advice: warn when sparse index expands

Typically, forcing a sparse index to expand to a full index means that
Git could not determine the status of a file outside of the
sparse-checkout and needed to expand sparse trees into the full list of
sparse blobs. This operation can be very slow when the sparse-checkout
is much smaller than the full tree at HEAD.

When users are in this state, there is usually a modified or untracked
file outside of the sparse-checkout mentioned by the output of 'git
status'. There are a number of reasons why this is insufficient:

 1. Users may not have a full understanding of which files are inside or
    outside of their sparse-checkout. This is more common in monorepos
    that manage the sparse-checkout using custom tools that map build
    dependencies into sparse-checkout definitions.

 2. In some cases, an empty directory could exist outside the
    sparse-checkout and these empty directories are not reported by 'git
    status' and friends.

 3. If the user has '.gitignore' or 'exclude' files, then 'git status'
    will squelch the warnings and not demonstrate any problems.

In order to help users who are in this state, add a new advice message
to indicate that a sparse index is expanded to a full index. This
message should be written at most once per process, so add a static
global 'give_advice_on_expansion' to sparse-index.c. Further, there is a
case in 'git sparse-checkout set' that uses the sparse index as an
in-memory data structure (even when writing a full index) so we need to
disable the message in that kind of case.

The t1092-sparse-checkout-compatibility.sh test script compares the
behavior of several Git commands across full and sparse repositories,
including sparse repositories with and without a sparse index. We need
to disable the advice in the sparse-index repo to avoid differences in
stderr. By leaving the advice on in the sparse-checkout repo (without
the sparse index), we can test the behavior of disabling the advice in
convert_to_sparse(). (Indeed, these tests are how that necessity was
discovered.) Add a test that reenables the advice and demonstrates that
the message is output.

The advice message is defined outside of expand_index() to avoid super-
wide lines. It is also defined as a macro to avoid compile issues with
-Werror=format-security.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/advice.txt

@@ -109,6 +109,10 @@ advice.*::
 	skippedCherryPicks::
 		Shown when linkgit:git-rebase[1] skips a commit that has already
 		been cherry-picked onto the upstream branch.
+	sparseIndexExpanded::
+		Shown when a sparse index is expanded to a full index, which is likely
+		due to an unexpected set of files existing outside of the
+		sparse-checkout.
 	statusAheadBehind::
 		Shown when linkgit:git-status[1] computes the ahead/behind
 		counts for a local ref compared to its remote tracking ref,

advice.c

@@ -74,6 +74,7 @@ static struct {
 	[ADVICE_SEQUENCER_IN_USE]			= { "sequencerInUse" },
 	[ADVICE_SET_UPSTREAM_FAILURE]			= { "setUpstreamFailure" },
 	[ADVICE_SKIPPED_CHERRY_PICKS]			= { "skippedCherryPicks" },
+	[ADVICE_SPARSE_INDEX_EXPANDED]			= { "sparseIndexExpanded" },
 	[ADVICE_STATUS_AHEAD_BEHIND_WARNING]		= { "statusAheadBehindWarning" },
 	[ADVICE_STATUS_HINTS]				= { "statusHints" },
 	[ADVICE_STATUS_U_OPTION]			= { "statusUoption" },

advice.h

@@ -42,6 +42,7 @@ enum advice_type {
 	ADVICE_SEQUENCER_IN_USE,
 	ADVICE_SET_UPSTREAM_FAILURE,
 	ADVICE_SKIPPED_CHERRY_PICKS,
+	ADVICE_SPARSE_INDEX_EXPANDED,
 	ADVICE_STATUS_AHEAD_BEHIND_WARNING,
 	ADVICE_STATUS_HINTS,
 	ADVICE_STATUS_U_OPTION,

sparse-index.c

@@ -12,6 +12,22 @@
 #include "config.h"
 #include "dir.h"
 #include "fsmonitor-ll.h"
+#include "advice.h"
+
+/**
+ * This global is used by expand_index() to determine if we should give the
+ * advice for advice.sparseIndexExpanded when expanding a sparse index to a full
+ * one. However, this is sometimes done on purpose, such as in the sparse-checkout
+ * builtin, even when index.sparse=false. This may be disabled in
+ * convert_to_sparse().
+ */
+static int give_advice_on_expansion = 1;
+#define ADVICE_MSG \
+	"The sparse index is expanding to a full index, a slow operation.\n"   \
+	"Your working directory likely has contents that are outside of\n"     \
+	"your sparse-checkout patterns. Use 'git sparse-checkout list' to\n"   \
+	"see your sparse-checkout definition and compare it to your working\n" \
+	"directory contents. Running 'git clean' may assist in this cleanup."
 
 struct modify_index_context {
 	struct index_state *write;
@@ -183,6 +199,12 @@ int convert_to_sparse(struct index_state *istate, int flags)
 	    !is_sparse_index_allowed(istate, flags))
 		return 0;
 
+	/*
+	 * If we are purposefully collapsing a full index, then don't give
+	 * advice when it is expanded later.
+	 */
+	give_advice_on_expansion = 0;
+
 	/*
 	 * NEEDSWORK: If we have unmerged entries, then stay full.
 	 * Unmerged entries prevent the cache-tree extension from working.
@@ -328,6 +350,12 @@ void expand_index(struct index_state *istate, struct pattern_list *pl)
 			pl = NULL;
 	}
 
+	if (!pl && give_advice_on_expansion) {
+		give_advice_on_expansion = 0;
+		advise_if_enabled(ADVICE_SPARSE_INDEX_EXPANDED,
+				  _(ADVICE_MSG));
+	}
+
 	/*
 	 * A NULL pattern set indicates we are expanding a full index, so
 	 * we use a special region name that indicates the full expansion.

t/t1092-sparse-checkout-compatibility.sh

@@ -159,7 +159,10 @@ init_repos () {
 	git -C sparse-checkout sparse-checkout set deep &&
 	git -C sparse-index sparse-checkout init --cone --sparse-index &&
 	test_cmp_config -C sparse-index true index.sparse &&
-	git -C sparse-index sparse-checkout set deep
+	git -C sparse-index sparse-checkout set deep &&
+
+	# Disable this message to keep stderr the same.
+	git -C sparse-index config advice.sparseIndexExpanded false
 }
 
 init_repos_as_submodules () {
@@ -2331,4 +2334,15 @@ test_expect_success 'sparse-index is not expanded: check-attr' '
 	ensure_not_expanded check-attr -a --cached -- folder1/a
 '
 
+test_expect_success 'advice.sparseIndexExpanded' '
+	init_repos &&
+
+	git -C sparse-index config --unset advice.sparseIndexExpanded &&
+	git -C sparse-index sparse-checkout set deep/deeper1 &&
+	mkdir -p sparse-index/deep/deeper2/deepest &&
+	touch sparse-index/deep/deeper2/deepest/bogus &&
+	git -C sparse-index status 2>err &&
+	grep "The sparse index is expanding to a full index" err
+'
+
 test_done