[PATCH 1/4] fsmonitor: use fsmonitor data in `git diff`

["Alex Vandiver via GitGitGadget" <gitgitgadget@gmail.com>]

From: 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.

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

Signed-off-by: Alex Vandiver <alexmv@dropbox.com>
Signed-off-by: Nipunn Koorapati <nipunn@dropbox.com>
---
 diff-lib.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/diff-lib.c b/diff-lib.c
index f95c6de75f..b7ee1b89ef 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -97,6 +97,8 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 
 	diff_set_mnemonic_prefix(&revs->diffopt, "i/", "w/");
 
+	refresh_fsmonitor(istate);
+
 	if (diff_unmerged_stage < 0)
 		diff_unmerged_stage = 2;
 	entries = istate->cache_nr;
@@ -197,8 +199,19 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 		if (ce_uptodate(ce) || ce_skip_worktree(ce))
 			continue;
 
-		/* If CE_VALID is set, don't look at workdir for file removal */
-		if (ce->ce_flags & CE_VALID) {
+		/*
+		 * If CE_VALID is set, the user has promised us that the workdir
+		 * hasn't changed compared to index, so don't stat workdir
+		 * for file removal
+		 *  eg - via git udpate-index --assume-unchanged
+		 *  eg - via core.ignorestat=true
+		 *
+		 * When using FSMONITOR:
+		 * If CE_FSMONITOR_VALID is set, then we know the metadata on disk
+		 * has not changed since the last refresh, and we can skip the
+		 * file-removal checks without doing the stat in check_removed.
+		 */
+		if (ce->ce_flags & CE_VALID || ce->ce_flags & CE_FSMONITOR_VALID) {
 			changed = 0;
 			newmode = ce->ce_mode;
 		} else {
-- 
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