pack-revindex: introduce `pack.readReverseIndex`

Since 1615c567b8 (Documentation/config/pack.txt: advertise
'pack.writeReverseIndex', 2021-01-25), we have had the
`pack.writeReverseIndex` configuration option, which tells Git whether
or not it is allowed to write a ".rev" file when indexing a pack.

Introduce a complementary configuration knob, `pack.readReverseIndex` to
control whether or not Git will read any ".rev" file(s) that may be
available on disk.

This option is useful for debugging, as well as disabling the effect of
".rev" files in certain instances.

This is useful because of the trade-off[^1] between the time it takes to
generate a reverse index (slow from scratch, fast when reading an
existing ".rev" file), and the time it takes to access a record (the
opposite).

For example, even though it is faster to use the on-disk reverse index
when computing the on-disk size of a packed object, it is slower to
enumerate the same value for all objects.

Here are a couple of examples from linux.git. When computing the above
for a single object, using the on-disk reverse index is significantly
faster:

    $ git rev-parse HEAD >in
    $ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} cat-file --batch-check="%(objectsize:disk)" <in'
    Benchmark 1: git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" <in
      Time (mean ± σ):     302.5 ms ±  12.5 ms    [User: 258.7 ms, System: 43.6 ms]
      Range (min … max):   291.1 ms … 328.1 ms    10 runs

    Benchmark 2: git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" <in
      Time (mean ± σ):       3.9 ms ±   0.3 ms    [User: 1.6 ms, System: 2.4 ms]
      Range (min … max):     2.0 ms …   4.4 ms    801 runs

    Summary
      'git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" <in' ran
       77.29 ± 7.14 times faster than 'git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" <in'

, but when instead trying to compute the on-disk object size for all
objects in the repository, using the ".rev" file is a disadvantage over
creating the reverse index from scratch:

    $ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} cat-file --batch-check="%(objectsize:disk)" --batch-all-objects'
    Benchmark 1: git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" --batch-all-objects
      Time (mean ± σ):      8.258 s ±  0.035 s    [User: 7.949 s, System: 0.308 s]
      Range (min … max):    8.199 s …  8.293 s    10 runs

    Benchmark 2: git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" --batch-all-objects
      Time (mean ± σ):     16.976 s ±  0.107 s    [User: 16.706 s, System: 0.268 s]
      Range (min … max):   16.839 s … 17.105 s    10 runs

    Summary
      'git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" --batch-all-objects' ran
	2.06 ± 0.02 times faster than 'git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" --batch-all-objects'

Luckily, the results when running `git cat-file` with `--unordered` are
closer together:

    $ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects'
    Benchmark 1: git.compile -c pack.readReverseIndex=false cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects
      Time (mean ± σ):      5.066 s ±  0.105 s    [User: 4.792 s, System: 0.274 s]
      Range (min … max):    4.943 s …  5.220 s    10 runs

    Benchmark 2: git.compile -c pack.readReverseIndex=true cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects
      Time (mean ± σ):      6.193 s ±  0.069 s    [User: 5.937 s, System: 0.255 s]
      Range (min … max):    6.145 s …  6.356 s    10 runs

    Summary
      'git.compile -c pack.readReverseIndex=false cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects' ran
        1.22 ± 0.03 times faster than 'git.compile -c pack.readReverseIndex=true cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects'

Because the equilibrium point between these two is highly machine- and
repository-dependent, allow users to configure whether or not they will
read any ".rev" file(s) with this configuration knob.

[^1]: Generating a reverse index in memory takes O(N) time (where N is
  the number of objects in the repository), since we use a radix sort.
  Reading an entry from an on-disk ".rev" file is slower since each
  operation is bound by disk I/O instead of memory I/O.

  In order to compute the on-disk size of a packed object, we need to
  find the offset of our object, and the adjacent object (the on-disk
  size difference of these two). Finding the first offset requires a
  binary search. Finding the latter involves a single .rev lookup.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/pack.txt

@@ -171,6 +171,12 @@ pack.writeBitmapLookupTable::
 	beneficial in repositories that have relatively large bitmap
 	indexes. Defaults to false.
 
+pack.readReverseIndex::
+	When true, git will read any .rev file(s) that may be available
+	(see: linkgit:gitformat-pack[5]). When false, the reverse index
+	will be generated from scratch and stored in memory. Defaults to
+	true.
+
 pack.writeReverseIndex::
 	When true, git will write a corresponding .rev file (see:
 	linkgit:gitformat-pack[5])

pack-revindex.c

@@ -291,7 +291,10 @@ int load_pack_revindex(struct repository *r, struct packed_git *p)
 	if (p->revindex || p->revindex_data)
 		return 0;
 
-	if (!load_pack_revindex_from_disk(p))
+	prepare_repo_settings(r);
+
+	if (r->settings.pack_read_reverse_index &&
+	    !load_pack_revindex_from_disk(p))
 		return 0;
 	else if (!create_pack_revindex_in_memory(p))
 		return 0;

repo-settings.c

@@ -63,6 +63,7 @@ void prepare_repo_settings(struct repository *r)
 	repo_cfg_bool(r, "core.multipackindex", &r->settings.core_multi_pack_index, 1);
 	repo_cfg_bool(r, "index.sparse", &r->settings.sparse_index, 0);
 	repo_cfg_bool(r, "index.skiphash", &r->settings.index_skip_hash, r->settings.index_skip_hash);
+	repo_cfg_bool(r, "pack.readreverseindex", &r->settings.pack_read_reverse_index, 1);
 
 	/*
 	 * The GIT_TEST_MULTI_PACK_INDEX variable is special in that

repository.h

@@ -37,6 +37,7 @@ struct repo_settings {
 	int fetch_write_commit_graph;
 	int command_requires_full_index;
 	int sparse_index;
+	int pack_read_reverse_index;
 
 	struct fsmonitor_settings *fsmonitor; /* lazily loaded */
 

t/t5325-reverse-index.sh

@@ -96,6 +96,17 @@ test_expect_success 'reverse index is not generated when available on disk' '
 		--batch-check="%(objectsize:disk)" <tip
 '
 
+test_expect_success 'reverse index is ignored when pack.readReverseIndex is false' '
+	test_index_pack true &&
+	test_path_is_file $rev &&
+
+	test_config pack.readReverseIndex false &&
+
+	git rev-parse HEAD >tip &&
+	GIT_TEST_REV_INDEX_DIE_ON_DISK=1 git cat-file \
+		--batch-check="%(objectsize:disk)" <tip
+'
+
 test_expect_success 'revindex in-memory vs on-disk' '
 	git init repo &&
 	test_when_finished "rm -fr repo" &&