refs.c: make resolve_ref_unsafe set errno to something meaningful on …

…error

Making errno when returning from resolve_ref_unsafe() meaningful,
which should fix

 * a bug in lock_ref_sha1_basic, where it assumes EISDIR
   means it failed due to a directory being in the way

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Michael Haggerty <mhagger@alum.mit.edu>

cache.h

@@ -979,7 +979,7 @@ extern int read_ref(const char *refname, unsigned char *sha1);
  * NULL.  If more than MAXDEPTH recursive symbolic lookups are needed,
  * give up and return NULL.
  *
- * errno is sometimes set on errors, but not always.
+ * errno is set to something meaningful on error.
  */
 extern const char *resolve_ref_unsafe(const char *ref, unsigned char *sha1, int reading, int *flag);
 extern char *resolve_refdup(const char *ref, unsigned char *sha1, int reading, int *flag);

refs.c

@@ -1334,6 +1334,7 @@ static const char *handle_missing_loose_ref(const char *refname,
 	}
 }
 
+/* This function needs to return a meaningful errno on failure */
 const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int reading, int *flag)
 {
 	int depth = MAXDEPTH;
@@ -1344,17 +1345,21 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea
 	if (flag)
 		*flag = 0;
 
-	if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL))
+	if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
+		errno = EINVAL;
 		return NULL;
+	}
 
 	for (;;) {
 		char path[PATH_MAX];
 		struct stat st;
 		char *buf;
 		int fd;
 
-		if (--depth < 0)
+		if (--depth < 0) {
+			errno = ELOOP;
 			return NULL;
+		}
 
 		git_snpath(path, sizeof(path), "%s", refname);
 
@@ -1416,9 +1421,13 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea
 				return NULL;
 		}
 		len = read_in_full(fd, buffer, sizeof(buffer)-1);
-		close(fd);
-		if (len < 0)
+		if (len < 0) {
+			int save_errno = errno;
+			close(fd);
+			errno = save_errno;
 			return NULL;
+		}
+		close(fd);
 		while (len && isspace(buffer[len-1]))
 			len--;
 		buffer[len] = '\0';
@@ -1435,6 +1444,7 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea
 			    (buffer[40] != '\0' && !isspace(buffer[40]))) {
 				if (flag)
 					*flag |= REF_ISBROKEN;
+				errno = EINVAL;
 				return NULL;
 			}
 			return refname;
@@ -1447,6 +1457,7 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea
 		if (check_refname_format(buf, REFNAME_ALLOW_ONELEVEL)) {
 			if (flag)
 				*flag |= REF_ISBROKEN;
+			errno = EINVAL;
 			return NULL;
 		}
 		refname = strcpy(refname_buffer, buf);