Merge branch 'mh/loose-refs-race-with-pack-ref'

We read loose and packed rerferences in two steps, but after
deciding to read a loose ref but before actually opening it to read
it, another process racing with us can unlink it, which would cause
us to barf. Update the codepath to retry when such a race is
detected.

* mh/loose-refs-race-with-pack-ref:
  resolve_ref_unsafe(): close race condition reading loose refs
  resolve_ref_unsafe(): handle the case of an SHA-1 within loop
  resolve_ref_unsafe(): extract function handle_missing_loose_ref()

refs.c

@@ -1197,6 +1197,37 @@ static struct ref_entry *get_packed_ref(const char *refname)
 	return find_ref(get_packed_refs(&ref_cache), refname);
 }
 
+/*
+ * A loose ref file doesn't exist; check for a packed ref.  The
+ * options are forwarded from resolve_safe_unsafe().
+ */
+static const char *handle_missing_loose_ref(const char *refname,
+					    unsigned char *sha1,
+					    int reading,
+					    int *flag)
+{
+	struct ref_entry *entry;
+
+	/*
+	 * The loose reference file does not exist; check for a packed
+	 * reference.
+	 */
+	entry = get_packed_ref(refname);
+	if (entry) {
+		hashcpy(sha1, entry->u.value.sha1);
+		if (flag)
+			*flag |= REF_ISPACKED;
+		return refname;
+	}
+	/* The reference is not a packed reference, either. */
+	if (reading) {
+		return NULL;
+	} else {
+		hashclr(sha1);
+		return refname;
+	}
+}
+
 const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int reading, int *flag)
 {
 	int depth = MAXDEPTH;
@@ -1221,36 +1252,34 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea
 
 		git_snpath(path, sizeof(path), "%s", refname);
 
+		/*
+		 * We might have to loop back here to avoid a race
+		 * condition: first we lstat() the file, then we try
+		 * to read it as a link or as a file.  But if somebody
+		 * changes the type of the file (file <-> directory
+		 * <-> symlink) between the lstat() and reading, then
+		 * we don't want to report that as an error but rather
+		 * try again starting with the lstat().
+		 */
+	stat_ref:
 		if (lstat(path, &st) < 0) {
-			struct ref_entry *entry;
-
-			if (errno != ENOENT)
+			if (errno == ENOENT)
+				return handle_missing_loose_ref(refname, sha1,
+								reading, flag);
+			else
 				return NULL;
-			/*
-			 * The loose reference file does not exist;
-			 * check for a packed reference.
-			 */
-			entry = get_packed_ref(refname);
-			if (entry) {
-				hashcpy(sha1, entry->u.value.sha1);
-				if (flag)
-					*flag |= REF_ISPACKED;
-				return refname;
-			}
-			/* The reference is not a packed reference, either. */
-			if (reading) {
-				return NULL;
-			} else {
-				hashclr(sha1);
-				return refname;
-			}
 		}
 
 		/* Follow "normalized" - ie "refs/.." symlinks by hand */
 		if (S_ISLNK(st.st_mode)) {
 			len = readlink(path, buffer, sizeof(buffer)-1);
-			if (len < 0)
-				return NULL;
+			if (len < 0) {
+				if (errno == ENOENT || errno == EINVAL)
+					/* inconsistent with lstat; retry */
+					goto stat_ref;
+				else
+					return NULL;
+			}
 			buffer[len] = 0;
 			if (!prefixcmp(buffer, "refs/") &&
 					!check_refname_format(buffer, 0)) {
@@ -1273,8 +1302,13 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea
 		 * a ref
 		 */
 		fd = open(path, O_RDONLY);
-		if (fd < 0)
-			return NULL;
+		if (fd < 0) {
+			if (errno == ENOENT)
+				/* inconsistent with lstat; retry */
+				goto stat_ref;
+			else
+				return NULL;
+		}
 		len = read_in_full(fd, buffer, sizeof(buffer)-1);
 		close(fd);
 		if (len < 0)
@@ -1286,8 +1320,19 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea
 		/*
 		 * Is it a symbolic ref?
 		 */
-		if (prefixcmp(buffer, "ref:"))
-			break;
+		if (prefixcmp(buffer, "ref:")) {
+			/*
+			 * Please note that FETCH_HEAD has a second
+			 * line containing other data.
+			 */
+			if (get_sha1_hex(buffer, sha1) ||
+			    (buffer[40] != '\0' && !isspace(buffer[40]))) {
+				if (flag)
+					*flag |= REF_ISBROKEN;
+				return NULL;
+			}
+			return refname;
+		}
 		if (flag)
 			*flag |= REF_ISSYMREF;
 		buf = buffer + 4;
@@ -1300,13 +1345,6 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea
 		}
 		refname = strcpy(refname_buffer, buf);
 	}
-	/* Please note that FETCH_HEAD has a second line containing other data. */
-	if (get_sha1_hex(buffer, sha1) || (buffer[40] != '\0' && !isspace(buffer[40]))) {
-		if (flag)
-			*flag |= REF_ISBROKEN;
-		return NULL;
-	}
-	return refname;
 }
 
 char *resolve_refdup(const char *ref, unsigned char *sha1, int reading, int *flag)