[PATCH 6/6] fsmonitor: Use fsmonitor data in `git diff`

[Alex Vandiver <alexmv@dropbox.com>]

With fsmonitor enabled, the first call to match_stat_with_submodule
calls refresh_fsmonitor, incurring the overhead of reading the list of
updated files -- but run_diff_files does not respect the
CE_FSMONITOR_VALID flag.

Make use of the fsmonitor extension to skip lstat() calls on files
that fsmonitor judged as unmodified.  Skip use of the fsmonitor
extension when called by "add", as the format_callback in such cases
expects to be called even when the file is believed to be "up to date"
with the index.

Notably, this change improves performance of the git shell prompt when
GIT_PS1_SHOWDIRTYSTATE is set.

Signed-off-by: Alex Vandiver <alexmv@dropbox.com>
---
 builtin/add.c | 2 +-
 diff-lib.c    | 6 ++++++
 diff.h        | 2 ++
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/builtin/add.c b/builtin/add.c
index bf01d89e2..bba20b46e 100644
--- a/builtin/add.c
+++ b/builtin/add.c
@@ -119,7 +119,7 @@ int add_files_to_cache(const char *prefix,
 	rev.diffopt.format_callback_data = &data;
 	rev.diffopt.flags.override_submodule_config = 1;
 	rev.max_count = 0; /* do not compare unmerged paths with stage #2 */
-	run_diff_files(&rev, DIFF_RACY_IS_MODIFIED);
+	run_diff_files(&rev, DIFF_RACY_IS_MODIFIED | DIFF_SKIP_FSMONITOR);
 	clear_pathspec(&rev.prune_data);
 	return !!data.add_errors;
 }
diff --git a/diff-lib.c b/diff-lib.c
index 8104603a3..13ff00d81 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -95,6 +95,9 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 
 	diff_set_mnemonic_prefix(&revs->diffopt, "i/", "w/");
 
+	if (!(option & DIFF_SKIP_FSMONITOR))
+		refresh_fsmonitor(&the_index);
+
 	if (diff_unmerged_stage < 0)
 		diff_unmerged_stage = 2;
 	entries = active_nr;
@@ -197,6 +200,9 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 		if (ce_uptodate(ce) || ce_skip_worktree(ce))
 			continue;
 
+		if (ce->ce_flags & CE_FSMONITOR_VALID && !(option & DIFF_SKIP_FSMONITOR))
+			continue;
+
 		/* If CE_VALID is set, don't look at workdir for file removal */
 		if (ce->ce_flags & CE_VALID) {
 			changed = 0;
diff --git a/diff.h b/diff.h
index 36a09624f..1060bc495 100644
--- a/diff.h
+++ b/diff.h
@@ -395,6 +395,8 @@ extern const char *diff_aligned_abbrev(const struct object_id *sha1, int);
 #define DIFF_SILENT_ON_REMOVED 01
 /* report racily-clean paths as modified */
 #define DIFF_RACY_IS_MODIFIED 02
+/* skip loading the fsmonitor data */
+#define DIFF_SKIP_FSMONITOR 04
 extern int run_diff_files(struct rev_info *revs, unsigned int option);
 extern int run_diff_index(struct rev_info *revs, int cached);
 
-- 
2.15.1.626.gc4617b774