fully resolve symlinks when creating lockfiles

Make the code for resolving symlinks in lockfile.c more robust as
follows:

1. Handle relative symlinks
2. recursively resolve symlink chains up to 5

[jc: removed lstat/stat calls to do things stupid way]

Signed-off-by: Bradford C. Smith <bradford.carl.smith@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

lockfile.c

@@ -25,23 +25,111 @@ static void remove_lock_file_on_signal(int signo)
 	raise(signo);
 }
 
+/*
+ * p = absolute or relative path name
+ *
+ * Return a pointer into p showing the beginning of the last path name
+ * element.  If p is empty or the root directory ("/"), just return p.
+ */
+static char *last_path_elm(char *p)
+{
+	/* r starts pointing to null at the end of the string */
+	char *r = strchr(p, '\0');
+
+	if (r == p)
+		return p; /* just return empty string */
+
+	r--; /* back up to last non-null character */
+
+	/* back up past trailing slashes, if any */
+	while (r > p && *r == '/')
+		r--;
+
+	/*
+	 * then go backwards until I hit a slash, or the beginning of
+	 * the string
+	 */
+	while (r > p && *(r-1) != '/')
+		r--;
+	return r;
+}
+
+
+/* We allow "recursive" symbolic links. Only within reason, though */
+#define MAXDEPTH 5
+
+/*
+ * p = path that may be a symlink
+ * s = full size of p
+ *
+ * If p is a symlink, attempt to overwrite p with a path to the real
+ * file or directory (which may or may not exist), following a chain of
+ * symlinks if necessary.  Otherwise, leave p unmodified.
+ *
+ * This is a best-effort routine.  If an error occurs, p will either be
+ * left unmodified or will name a different symlink in a symlink chain
+ * that started with p's initial contents.
+ *
+ * Always returns p.
+ */
+
+static char *resolve_symlink(char *p, size_t s)
+{
+	int depth = MAXDEPTH;
+
+	while (depth--) {
+		char link[PATH_MAX];
+		int link_len = readlink(p, link, sizeof(link));
+		if (link_len < 0) {
+			/* not a symlink anymore */
+			return p;
+		}
+		else if (link_len < sizeof(link))
+			/* readlink() never null-terminates */
+			link[link_len] = '\0';
+		else {
+			warning("%s: symlink too long", p);
+			return p;
+		}
+
+		if (link[0] == '/') {
+			/* absolute path simply replaces p */
+			if (link_len < s)
+				strcpy(p, link);
+			else {
+				warning("%s: symlink too long", p);
+				return p;
+			}
+		} else {
+			/*
+			 * link is a relative path, so I must replace the
+			 * last element of p with it.
+			 */
+			char *r = (char*)last_path_elm(p);
+			if (r - p + link_len < s)
+				strcpy(r, link);
+			else {
+				warning("%s: symlink too long", p);
+				return p;
+			}
+		}
+	}
+	return p;
+}
+
+
 static int lock_file(struct lock_file *lk, const char *path)
 {
 	int fd;
-	struct stat st;
 
-	if ((!lstat(path, &st)) && S_ISLNK(st.st_mode)) {
-		ssize_t sz;
-		static char target[PATH_MAX];
-		sz = readlink(path, target, sizeof(target));
-		if (sz < 0)
-			warning("Cannot readlink %s", path);
-		else if (target[0] != '/')
-			warning("Cannot lock target of relative symlink %s", path);
-		else
-			path = target;
-	}
-	sprintf(lk->filename, "%s.lock", path);
+	if (strlen(path) >= sizeof(lk->filename)) return -1;
+	strcpy(lk->filename, path);
+	/*
+	 * subtract 5 from size to make sure there's room for adding
+	 * ".lock" for the lock file name
+	 */
+	resolve_symlink(lk->filename, sizeof(lk->filename)-5);
+	strcat(lk->filename, ".lock");
 	fd = open(lk->filename, O_RDWR | O_CREAT | O_EXCL, 0666);
 	if (0 <= fd) {
 		if (!lock_file_list) {