_CHAR_BY_DEFAULT
+fi
+test_expect_success SIGNED_CHAR_BY_DEFAULT 'check value of version 1 changed-path' '
+	# Only the prereq matters for this test.
+	true
+'
+
+test_expect_success 'setup make another commit' '
+	# "git log" does not use Bloom filters for root commits - see how, in
+	# revision.c, rev_compare_tree() (the only code path that eventually calls
+	# get_bloom_filter()) is only called by try_to_simplify_commit() when the commit
+	# has one parent. Therefore, make another commit so that we perform the tests on
+	# a non-root commit.
+	test_commit -C highbit1 anotherc1 "another$CENT"
+'
+
+test_expect_success 'version 1 changed-path used when version 1 requested' '
+	(
+		cd highbit1 &&
+		test_bloom_filters_used "-- another$CENT"
+	)
+'
+
 corrupt_graph () {
 	test_when_finished "rm -rf $graph" &&
 	git commit-graph write --reachable --changed-paths &&

From ea0024deb9faa5474673a3db8f4f98812b48f44d Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:39:50 -0400
Subject: [PATCH 09/16] repo-settings: introduce
 commitgraph.changedPathsVersion

A subsequent commit will introduce another version of the changed-path
filter in the commit graph file. In order to control which version to
write (and read), a config variable is needed.

Therefore, introduce this config variable. For forwards compatibility,
teach Git to not read commit graphs when the config variable
is set to an unsupported version. Because we teach Git this,
commitgraph.readChangedPaths is now redundant, so deprecate it and
define its behavior in terms of the config variable we introduce.

This commit does not change the behavior of writing (Git writes changed
path filters when explicitly instructed regardless of any config
variable), but a subsequent commit will restrict Git such that it will
only write when commitgraph.changedPathsVersion is a recognized value.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 Documentation/config/commitgraph.txt | 26 +++++++++++++++++++++++---
 commit-graph.c                       |  5 +++--
 oss-fuzz/fuzz-commit-graph.c         |  2 +-
 repo-settings.c                      |  6 +++++-
 repository.h                         |  2 +-
 5 files changed, 33 insertions(+), 8 deletions(-)

diff --git a/Documentation/config/commitgraph.txt b/Documentation/config/commitgraph.txt
index 30604e4a4c..e68cdededa 100644
--- a/Documentation/config/commitgraph.txt
+++ b/Documentation/config/commitgraph.txt
@@ -9,6 +9,26 @@ commitGraph.maxNewFilters::
 	commit-graph write` (c.f., linkgit:git-commit-graph[1]).
 
 commitGraph.readChangedPaths::
-	If true, then git will use the changed-path Bloom filters in the
-	commit-graph file (if it exists, and they are present). Defaults to
-	true. See linkgit:git-commit-graph[1] for more information.
+	Deprecated. Equivalent to commitGraph.changedPathsVersion=-1 if true, and
+	commitGraph.changedPathsVersion=0 if false. (If commitGraph.changedPathVersion
+	is also set, commitGraph.changedPathsVersion takes precedence.)
+
+commitGraph.changedPathsVersion::
+	Specifies the version of the changed-path Bloom filters that Git will read and
+	write. May be -1, 0 or 1. Note that values greater than 1 may be
+	incompatible with older versions of Git which do not yet understand
+	those versions. Use caution when operating in a mixed-version
+	environment.
++
+Defaults to -1.
++
+If -1, Git will use the version of the changed-path Bloom filters in the
+repository, defaulting to 1 if there are none.
++
+If 0, Git will not read any Bloom filters, and will write version 1 Bloom
+filters when instructed to write.
++
+If 1, Git will only read version 1 Bloom filters, and will write version 1
+Bloom filters.
++
+See linkgit:git-commit-graph[1] for more information.
diff --git a/commit-graph.c b/commit-graph.c
index 00113b0f62..91c98ebc6c 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -459,7 +459,7 @@ struct commit_graph *parse_commit_graph(struct repo_settings *s,
 			graph->read_generation_data = 1;
 	}
 
-	if (s->commit_graph_read_changed_paths) {
+	if (s->commit_graph_changed_paths_version) {
 		read_chunk(cf, GRAPH_CHUNKID_BLOOMINDEXES,
 			   graph_read_bloom_index, graph);
 		read_chunk(cf, GRAPH_CHUNKID_BLOOMDATA,
@@ -555,7 +555,8 @@ static void validate_mixed_bloom_settings(struct commit_graph *g)
 		}
 
 		if (g->bloom_filter_settings->bits_per_entry != settings->bits_per_entry ||
-		    g->bloom_filter_settings->num_hashes != settings->num_hashes) {
+		    g->bloom_filter_settings->num_hashes != settings->num_hashes ||
+		    g->bloom_filter_settings->hash_version != settings->hash_version) {
 			g->chunk_bloom_indexes = NULL;
 			g->chunk_bloom_data = NULL;
 			FREE_AND_NULL(g->bloom_filter_settings);
diff --git a/oss-fuzz/fuzz-commit-graph.c b/oss-fuzz/fuzz-commit-graph.c
index 2992079dd9..325c0b991a 100644
--- a/oss-fuzz/fuzz-commit-graph.c
+++ b/oss-fuzz/fuzz-commit-graph.c
@@ -19,7 +19,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 	 * possible.
 	 */
 	the_repository->settings.commit_graph_generation_version = 2;
-	the_repository->settings.commit_graph_read_changed_paths = 1;
+	the_repository->settings.commit_graph_changed_paths_version = 1;
 	g = parse_commit_graph(&the_repository->settings, (void *)data, size);
 	repo_clear(the_repository);
 	free_commit_graph(g);
diff --git a/repo-settings.c b/repo-settings.c
index 30cd478762..c821583fe5 100644
--- a/repo-settings.c
+++ b/repo-settings.c
@@ -23,6 +23,7 @@ void prepare_repo_settings(struct repository *r)
 	int value;
 	const char *strval;
 	int manyfiles;
+	int read_changed_paths;
 
 	if (!r->gitdir)
 		BUG("Cannot add settings for uninitialized repository");
@@ -53,7 +54,10 @@ void prepare_repo_settings(struct repository *r)
 	/* Commit graph config or default, does not cascade (simple) */
 	repo_cfg_bool(r, "core.commitgraph", &r->settings.core_commit_graph, 1);
 	repo_cfg_int(r, "commitgraph.generationversion", &r->settings.commit_graph_generation_version, 2);
-	repo_cfg_bool(r, "commitgraph.readchangedpaths", &r->settings.commit_graph_read_changed_paths, 1);
+	repo_cfg_bool(r, "commitgraph.readchangedpaths", &read_changed_paths, 1);
+	repo_cfg_int(r, "commitgraph.changedpathsversion",
+		     &r->settings.commit_graph_changed_paths_version,
+		     read_changed_paths ? -1 : 0);
 	repo_cfg_bool(r, "gc.writecommitgraph", &r->settings.gc_write_commit_graph, 1);
 	repo_cfg_bool(r, "fetch.writecommitgraph", &r->settings.fetch_write_commit_graph, 0);
 
diff --git a/repository.h b/repository.h
index 5f18486f64..f71154e12c 100644
--- a/repository.h
+++ b/repository.h
@@ -29,7 +29,7 @@ struct repo_settings {
 
 	int core_commit_graph;
 	int commit_graph_generation_version;
-	int commit_graph_read_changed_paths;
+	int commit_graph_changed_paths_version;
 	int gc_write_commit_graph;
 	int fetch_write_commit_graph;
 	int command_requires_full_index;

From 5b5d5b598ccb8d5eb8a1de3abbf7b5829f9ac4fe Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:39:54 -0400
Subject: [PATCH 10/16] bloom: annotate filters with hash version

In subsequent commits, we will want to load existing Bloom filters out
of a commit-graph, even when the hash version they were computed with
does not match the value of `commitGraph.changedPathVersion`.

In order to differentiate between the two, add a "version" field to each
Bloom filter.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 bloom.c | 11 ++++++++---
 bloom.h |  1 +
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/bloom.c b/bloom.c
index 401999ed3c..e64e53bc4c 100644
--- a/bloom.c
+++ b/bloom.c
@@ -88,6 +88,7 @@ int load_bloom_filter_from_graph(struct commit_graph *g,
 	filter->data = (unsigned char *)(g->chunk_bloom_data +
 					sizeof(unsigned char) * start_index +
 					BLOOMDATA_CHUNK_HEADER_SIZE);
+	filter->version = g->bloom_filter_settings->hash_version;
 
 	return 1;
 }
@@ -210,11 +211,13 @@ static int pathmap_cmp(const void *hashmap_cmp_fn_data UNUSED,
 	return strcmp(e1->path, e2->path);
 }
 
-static void init_truncated_large_filter(struct bloom_filter *filter)
+static void init_truncated_large_filter(struct bloom_filter *filter,
+					int version)
 {
 	filter->data = xmalloc(1);
 	filter->data[0] = 0xFF;
 	filter->len = 1;
+	filter->version = version;
 }
 
 struct bloom_filter *get_or_compute_bloom_filter(struct repository *r,
@@ -299,13 +302,15 @@ struct bloom_filter *get_or_compute_bloom_filter(struct repository *r,
 		}
 
 		if (hashmap_get_size(&pathmap) > settings->max_changed_paths) {
-			init_truncated_large_filter(filter);
+			init_truncated_large_filter(filter,
+						    settings->hash_version);
 			if (computed)
 				*computed |= BLOOM_TRUNC_LARGE;
 			goto cleanup;
 		}
 
 		filter->len = (hashmap_get_size(&pathmap) * settings->bits_per_entry + BITS_PER_WORD - 1) / BITS_PER_WORD;
+		filter->version = settings->hash_version;
 		if (!filter->len) {
 			if (computed)
 				*computed |= BLOOM_TRUNC_EMPTY;
@@ -325,7 +330,7 @@ struct bloom_filter *get_or_compute_bloom_filter(struct repository *r,
 	} else {
 		for (i = 0; i < diff_queued_diff.nr; i++)
 			diff_free_filepair(diff_queued_diff.queue[i]);
-		init_truncated_large_filter(filter);
+		init_truncated_large_filter(filter, settings->hash_version);
 
 		if (computed)
 			*computed |= BLOOM_TRUNC_LARGE;
diff --git a/bloom.h b/bloom.h
index 1e4f612d2c..c9dd7d4022 100644
--- a/bloom.h
+++ b/bloom.h
@@ -53,6 +53,7 @@ struct bloom_filter_settings {
 struct bloom_filter {
 	unsigned char *data;
 	size_t len;
+	int version;
 };
 
 /*

From b2cf331057c5862526c26e610113b2ee804192aa Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:39:57 -0400
Subject: [PATCH 11/16] bloom: prepare to discard incompatible Bloom filters

Callers use the inline `get_bloom_filter()` implementation as a thin
wrapper around `get_or_compute_bloom_filter()`. The former calls the
latter with a value of "0" for `compute_if_not_present`, making
`get_bloom_filter()` the default read-only path for fetching an existing
Bloom filter.

Callers expect the value returned from `get_bloom_filter()` is usable,
that is that it's compatible with the configured value corresponding to
`commitGraph.changedPathsVersion`.

This is OK, since the commit-graph machinery only initializes its BDAT
chunk (thereby enabling it to service Bloom filter queries) when the
Bloom filter hash_version is compatible with our settings. So any value
returned by `get_bloom_filter()` is trivially useable.

However, subsequent commits will load the BDAT chunk even when the Bloom
filters are built with incompatible hash versions. Prepare to handle
this by teaching `get_bloom_filter()` to discard filters that are
incompatible with the configured hash version.

Callers who wish to read incompatible filters (e.g., for upgrading
filters from v1 to v2) may use the lower level routine,
`get_or_compute_bloom_filter()`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 bloom.c | 20 +++++++++++++++++++-
 bloom.h | 20 ++++++++++++++++++--
 2 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/bloom.c b/bloom.c
index e64e53bc4c..c24489dbcf 100644
--- a/bloom.c
+++ b/bloom.c
@@ -220,6 +220,23 @@ static void init_truncated_large_filter(struct bloom_filter *filter,
 	filter->version = version;
 }
 
+struct bloom_filter *get_bloom_filter(struct repository *r, struct commit *c)
+{
+	struct bloom_filter *filter;
+	int hash_version;
+
+	filter = get_or_compute_bloom_filter(r, c, 0, NULL, NULL);
+	if (!filter)
+		return NULL;
+
+	prepare_repo_settings(r);
+	hash_version = r->settings.commit_graph_changed_paths_version;
+
+	if (!(hash_version == -1 || hash_version == filter->version))
+		return NULL; /* unusable filter */
+	return filter;
+}
+
 struct bloom_filter *get_or_compute_bloom_filter(struct repository *r,
 						 struct commit *c,
 						 int compute_if_not_present,
@@ -245,7 +262,8 @@ struct bloom_filter *get_or_compute_bloom_filter(struct repository *r,
 						     filter, graph_pos);
 	}
 
-	if (filter->data && filter->len)
+	if ((filter->data && filter->len) &&
+	    (!settings || settings->hash_version == filter->version))
 		return filter;
 	if (!compute_if_not_present)
 		return NULL;
diff --git a/bloom.h b/bloom.h
index c9dd7d4022..052a993aab 100644
--- a/bloom.h
+++ b/bloom.h
@@ -108,8 +108,24 @@ struct bloom_filter *get_or_compute_bloom_filter(struct repository *r,
 						 const struct bloom_filter_settings *settings,
 						 enum bloom_filter_computed *computed);
 
-#define get_bloom_filter(r, c) get_or_compute_bloom_filter( \
-	(r), (c), 0, NULL, NULL)
+/*
+ * Find the Bloom filter associated with the given commit "c".
+ *
+ * If any of the following are true
+ *
+ *   - the repository does not have a commit-graph, or
+ *   - the repository disables reading from the commit-graph, or
+ *   - the given commit does not have a Bloom filter computed, or
+ *   - there is a Bloom filter for commit "c", but it cannot be read
+ *     because the filter uses an incompatible version of murmur3
+ *
+ * , then `get_bloom_filter()` will return NULL. Otherwise, the corresponding
+ * Bloom filter will be returned.
+ *
+ * For callers who wish to inspect Bloom filters with incompatible hash
+ * versions, use get_or_compute_bloom_filter().
+ */
+struct bloom_filter *get_bloom_filter(struct repository *r, struct commit *c);
 
 int bloom_filter_contains(const struct bloom_filter *filter,
 			  const struct bloom_key *key,

From 638e1702d7cf3ca000f355432b7c12bd4a27b244 Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:40:00 -0400
Subject: [PATCH 12/16] commit-graph: unconditionally load Bloom filters

In an earlier commit, we began ignoring the Bloom data ("BDAT") chunk
for commit-graphs whose Bloom filters were computed using a hash version
incompatible with the value of `commitGraph.changedPathVersion`.

Now that the Bloom API has been hardened to discard these incompatible
filters (with the exception of low-level APIs), we can safely load these
Bloom filters unconditionally.

We no longer want to return early from `graph_read_bloom_data()`, and
similarly do not want to set the bloom_settings' `hash_version` field as
a side-effect. The latter is because we want to wait until we know which
Bloom settings we're using (either the defaults, from the GIT_TEST
variables, or from the previous commit-graph layer) before deciding what
hash_version to use.

If we detect an existing BDAT chunk, we'll infer the rest of the
settings (e.g., number of hashes, bits per entry, and maximum number of
changed paths) from the earlier graph layer. The hash_version will be
inferred from the previous layer as well, unless one has already been
specified via configuration.

Once all of that is done, we normalize the value of the hash_version to
either "1" or "2".

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 commit-graph.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index 91c98ebc6c..d6fb714f32 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -358,9 +358,6 @@ static int graph_read_bloom_data(const unsigned char *chunk_start,
 	g->chunk_bloom_data_size = chunk_size;
 	hash_version = get_be32(chunk_start);
 
-	if (hash_version != 1)
-		return 0;
-
 	g->bloom_filter_settings = xmalloc(sizeof(struct bloom_filter_settings));
 	g->bloom_filter_settings->hash_version = hash_version;
 	g->bloom_filter_settings->num_hashes = get_be32(chunk_start + 4);
@@ -2513,6 +2510,7 @@ int write_commit_graph(struct object_directory *odb,
 	ctx->write_generation_data = (get_configured_generation_version(r) == 2);
 	ctx->num_generation_data_overflows = 0;
 
+	bloom_settings.hash_version = r->settings.commit_graph_changed_paths_version;
 	bloom_settings.bits_per_entry = git_env_ulong("GIT_TEST_BLOOM_SETTINGS_BITS_PER_ENTRY",
 						      bloom_settings.bits_per_entry);
 	bloom_settings.num_hashes = git_env_ulong("GIT_TEST_BLOOM_SETTINGS_NUM_HASHES",
@@ -2544,10 +2542,18 @@ int write_commit_graph(struct object_directory *odb,
 		/* We have changed-paths already. Keep them in the next graph */
 		if (g && g->chunk_bloom_data) {
 			ctx->changed_paths = 1;
-			ctx->bloom_settings = g->bloom_filter_settings;
+
+			/* don't propagate the hash_version unless unspecified */
+			if (bloom_settings.hash_version == -1)
+				bloom_settings.hash_version = g->bloom_filter_settings->hash_version;
+			bloom_settings.bits_per_entry = g->bloom_filter_settings->bits_per_entry;
+			bloom_settings.num_hashes = g->bloom_filter_settings->num_hashes;
+			bloom_settings.max_changed_paths = g->bloom_filter_settings->max_changed_paths;
 		}
 	}
 
+	bloom_settings.hash_version = bloom_settings.hash_version == 2 ? 2 : 1;
+
 	if (ctx->split) {
 		struct commit_graph *g = ctx->r->objects->commit_graph;
 

From ba5a81d52b9a4c4608b6a645d2cc6f508bcb66b4 Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:40:04 -0400
Subject: [PATCH 13/16] commit-graph: new Bloom filter version that fixes
 murmur3

The murmur3 implementation in bloom.c has a bug when converting series
of 4 bytes into network-order integers when char is signed (which is
controllable by a compiler option, and the default signedness of char is
platform-specific). When a string contains characters with the high bit
set, this bug causes results that, although internally consistent within
Git, does not accord with other implementations of murmur3 (thus,
the changed path filters wouldn't be readable by other off-the-shelf
implementatios of murmur3) and even with Git binaries that were compiled
with different signedness of char. This bug affects both how Git writes
changed path filters to disk and how Git interprets changed path filters
on disk.

Therefore, introduce a new version (2) of changed path filters that
corrects this problem. The existing version (1) is still supported and
is still the default, but users should migrate away from it as soon
as possible.

Because this bug only manifests with characters that have the high bit
set, it may be possible that some (or all) commits in a given repo would
have the same changed path filter both before and after this fix is
applied. However, in order to determine whether this is the case, the
changed paths would first have to be computed, at which point it is not
much more expensive to just compute a new changed path filter.

So this patch does not include any mechanism to "salvage" changed path
filters from repositories. There is also no "mixed" mode - for each
invocation of Git, reading and writing changed path filters are done
with the same version number; this version number may be explicitly
stated (typically if the user knows which version they need) or
automatically determined from the version of the existing changed path
filters in the repository.

There is a change in write_commit_graph(). graph_read_bloom_data()
makes it possible for chunk_bloom_data to be non-NULL but
bloom_filter_settings to be NULL, which causes a segfault later on. I
produced such a segfault while developing this patch, but couldn't find
a way to reproduce it neither after this complete patch (or before),
but in any case it seemed like a good thing to include that might help
future patch authors.

The value in t0095 was obtained from another murmur3 implementation
using the following Go source code:

  package main

  import "fmt"
  import "github.com/spaolacci/murmur3"

  func main() {
          fmt.Printf("%x\n", murmur3.Sum32([]byte("Hello world!")))
          fmt.Printf("%x\n", murmur3.Sum32([]byte{0x99, 0xaa, 0xbb, 0xcc, 0xdd, 0xee, 0xff}))
  }

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 Documentation/config/commitgraph.txt |   5 +-
 bloom.c                              |  69 +++++++++++-
 bloom.h                              |   8 +-
 commit-graph.c                       |  13 ++-
 t/helper/test-bloom.c                |   9 +-
 t/t0095-bloom.sh                     |   8 ++
 t/t4216-log-bloom.sh                 | 155 ++++++++++++++++++++++++++-
 7 files changed, 252 insertions(+), 15 deletions(-)

diff --git a/Documentation/config/commitgraph.txt b/Documentation/config/commitgraph.txt
index e68cdededa..7f8c9d6638 100644
--- a/Documentation/config/commitgraph.txt
+++ b/Documentation/config/commitgraph.txt
@@ -15,7 +15,7 @@ commitGraph.readChangedPaths::
 
 commitGraph.changedPathsVersion::
 	Specifies the version of the changed-path Bloom filters that Git will read and
-	write. May be -1, 0 or 1. Note that values greater than 1 may be
+	write. May be -1, 0, 1, or 2. Note that values greater than 1 may be
 	incompatible with older versions of Git which do not yet understand
 	those versions. Use caution when operating in a mixed-version
 	environment.
@@ -31,4 +31,7 @@ filters when instructed to write.
 If 1, Git will only read version 1 Bloom filters, and will write version 1
 Bloom filters.
 +
+If 2, Git will only read version 2 Bloom filters, and will write version 2
+Bloom filters.
++
 See linkgit:git-commit-graph[1] for more information.
diff --git a/bloom.c b/bloom.c
index c24489dbcf..323d8012b8 100644
--- a/bloom.c
+++ b/bloom.c
@@ -100,7 +100,64 @@ int load_bloom_filter_from_graph(struct commit_graph *g,
  * Not considered to be cryptographically secure.
  * Implemented as described in https://en.wikipedia.org/wiki/MurmurHash#Algorithm
  */
-uint32_t murmur3_seeded(uint32_t seed, const char *data, size_t len)
+uint32_t murmur3_seeded_v2(uint32_t seed, const char *data, size_t len)
+{
+	const uint32_t c1 = 0xcc9e2d51;
+	const uint32_t c2 = 0x1b873593;
+	const uint32_t r1 = 15;
+	const uint32_t r2 = 13;
+	const uint32_t m = 5;
+	const uint32_t n = 0xe6546b64;
+	int i;
+	uint32_t k1 = 0;
+	const char *tail;
+
+	int len4 = len / sizeof(uint32_t);
+
+	uint32_t k;
+	for (i = 0; i < len4; i++) {
+		uint32_t byte1 = (uint32_t)(unsigned char)data[4*i];
+		uint32_t byte2 = ((uint32_t)(unsigned char)data[4*i + 1]) << 8;
+		uint32_t byte3 = ((uint32_t)(unsigned char)data[4*i + 2]) << 16;
+		uint32_t byte4 = ((uint32_t)(unsigned char)data[4*i + 3]) << 24;
+		k = byte1 | byte2 | byte3 | byte4;
+		k *= c1;
+		k = rotate_left(k, r1);
+		k *= c2;
+
+		seed ^= k;
+		seed = rotate_left(seed, r2) * m + n;
+	}
+
+	tail = (data + len4 * sizeof(uint32_t));
+
+	switch (len & (sizeof(uint32_t) - 1)) {
+	case 3:
+		k1 ^= ((uint32_t)(unsigned char)tail[2]) << 16;
+		/*-fallthrough*/
+	case 2:
+		k1 ^= ((uint32_t)(unsigned char)tail[1]) << 8;
+		/*-fallthrough*/
+	case 1:
+		k1 ^= ((uint32_t)(unsigned char)tail[0]) << 0;
+		k1 *= c1;
+		k1 = rotate_left(k1, r1);
+		k1 *= c2;
+		seed ^= k1;
+		break;
+	}
+
+	seed ^= (uint32_t)len;
+	seed ^= (seed >> 16);
+	seed *= 0x85ebca6b;
+	seed ^= (seed >> 13);
+	seed *= 0xc2b2ae35;
+	seed ^= (seed >> 16);
+
+	return seed;
+}
+
+static uint32_t murmur3_seeded_v1(uint32_t seed, const char *data, size_t len)
 {
 	const uint32_t c1 = 0xcc9e2d51;
 	const uint32_t c2 = 0x1b873593;
@@ -165,8 +222,14 @@ void fill_bloom_key(const char *data,
 	int i;
 	const uint32_t seed0 = 0x293ae76f;
 	const uint32_t seed1 = 0x7e646e2c;
-	const uint32_t hash0 = murmur3_seeded(seed0, data, len);
-	const uint32_t hash1 = murmur3_seeded(seed1, data, len);
+	uint32_t hash0, hash1;
+	if (settings->hash_version == 2) {
+		hash0 = murmur3_seeded_v2(seed0, data, len);
+		hash1 = murmur3_seeded_v2(seed1, data, len);
+	} else {
+		hash0 = murmur3_seeded_v1(seed0, data, len);
+		hash1 = murmur3_seeded_v1(seed1, data, len);
+	}
 
 	key->hashes = (uint32_t *)xcalloc(settings->num_hashes, sizeof(uint32_t));
 	for (i = 0; i < settings->num_hashes; i++)
diff --git a/bloom.h b/bloom.h
index 052a993aab..bfe389e29c 100644
--- a/bloom.h
+++ b/bloom.h
@@ -8,9 +8,11 @@ struct commit_graph;
 struct bloom_filter_settings {
 	/*
 	 * The version of the hashing technique being used.
-	 * We currently only support version = 1 which is
+	 * The newest version is 2, which is
 	 * the seeded murmur3 hashing technique implemented
-	 * in bloom.c.
+	 * in bloom.c. Bloom filters of version 1 were created
+	 * with prior versions of Git, which had a bug in the
+	 * implementation of the hash function.
 	 */
 	uint32_t hash_version;
 
@@ -81,7 +83,7 @@ int load_bloom_filter_from_graph(struct commit_graph *g,
  * Not considered to be cryptographically secure.
  * Implemented as described in https://en.wikipedia.org/wiki/MurmurHash#Algorithm
  */
-uint32_t murmur3_seeded(uint32_t seed, const char *data, size_t len);
+uint32_t murmur3_seeded_v2(uint32_t seed, const char *data, size_t len);
 
 void fill_bloom_key(const char *data,
 		    size_t len,
diff --git a/commit-graph.c b/commit-graph.c
index d6fb714f32..ce4524e2b0 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -344,7 +344,6 @@ static int graph_read_bloom_data(const unsigned char *chunk_start,
 				  size_t chunk_size, void *data)
 {
 	struct commit_graph *g = data;
-	uint32_t hash_version;
 
 	if (chunk_size < BLOOMDATA_CHUNK_HEADER_SIZE) {
 		warning(_("ignoring too-small changed-path chunk"
@@ -356,10 +355,9 @@ static int graph_read_bloom_data(const unsigned char *chunk_start,
 
 	g->chunk_bloom_data = chunk_start;
 	g->chunk_bloom_data_size = chunk_size;
-	hash_version = get_be32(chunk_start);
 
 	g->bloom_filter_settings = xmalloc(sizeof(struct bloom_filter_settings));
-	g->bloom_filter_settings->hash_version = hash_version;
+	g->bloom_filter_settings->hash_version = get_be32(chunk_start);
 	g->bloom_filter_settings->num_hashes = get_be32(chunk_start + 4);
 	g->bloom_filter_settings->bits_per_entry = get_be32(chunk_start + 8);
 	g->bloom_filter_settings->max_changed_paths = DEFAULT_BLOOM_MAX_CHANGES;
@@ -2498,6 +2496,13 @@ int write_commit_graph(struct object_directory *odb,
 	}
 	if (!commit_graph_compatible(r))
 		return 0;
+	if (r->settings.commit_graph_changed_paths_version < -1
+	    || r->settings.commit_graph_changed_paths_version > 2) {
+		warning(_("attempting to write a commit-graph, but "
+			  "'commitGraph.changedPathsVersion' (%d) is not supported"),
+			r->settings.commit_graph_changed_paths_version);
+		return 0;
+	}
 
 	CALLOC_ARRAY(ctx, 1);
 	ctx->r = r;
@@ -2540,7 +2545,7 @@ int write_commit_graph(struct object_directory *odb,
 		g = ctx->r->objects->commit_graph;
 
 		/* We have changed-paths already. Keep them in the next graph */
-		if (g && g->chunk_bloom_data) {
+		if (g && g->bloom_filter_settings) {
 			ctx->changed_paths = 1;
 
 			/* don't propagate the hash_version unless unspecified */
diff --git a/t/helper/test-bloom.c b/t/helper/test-bloom.c
index 1281e66876..eefc1668c7 100644
--- a/t/helper/test-bloom.c
+++ b/t/helper/test-bloom.c
@@ -49,6 +49,7 @@ static void get_bloom_filter_for_commit(const struct object_id *commit_oid)
 
 static const char *bloom_usage = "\n"
 "  test-tool bloom get_murmur3 <string>\n"
+"  test-tool bloom get_murmur3_seven_highbit\n"
 "  test-tool bloom generate_filter <string> [<string>...]\n"
 "  test-tool bloom get_filter_for_commit <commit-hex>\n";
 
@@ -63,7 +64,13 @@ int cmd__bloom(int argc, const char **argv)
 		uint32_t hashed;
 		if (argc < 3)
 			usage(bloom_usage);
-		hashed = murmur3_seeded(0, argv[2], strlen(argv[2]));
+		hashed = murmur3_seeded_v2(0, argv[2], strlen(argv[2]));
+		printf("Murmur3 Hash with seed=0:0x%08x\n", hashed);
+	}
+
+	if (!strcmp(argv[1], "get_murmur3_seven_highbit")) {
+		uint32_t hashed;
+		hashed = murmur3_seeded_v2(0, "\x99\xaa\xbb\xcc\xdd\xee\xff", 7);
 		printf("Murmur3 Hash with seed=0:0x%08x\n", hashed);
 	}
 
diff --git a/t/t0095-bloom.sh b/t/t0095-bloom.sh
index b567383eb8..c8d84ab606 100755
--- a/t/t0095-bloom.sh
+++ b/t/t0095-bloom.sh
@@ -29,6 +29,14 @@ test_expect_success 'compute unseeded murmur3 hash for test string 2' '
 	test_cmp expect actual
 '
 
+test_expect_success 'compute unseeded murmur3 hash for test string 3' '
+	cat >expect <<-\EOF &&
+	Murmur3 Hash with seed=0:0xa183ccfd
+	EOF
+	test-tool bloom get_murmur3_seven_highbit >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'compute bloom key for empty string' '
 	cat >expect <<-\EOF &&
 	Hashes:0x5615800c|0x5b966560|0x61174ab4|0x66983008|0x6c19155c|0x7199fab0|0x771ae004|
diff --git a/t/t4216-log-bloom.sh b/t/t4216-log-bloom.sh
index 49d1113171..cc6e5733f6 100755
--- a/t/t4216-log-bloom.sh
+++ b/t/t4216-log-bloom.sh
@@ -488,14 +488,49 @@ test_expect_success 'merge graph layers with incompatible Bloom settings' '
 	test_must_be_empty err
 '
 
+# chosen to be the same under all Unicode normalization forms
+CENT=$(printf "\302\242")
+
+test_expect_success 'ensure Bloom filter with incompatible versions are ignored' '
+	rm "$repo/$graph" &&
+
+	git -C $repo log --oneline --no-decorate -- $CENT >expect &&
+
+	# Compute v1 Bloom filters for commits at the bottom.
+	git -C $repo rev-parse HEAD^ >in &&
+	git -C $repo commit-graph write --stdin-commits --changed-paths \
+		--split <in &&
+
+	# Compute v2 Bloomfilters for the rest of the commits at the top.
+	git -C $repo rev-parse HEAD >in &&
+	git -C $repo -c commitGraph.changedPathsVersion=2 commit-graph write \
+		--stdin-commits --changed-paths --split=no-merge <in &&
+
+	test_line_count = 2 $repo/$chain &&
+
+	git -C $repo log --oneline --no-decorate -- $CENT >actual 2>err &&
+	test_cmp expect actual &&
+
+	layer="$(head -n 1 $repo/$chain)" &&
+	cat >expect.err <<-EOF &&
+	warning: disabling Bloom filters for commit-graph layer $SQ$layer$SQ due to incompatible settings
+	EOF
+	test_cmp expect.err err &&
+
+	# Merge the two layers with incompatible bloom filter versions,
+	# ensuring that the v2 filters are used.
+	>trace2.txt &&
+	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \
+		git -C $repo -c commitGraph.changedPathsVersion=2 commit-graph write --reachable --changed-paths 2>err &&
+	grep "disabling Bloom filters for commit-graph layer .$layer." err &&
+	grep "{\"hash_version\":2,\"num_hashes\":7,\"bits_per_entry\":10,\"max_changed_paths\":512" trace2.txt
+'
+
 get_first_changed_path_filter () {
 	test-tool read-graph bloom-filters >filters.dat &&
 	head -n 1 filters.dat
 }
 
-# chosen to be the same under all Unicode normalization forms
-CENT=$(printf "\302\242")
-
 test_expect_success 'set up repo with high bit path, version 1 changed-path' '
 	git init highbit1 &&
 	test_commit -C highbit1 c1 "$CENT" &&
@@ -539,6 +574,120 @@ test_expect_success 'version 1 changed-path used when version 1 requested' '
 	)
 '
 
+test_expect_success 'version 1 changed-path not used when version 2 requested' '
+	(
+		cd highbit1 &&
+		git config --add commitGraph.changedPathsVersion 2 &&
+		test_bloom_filters_not_used "-- another$CENT"
+	)
+'
+
+test_expect_success 'version 1 changed-path used when autodetect requested' '
+	(
+		cd highbit1 &&
+		git config --add commitGraph.changedPathsVersion -1 &&
+		test_bloom_filters_used "-- another$CENT"
+	)
+'
+
+test_expect_success 'when writing another commit graph, preserve existing version 1 of changed-path' '
+	test_commit -C highbit1 c1double "$CENT$CENT" &&
+	git -C highbit1 commit-graph write --reachable --changed-paths &&
+	(
+		cd highbit1 &&
+		git config --add commitGraph.changedPathsVersion -1 &&
+		echo "options: bloom(1,10,7) read_generation_data" >expect &&
+		test-tool read-graph >full &&
+		grep options full >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'set up repo with high bit path, version 2 changed-path' '
+	git init highbit2 &&
+	git -C highbit2 config --add commitGraph.changedPathsVersion 2 &&
+	test_commit -C highbit2 c2 "$CENT" &&
+	git -C highbit2 commit-graph write --reachable --changed-paths
+'
+
+test_expect_success 'check value of version 2 changed-path' '
+	(
+		cd highbit2 &&
+		echo "c01f" >expect &&
+		get_first_changed_path_filter >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'setup make another commit' '
+	# "git log" does not use Bloom filters for root commits - see how, in
+	# revision.c, rev_compare_tree() (the only code path that eventually calls
+	# get_bloom_filter()) is only called by try_to_simplify_commit() when the commit
+	# has one parent. Therefore, make another commit so that we perform the tests on
+	# a non-root commit.
+	test_commit -C highbit2 anotherc2 "another$CENT"
+'
+
+test_expect_success 'version 2 changed-path used when version 2 requested' '
+	(
+		cd highbit2 &&
+		test_bloom_filters_used "-- another$CENT"
+	)
+'
+
+test_expect_success 'version 2 changed-path not used when version 1 requested' '
+	(
+		cd highbit2 &&
+		git config --add commitGraph.changedPathsVersion 1 &&
+		test_bloom_filters_not_used "-- another$CENT"
+	)
+'
+
+test_expect_success 'version 2 changed-path used when autodetect requested' '
+	(
+		cd highbit2 &&
+		git config --add commitGraph.changedPathsVersion -1 &&
+		test_bloom_filters_used "-- another$CENT"
+	)
+'
+
+test_expect_success 'when writing another commit graph, preserve existing version 2 of changed-path' '
+	test_commit -C highbit2 c2double "$CENT$CENT" &&
+	git -C highbit2 commit-graph write --reachable --changed-paths &&
+	(
+		cd highbit2 &&
+		git config --add commitGraph.changedPathsVersion -1 &&
+		echo "options: bloom(2,10,7) read_generation_data" >expect &&
+		test-tool read-graph >full &&
+		grep options full >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'when writing commit graph, do not reuse changed-path of another version' '
+	git init doublewrite &&
+	test_commit -C doublewrite c "$CENT" &&
+	git -C doublewrite config --add commitGraph.changedPathsVersion 1 &&
+	git -C doublewrite commit-graph write --reachable --changed-paths &&
+	for v in -2 3
+	do
+		git -C doublewrite config --add commitGraph.changedPathsVersion $v &&
+		git -C doublewrite commit-graph write --reachable --changed-paths 2>err &&
+		cat >expect <<-EOF &&
+		warning: attempting to write a commit-graph, but ${SQ}commitGraph.changedPathsVersion${SQ} ($v) is not supported
+		EOF
+		test_cmp expect err || return 1
+	done &&
+	git -C doublewrite config --add commitGraph.changedPathsVersion 2 &&
+	git -C doublewrite commit-graph write --reachable --changed-paths &&
+	(
+		cd doublewrite &&
+		echo "c01f" >expect &&
+		get_first_changed_path_filter >actual &&
+		test_cmp expect actual
+	)
+'
+
 corrupt_graph () {
 	test_when_finished "rm -rf $graph" &&
 	git commit-graph write --reachable --changed-paths &&

From df3df2dcf49d3d2c379354486ea5e3e64d935984 Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:40:07 -0400
Subject: [PATCH 14/16] object.h: fix mis-aligned flag bits table

Bit position 23 is one column too far to the left.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 object.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/object.h b/object.h
index 114d45954d..db25714b4e 100644
--- a/object.h
+++ b/object.h
@@ -62,7 +62,7 @@ void object_array_init(struct object_array *array);
 
 /*
  * object flag allocation:
- * revision.h:               0---------10         15             23------27
+ * revision.h:               0---------10         15               23------27
  * fetch-pack.c:             01    67
  * negotiator/default.c:       2--5
  * walker.c:                 0-2

From 5421e7c3a119bc869c25e2e3d5692c86bb8aa5d9 Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:40:11 -0400
Subject: [PATCH 15/16] commit-graph: reuse existing Bloom filters where
 possible
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

In an earlier commit, a bug was described where it's possible for Git to
produce non-murmur3 hashes when the platform's "char" type is signed,
and there are paths with characters whose highest bit is set (i.e. all
characters >= 0x80).

That patch allows the caller to control which version of Bloom filters
are read and written. However, even on platforms with a signed "char"
type, it is possible to reuse existing Bloom filters if and only if
there are no changed paths in any commit's first parent tree-diff whose
characters have their highest bit set.

When this is the case, we can reuse the existing filter without having
to compute a new one. This is done by marking trees which are known to
have (or not have) any such paths. When a commit's root tree is verified
to not have any such paths, we mark it as such and declare that the
commit's Bloom filter is reusable.

Note that this heuristic only goes in one direction. If neither a commit
nor its first parent have any paths in their trees with non-ASCII
characters, then we know for certain that a path with non-ASCII
characters will not appear in a tree-diff against that commit's first
parent. The reverse isn't necessarily true: just because the tree-diff
doesn't contain any such paths does not imply that no such paths exist
in either tree.

So we end up recomputing some Bloom filters that we don't strictly have
to (i.e. their bits are the same no matter which version of murmur3 we
use). But culling these out is impossible, since we'd have to perform
the full tree-diff, which is the same effort as computing the Bloom
filter from scratch.

But because we can cache our results in each tree's flag bits, we can
often avoid recomputing many filters, thereby reducing the time it takes
to run

    $ git commit-graph write --changed-paths --reachable

when upgrading from v1 to v2 Bloom filters.

To benchmark this, let's generate a commit-graph in linux.git with v1
changed-paths in generation order[^1]:

    $ git clone git@github.com:torvalds/linux.git
    $ cd linux
    $ git commit-graph write --reachable --changed-paths
    $ graph=".git/objects/info/commit-graph"
    $ mv $graph{,.bak}

Then let's time how long it takes to go from v1 to v2 filters (with and
without the upgrade path enabled), resetting the state of the
commit-graph each time:

    $ git config commitGraph.changedPathsVersion 2
    $ hyperfine -p 'cp -f $graph.bak $graph' -L v 0,1 \
        'GIT_TEST_UPGRADE_BLOOM_FILTERS={v} git.compile commit-graph write --reachable --changed-paths'

On linux.git (where there aren't any non-ASCII paths), the timings
indicate that this patch represents a speed-up over recomputing all
Bloom filters from scratch:

    Benchmark 1: GIT_TEST_UPGRADE_BLOOM_FILTERS=0 git.compile commit-graph write --reachable --changed-paths
      Time (mean ± σ):     124.873 s ±  0.316 s    [User: 124.081 s, System: 0.643 s]
      Range (min … max):   124.621 s … 125.227 s    3 runs

    Benchmark 2: GIT_TEST_UPGRADE_BLOOM_FILTERS=1 git.compile commit-graph write --reachable --changed-paths
      Time (mean ± σ):     79.271 s ±  0.163 s    [User: 74.611 s, System: 4.521 s]
      Range (min … max):   79.112 s … 79.437 s    3 runs

    Summary
      'GIT_TEST_UPGRADE_BLOOM_FILTERS=1 git.compile commit-graph write --reachable --changed-paths' ran
        1.58 ± 0.01 times faster than 'GIT_TEST_UPGRADE_BLOOM_FILTERS=0 git.compile commit-graph write --reachable --changed-paths'

On git.git, we do have some non-ASCII paths, giving us a more modest
improvement from 4.163 seconds to 3.348 seconds, for a 1.24x speed-up.
On my machine, the stats for git.git are:

  - 8,285 Bloom filters computed from scratch
  - 10 Bloom filters generated as empty
  - 4 Bloom filters generated as truncated due to too many changed paths
  - 65,114 Bloom filters were reused when transitioning from v1 to v2.

[^1]: Note that this is is important, since `--stdin-packs` or
  `--stdin-commits` orders commits in the commit-graph by their pack
  position (with `--stdin-packs`) or in the raw input (with
  `--stdin-commits`).

  Since we compute Bloom filters in the same order that commits appear
  in the graph, we must see a commit's (first) parent before we process
  the commit itself. This is only guaranteed to happen when sorting
  commits by their generation number.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 bloom.c              | 90 ++++++++++++++++++++++++++++++++++++++++++--
 bloom.h              |  1 +
 commit-graph.c       |  5 +++
 object.h             |  1 +
 t/t4216-log-bloom.sh | 39 ++++++++++++++++++-
 5 files changed, 132 insertions(+), 4 deletions(-)

diff --git a/bloom.c b/bloom.c
index 323d8012b8..740c1767ea 100644
--- a/bloom.c
+++ b/bloom.c
@@ -6,6 +6,9 @@
 #include "commit-graph.h"
 #include "commit.h"
 #include "commit-slab.h"
+#include "tree.h"
+#include "tree-walk.h"
+#include "config.h"
 
 define_commit_slab(bloom_filter_slab, struct bloom_filter);
 
@@ -283,6 +286,73 @@ static void init_truncated_large_filter(struct bloom_filter *filter,
 	filter->version = version;
 }
 
+#define VISITED   (1u<<21)
+#define HIGH_BITS (1u<<22)
+
+static int has_entries_with_high_bit(struct repository *r, struct tree *t)
+{
+	if (parse_tree(t))
+		return 1;
+
+	if (!(t->object.flags & VISITED)) {
+		struct tree_desc desc;
+		struct name_entry entry;
+
+		init_tree_desc(&desc, &t->object.oid, t->buffer, t->size);
+		while (tree_entry(&desc, &entry)) {
+			size_t i;
+			for (i = 0; i < entry.pathlen; i++) {
+				if (entry.path[i] & 0x80) {
+					t->object.flags |= HIGH_BITS;
+					goto done;
+				}
+			}
+
+			if (S_ISDIR(entry.mode)) {
+				struct tree *sub = lookup_tree(r, &entry.oid);
+				if (sub && has_entries_with_high_bit(r, sub)) {
+					t->object.flags |= HIGH_BITS;
+					goto done;
+				}
+			}
+
+		}
+
+done:
+		t->object.flags |= VISITED;
+	}
+
+	return !!(t->object.flags & HIGH_BITS);
+}
+
+static int commit_tree_has_high_bit_paths(struct repository *r,
+					  struct commit *c)
+{
+	struct tree *t;
+	if (repo_parse_commit(r, c))
+		return 1;
+	t = repo_get_commit_tree(r, c);
+	if (!t)
+		return 1;
+	return has_entries_with_high_bit(r, t);
+}
+
+static struct bloom_filter *upgrade_filter(struct repository *r, struct commit *c,
+					   struct bloom_filter *filter,
+					   int hash_version)
+{
+	struct commit_list *p = c->parents;
+	if (commit_tree_has_high_bit_paths(r, c))
+		return NULL;
+
+	if (p && commit_tree_has_high_bit_paths(r, p->item))
+		return NULL;
+
+	filter->version = hash_version;
+
+	return filter;
+}
+
 struct bloom_filter *get_bloom_filter(struct repository *r, struct commit *c)
 {
 	struct bloom_filter *filter;
@@ -325,9 +395,23 @@ struct bloom_filter *get_or_compute_bloom_filter(struct repository *r,
 						     filter, graph_pos);
 	}
 
-	if ((filter->data && filter->len) &&
-	    (!settings || settings->hash_version == filter->version))
-		return filter;
+	if (filter->data && filter->len) {
+		struct bloom_filter *upgrade;
+		if (!settings || settings->hash_version == filter->version)
+			return filter;
+
+		/* version mismatch, see if we can upgrade */
+		if (compute_if_not_present &&
+		    git_env_bool("GIT_TEST_UPGRADE_BLOOM_FILTERS", 1)) {
+			upgrade = upgrade_filter(r, c, filter,
+						 settings->hash_version);
+			if (upgrade) {
+				if (computed)
+					*computed |= BLOOM_UPGRADED;
+				return upgrade;
+			}
+		}
+	}
 	if (!compute_if_not_present)
 		return NULL;
 
diff --git a/bloom.h b/bloom.h
index bfe389e29c..e3a9b68905 100644
--- a/bloom.h
+++ b/bloom.h
@@ -102,6 +102,7 @@ enum bloom_filter_computed {
 	BLOOM_COMPUTED     = (1 << 1),
 	BLOOM_TRUNC_LARGE  = (1 << 2),
 	BLOOM_TRUNC_EMPTY  = (1 << 3),
+	BLOOM_UPGRADED     = (1 << 4),
 };
 
 struct bloom_filter *get_or_compute_bloom_filter(struct repository *r,
diff --git a/commit-graph.c b/commit-graph.c
index ce4524e2b0..e401e31a78 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1165,6 +1165,7 @@ struct write_commit_graph_context {
 	int count_bloom_filter_not_computed;
 	int count_bloom_filter_trunc_empty;
 	int count_bloom_filter_trunc_large;
+	int count_bloom_filter_upgraded;
 };
 
 static int write_graph_chunk_fanout(struct hashfile *f,
@@ -1772,6 +1773,8 @@ static void trace2_bloom_filter_write_statistics(struct write_commit_graph_conte
 			   ctx->count_bloom_filter_trunc_empty);
 	trace2_data_intmax("commit-graph", ctx->r, "filter-trunc-large",
 			   ctx->count_bloom_filter_trunc_large);
+	trace2_data_intmax("commit-graph", ctx->r, "filter-upgraded",
+			   ctx->count_bloom_filter_upgraded);
 }
 
 static void compute_bloom_filters(struct write_commit_graph_context *ctx)
@@ -1813,6 +1816,8 @@ static void compute_bloom_filters(struct write_commit_graph_context *ctx)
 				ctx->count_bloom_filter_trunc_empty++;
 			if (computed & BLOOM_TRUNC_LARGE)
 				ctx->count_bloom_filter_trunc_large++;
+		} else if (computed & BLOOM_UPGRADED) {
+			ctx->count_bloom_filter_upgraded++;
 		} else if (computed & BLOOM_NOT_COMPUTED)
 			ctx->count_bloom_filter_not_computed++;
 		ctx->total_bloom_filter_data_size += filter
diff --git a/object.h b/object.h
index db25714b4e..2e5e08725f 100644
--- a/object.h
+++ b/object.h
@@ -75,6 +75,7 @@ void object_array_init(struct object_array *array);
  * commit-reach.c:                                  16-----19
  * sha1-name.c:                                              20
  * list-objects-filter.c:                                      21
+ * bloom.c:                                                    2122
  * builtin/fsck.c:           0--3
  * builtin/gc.c:             0
  * builtin/index-pack.c:                                     2021
diff --git a/t/t4216-log-bloom.sh b/t/t4216-log-bloom.sh
index cc6e5733f6..3f163dc396 100755
--- a/t/t4216-log-bloom.sh
+++ b/t/t4216-log-bloom.sh
@@ -222,6 +222,10 @@ test_filter_trunc_large () {
 	grep "\"key\":\"filter-trunc-large\",\"value\":\"$1\"" $2
 }
 
+test_filter_upgraded () {
+	grep "\"key\":\"filter-upgraded\",\"value\":\"$1\"" $2
+}
+
 test_expect_success 'correctly report changes over limit' '
 	git init limits &&
 	(
@@ -667,7 +671,14 @@ test_expect_success 'when writing another commit graph, preserve existing versio
 test_expect_success 'when writing commit graph, do not reuse changed-path of another version' '
 	git init doublewrite &&
 	test_commit -C doublewrite c "$CENT" &&
+
 	git -C doublewrite config --add commitGraph.changedPathsVersion 1 &&
+	>trace2.txt &&
+	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \
+		git -C doublewrite commit-graph write --reachable --changed-paths &&
+	test_filter_computed 1 trace2.txt &&
+	test_filter_upgraded 0 trace2.txt &&
+
 	git -C doublewrite commit-graph write --reachable --changed-paths &&
 	for v in -2 3
 	do
@@ -678,8 +689,14 @@ test_expect_success 'when writing commit graph, do not reuse changed-path of ano
 		EOF
 		test_cmp expect err || return 1
 	done &&
+
 	git -C doublewrite config --add commitGraph.changedPathsVersion 2 &&
-	git -C doublewrite commit-graph write --reachable --changed-paths &&
+	>trace2.txt &&
+	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \
+		git -C doublewrite commit-graph write --reachable --changed-paths &&
+	test_filter_computed 1 trace2.txt &&
+	test_filter_upgraded 0 trace2.txt &&
+
 	(
 		cd doublewrite &&
 		echo "c01f" >expect &&
@@ -688,6 +705,26 @@ test_expect_success 'when writing commit graph, do not reuse changed-path of ano
 	)
 '
 
+test_expect_success 'when writing commit graph, reuse changed-path of another version where possible' '
+	git init upgrade &&
+
+	test_commit -C upgrade base no-high-bits &&
+
+	git -C upgrade config --add commitGraph.changedPathsVersion 1 &&
+	>trace2.txt &&
+	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \
+		git -C upgrade commit-graph write --reachable --changed-paths &&
+	test_filter_computed 1 trace2.txt &&
+	test_filter_upgraded 0 trace2.txt &&
+
+	git -C upgrade config --add commitGraph.changedPathsVersion 2 &&
+	>trace2.txt &&
+	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \
+		git -C upgrade commit-graph write --reachable --changed-paths &&
+	test_filter_computed 0 trace2.txt &&
+	test_filter_upgraded 1 trace2.txt
+'
+
 corrupt_graph () {
 	test_when_finished "rm -rf $graph" &&
 	git commit-graph write --reachable --changed-paths &&

From 9c8a9ec787149dc4f4b278d9bd8ad94c96691a5f Mon Sep 17 00:00:00 2001
From: Taylor Blau <me@ttaylorr.com>
Date: Tue, 25 Jun 2024 13:40:15 -0400
Subject: [PATCH 16/16] bloom: introduce `deinit_bloom_filters()`

After we are done using Bloom filters, we do not currently clean up any
memory allocated by the commit slab used to store those filters in the
first place.

Besides the bloom_filter structures themselves, there is mostly nothing
to free() in the first place, since in the read-only path all Bloom
filter's `data` members point to a memory mapped region in the
commit-graph file itself.

But when generating Bloom filters from scratch (or initializing
truncated filters) we allocate additional memory to store the filter's
data.

Keep track of when we need to free() this additional chunk of memory by
using an extra pointer `to_free`. Most of the time this will be NULL
(indicating that we are representing an existing Bloom filter stored in
a memory mapped region). When it is non-NULL, free it before discarding
the Bloom filters slab.

Suggested-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 bloom.c        | 16 +++++++++++++++-
 bloom.h        |  3 +++
 commit-graph.c |  4 ++++
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/bloom.c b/bloom.c
index 740c1767ea..d080a1b616 100644
--- a/bloom.c
+++ b/bloom.c
@@ -92,6 +92,7 @@ int load_bloom_filter_from_graph(struct commit_graph *g,
 					sizeof(unsigned char) * start_index +
 					BLOOMDATA_CHUNK_HEADER_SIZE);
 	filter->version = g->bloom_filter_settings->hash_version;
+	filter->to_free = NULL;
 
 	return 1;
 }
@@ -264,6 +265,18 @@ void init_bloom_filters(void)
 	init_bloom_filter_slab(&bloom_filters);
 }
 
+static void free_one_bloom_filter(struct bloom_filter *filter)
+{
+	if (!filter)
+		return;
+	free(filter->to_free);
+}
+
+void deinit_bloom_filters(void)
+{
+	deep_clear_bloom_filter_slab(&bloom_filters, free_one_bloom_filter);
+}
+
 static int pathmap_cmp(const void *hashmap_cmp_fn_data UNUSED,
 		       const struct hashmap_entry *eptr,
 		       const struct hashmap_entry *entry_or_key,
@@ -280,7 +293,7 @@ static int pathmap_cmp(const void *hashmap_cmp_fn_data UNUSED,
 static void init_truncated_large_filter(struct bloom_filter *filter,
 					int version)
 {
-	filter->data = xmalloc(1);
+	filter->data = filter->to_free = xmalloc(1);
 	filter->data[0] = 0xFF;
 	filter->len = 1;
 	filter->version = version;
@@ -482,6 +495,7 @@ struct bloom_filter *get_or_compute_bloom_filter(struct repository *r,
 			filter->len = 1;
 		}
 		CALLOC_ARRAY(filter->data, filter->len);
+		filter->to_free = filter->data;
 
 		hashmap_for_each_entry(&pathmap, &iter, e, entry) {
 			struct bloom_key key;
diff --git a/bloom.h b/bloom.h
index e3a9b68905..d20e64bfbb 100644
--- a/bloom.h
+++ b/bloom.h
@@ -56,6 +56,8 @@ struct bloom_filter {
 	unsigned char *data;
 	size_t len;
 	int version;
+
+	void *to_free;
 };
 
 /*
@@ -96,6 +98,7 @@ void add_key_to_filter(const struct bloom_key *key,
 		       const struct bloom_filter_settings *settings);
 
 void init_bloom_filters(void);
+void deinit_bloom_filters(void);
 
 enum bloom_filter_computed {
 	BLOOM_NOT_COMPUTED = (1 << 0),
diff --git a/commit-graph.c b/commit-graph.c
index e401e31a78..0de0f38b53 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -828,6 +828,7 @@ struct bloom_filter_settings *get_bloom_filter_settings(struct repository *r)
 void close_commit_graph(struct raw_object_store *o)
 {
 	clear_commit_graph_data_slab(&commit_graph_data_slab);
+	deinit_bloom_filters();
 	free_commit_graph(o->commit_graph);
 	o->commit_graph = NULL;
 }
@@ -2646,6 +2647,9 @@ int write_commit_graph(struct object_directory *odb,
 
 	res = write_commit_graph_file(ctx);
 
+	if (ctx->changed_paths)
+		deinit_bloom_filters();
+
 	if (ctx->split)
 		mark_commit_graphs(ctx);