untracked cache: don't open non-existent .gitignore

This cuts down a signficant number of open(.gitignore) because most
directories usually don't have .gitignore files.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -1020,7 +1020,21 @@ static void prep_exclude(struct dir_struct *dir, const char *base, int baselen)
 		/* Try to read per-directory file */
 		hashclr(sha1_stat.sha1);
 		sha1_stat.valid = 0;
-		if (dir->exclude_per_dir) {
+		if (dir->exclude_per_dir &&
+		    /*
+		     * If we know that no files have been added in
+		     * this directory (i.e. valid_cached_dir() has
+		     * been executed and set untracked->valid) ..
+		     */
+		    (!untracked || !untracked->valid ||
+		     /*
+		      * .. and .gitignore does not exist before
+		      * (i.e. null exclude_sha1 and skip_worktree is
+		      * not set). Then we can skip loading .gitignore,
+		      * which would result in ENOENT anyway.
+		      * skip_worktree is taken care in read_directory()
+		      */
+		     !is_null_sha1(untracked->exclude_sha1))) {
 			/*
 			 * dir->basebuf gets reused by the traversal, but we
 			 * need fname to remain unchanged to ensure the src
@@ -1783,6 +1797,7 @@ static struct untracked_cache_dir *validate_untracked_cache(struct dir_struct *d
 						      const struct pathspec *pathspec)
 {
 	struct untracked_cache_dir *root;
+	int i;
 
 	if (!dir->untracked)
 		return NULL;
@@ -1834,6 +1849,15 @@ static struct untracked_cache_dir *validate_untracked_cache(struct dir_struct *d
 	if (dir->exclude_list_group[EXC_CMDL].nr)
 		return NULL;
 
+	/*
+	 * An optimization in prep_exclude() does not play well with
+	 * CE_SKIP_WORKTREE. It's a rare case anyway, if a single
+	 * entry has that bit set, disable the whole untracked cache.
+	 */
+	for (i = 0; i < active_nr; i++)
+		if (ce_skip_worktree(active_cache[i]))
+			return NULL;
+
 	if (!dir->untracked->root) {
 		const int len = sizeof(*dir->untracked->root);
 		dir->untracked->root = xmalloc(len);