wrapper.c: add and use warn_on_fopen_errors()

In many places, Git warns about an inaccessible file after a fopen()
failed. To discern these cases from other cases where we want to warn
about inaccessible files, introduce a new helper specifically to test
whether fopen() failed because the current user lacks the permission to
open file in question.

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

config.c

@@ -2640,6 +2640,9 @@ int git_config_rename_section_in_file(const char *config_filename,
 	}
 
 	if (!(config_file = fopen(config_filename, "rb"))) {
+		ret = warn_on_fopen_errors(config_filename);
+		if (ret)
+			goto out;
 		/* no config file means nothing to rename, no error */
 		goto commit_and_out;
 	}

dir.c

@@ -745,9 +745,9 @@ static int add_excludes(const char *fname, const char *base, int baselen,
 
 	fd = open(fname, O_RDONLY);
 	if (fd < 0 || fstat(fd, &st) < 0) {
-		if (errno != ENOENT)
-			warn_on_inaccessible(fname);
-		if (0 <= fd)
+		if (fd < 0)
+			warn_on_fopen_errors(fname);
+		else
 			close(fd);
 		if (!check_index ||
 		    (buf = read_skip_worktree_file_from_index(fname, &size, sha1_stat)) == NULL)

git-compat-util.h

@@ -1101,6 +1101,8 @@ int access_or_die(const char *path, int mode, unsigned flag);
 
 /* Warn on an inaccessible file that ought to be accessible */
 void warn_on_inaccessible(const char *path);
+/* Warn on an inaccessible file if errno indicates this is an error */
+int warn_on_fopen_errors(const char *path);
 
 #ifdef GMTIME_UNRELIABLE_ERRORS
 struct tm *git_gmtime(const time_t *);

t/t1308-config-set.sh

@@ -195,7 +195,8 @@ test_expect_success POSIXPERM,SANITY 'proper error on non-accessible files' '
 	chmod -r .git/config &&
 	test_when_finished "chmod +r .git/config" &&
 	echo "Error (-1) reading configuration file .git/config." >expect &&
-	test_expect_code 2 test-config configset_get_value foo.bar .git/config 2>actual &&
+	test_expect_code 2 test-config configset_get_value foo.bar .git/config 2>output &&
+	grep "^Error" output >actual &&
 	test_cmp expect actual
 '
 

wrapper.c

@@ -418,6 +418,16 @@ FILE *fopen_for_writing(const char *path)
 	return ret;
 }
 
+int warn_on_fopen_errors(const char *path)
+{
+	if (errno != ENOENT && errno != ENOTDIR) {
+		warn_on_inaccessible(path);
+		return -1;
+	}
+
+	return 0;
+}
+
 int xmkstemp(char *template)
 {
 	int fd;