Merge branch 'tk/empty-untracked-cache'

The untracked cache newly computed weren't written back to the
on-disk index file when there is no other change to the index,
which has been corrected.

* tk/empty-untracked-cache:
  untracked-cache: write index when populating empty untracked cache
  t7519: populate untracked cache before test
  t7519: avoid file to index mtime race for untracked cache

dir.c

@@ -2781,7 +2781,8 @@ void remove_untracked_cache(struct index_state *istate)
 
 static struct untracked_cache_dir *validate_untracked_cache(struct dir_struct *dir,
 						      int base_len,
-						      const struct pathspec *pathspec)
+						      const struct pathspec *pathspec,
+						      struct index_state *istate)
 {
 	struct untracked_cache_dir *root;
 	static int untracked_cache_disabled = -1;
@@ -2845,8 +2846,11 @@ static struct untracked_cache_dir *validate_untracked_cache(struct dir_struct *d
 		return NULL;
 	}
 
-	if (!dir->untracked->root)
+	if (!dir->untracked->root) {
+		/* Untracked cache existed but is not initialized; fix that */
 		FLEX_ALLOC_STR(dir->untracked->root, name, "");
+		istate->cache_changed |= UNTRACKED_CHANGED;
+	}
 
 	/* Validate $GIT_DIR/info/exclude and core.excludesfile */
 	root = dir->untracked->root;
@@ -2916,7 +2920,7 @@ int read_directory(struct dir_struct *dir, struct index_state *istate,
 		return dir->nr;
 	}
 
-	untracked = validate_untracked_cache(dir, len, pathspec);
+	untracked = validate_untracked_cache(dir, len, pathspec, istate);
 	if (!untracked)
 		/*
 		 * make sure untracked cache code path is disabled,

t/t7519-status-fsmonitor.sh

@@ -324,17 +324,24 @@ test_expect_success UNTRACKED_CACHE 'ignore .git changes when invalidating UNTR'
 		cd dot-git &&
 		mkdir -p .git/hooks &&
 		: >tracked &&
+		test-tool chmtime =-60 tracked &&
 		: >modified &&
+		test-tool chmtime =-60 modified &&
 		mkdir dir1 &&
 		: >dir1/tracked &&
+		test-tool chmtime =-60 dir1/tracked &&
 		: >dir1/modified &&
+		test-tool chmtime =-60 dir1/modified &&
 		mkdir dir2 &&
 		: >dir2/tracked &&
+		test-tool chmtime =-60 dir2/tracked &&
 		: >dir2/modified &&
+		test-tool chmtime =-60 dir2/modified &&
 		write_integration_script &&
 		git config core.fsmonitor .git/hooks/fsmonitor-test &&
 		git update-index --untracked-cache &&
 		git update-index --fsmonitor &&
+		git status &&
 		GIT_TRACE2_PERF="$TRASH_DIRECTORY/trace-before" \
 		git status &&
 		test-tool dump-untracked-cache >../before