clean: show an error message when the path is too long

Without an error message when `lstat()` failed, `git clean` would
abort without an error message, leaving the user quite puzzled.

In particular on Windows, where the default maximum path length is quite
small (yet there are ways to circumvent that limit in many cases), it is
very important that users be given an indication why their command
failed because of too long paths when it did.

This test case makes sure that a warning is issued that would have
helped the user who reported this issue:

	git-for-windows#521

Note that we temporarily set `core.longpaths = false` in the regression
test; This ensures forward-compatibility with the `core.longpaths`
feature that has not yet been upstreamed from Git for Windows.

Helped-by: René Scharfe <l.s.r@web.de>
Helped-by: SZEDER Gábor <szeder.dev@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

builtin/clean.c

@@ -34,6 +34,7 @@ static const char *msg_would_remove = N_("Would remove %s\n");
 static const char *msg_skip_git_dir = N_("Skipping repository %s\n");
 static const char *msg_would_skip_git_dir = N_("Would skip repository %s\n");
 static const char *msg_warn_remove_failed = N_("failed to remove %s");
+static const char *msg_warn_lstat_failed = N_("could not lstat %s\n");
 
 enum color_clean {
 	CLEAN_COLOR_RESET = 0,
@@ -194,7 +195,7 @@ static int remove_dirs(struct strbuf *path, const char *prefix, int force_flag,
 		strbuf_setlen(path, len);
 		strbuf_addstr(path, e->d_name);
 		if (lstat(path->buf, &st))
-			; /* fall thru */
+			warning_errno(_(msg_warn_lstat_failed), path->buf);
 		else if (S_ISDIR(st.st_mode)) {
 			if (remove_dirs(path, prefix, force_flag, dry_run, quiet, &gone))
 				ret = 1;

t/t7300-clean.sh

@@ -669,4 +669,16 @@ test_expect_success 'git clean -d skips untracked dirs containing ignored files'
 	test_path_is_missing foo/b/bb
 '
 
+test_expect_success MINGW 'handle clean & core.longpaths = false nicely' '
+	test_config core.longpaths false &&
+	a50=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa &&
+	mkdir -p $a50$a50/$a50$a50/$a50$a50 &&
+	: >"$a50$a50/test.txt" 2>"$a50$a50/$a50$a50/$a50$a50/test.txt" &&
+	# create a temporary outside the working tree to hide from "git clean"
+	test_must_fail git clean -xdf 2>.git/err &&
+	# grepping for a strerror string is unportable but it is OK here with
+	# MINGW prereq
+	test_i18ngrep "too long" .git/err
+'
+
 test_done