wrapper: implement xopen()

diff --git a/lockfile.c b/lockfile.c
index 988927775169487df7677a17465b512a567488dd..993bb8274833651159cec6f0571b5b555ea073ca 100644 (file)
--- a/lockfile.c
+++ b/lockfile.c
@@ -157,6 +157,67 @@ static int lock_file(struct lock_file *lk, const char *path, int flags)
        return lk->fd;
 }
 
+/*
+ * Constants defining the gaps between attempts to lock a file. The
+ * first backoff period is approximately INITIAL_BACKOFF_MS
+ * milliseconds. The longest backoff period is approximately
+ * (BACKOFF_MAX_MULTIPLIER * INITIAL_BACKOFF_MS) milliseconds.
+ */
+#define INITIAL_BACKOFF_MS 1L
+#define BACKOFF_MAX_MULTIPLIER 1000
+
+/*
+ * Try locking path, retrying with quadratic backoff for at least
+ * timeout_ms milliseconds. If timeout_ms is 0, try locking the file
+ * exactly once. If timeout_ms is -1, try indefinitely.
+ */
+static int lock_file_timeout(struct lock_file *lk, const char *path,
+                            int flags, long timeout_ms)
+{
+       int n = 1;
+       int multiplier = 1;
+       long remaining_ms = 0;
+       static int random_initialized = 0;
+
+       if (timeout_ms == 0)
+               return lock_file(lk, path, flags);
+
+       if (!random_initialized) {
+               srand((unsigned int)getpid());
+               random_initialized = 1;
+       }
+
+       if (timeout_ms > 0)
+               remaining_ms = timeout_ms;
+
+       while (1) {
+               long backoff_ms, wait_ms;
+               int fd;
+
+               fd = lock_file(lk, path, flags);
+
+               if (fd >= 0)
+                       return fd; /* success */
+               else if (errno != EEXIST)
+                       return -1; /* failure other than lock held */
+               else if (timeout_ms > 0 && remaining_ms <= 0)
+                       return -1; /* failure due to timeout */
+
+               backoff_ms = multiplier * INITIAL_BACKOFF_MS;
+               /* back off for between 0.75*backoff_ms and 1.25*backoff_ms */
+               wait_ms = (750 + rand() % 500) * backoff_ms / 1000;
+               sleep_millisec(wait_ms);
+               remaining_ms -= wait_ms;
+
+               /* Recursion: (n+1)^2 = n^2 + 2n + 1 */
+               multiplier += 2*n + 1;
+               if (multiplier > BACKOFF_MAX_MULTIPLIER)
+                       multiplier = BACKOFF_MAX_MULTIPLIER;
+               else
+                       n++;
+       }
+}
+
 void unable_to_lock_message(const char *path, int err, struct strbuf *buf)
 {
        if (err == EEXIST) {
@@ -179,9 +240,10 @@ NORETURN void unable_to_lock_die(const char *path, int err)
 }
 
 /* This should return a meaningful errno on failure */
-int hold_lock_file_for_update(struct lock_file *lk, const char *path, int flags)
+int hold_lock_file_for_update_timeout(struct lock_file *lk, const char *path,
+                                     int flags, long timeout_ms)
 {
-       int fd = lock_file(lk, path, flags);
+       int fd = lock_file_timeout(lk, path, flags, timeout_ms);
        if (fd < 0 && (flags & LOCK_DIE_ON_ERROR))
                unable_to_lock_die(path, errno);
        return fd;
@@ -214,7 +276,7 @@ int hold_lock_file_for_append(struct lock_file *lk, const char *path, int flags)
                int save_errno = errno;
 
                if (flags & LOCK_DIE_ON_ERROR)
-                       exit(128);
+                       die("failed to prepare '%s' for appending", path);
                close(orig_fd);
                rollback_lock_file(lk);
                errno = save_errno;