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authorMichael Haggerty <mhagger@alum.mit.edu>2015-05-11 10:35:25 (GMT)
committerJunio C Hamano <gitster@pobox.com>2015-05-14 21:51:08 (GMT)
commit044b6a9efecf9941073b99ced4cb2881b18aee62 (patch)
tree5cfbf39335b835c77d09d94250c2e4aaa8c2897a /lockfile.c
parent1ea28e149403ba92f95205f324cc3119fb92df07 (diff)
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lockfile: allow file locking to be retried with a timeout
Currently, there is only one attempt to lock a file. If it fails, the whole operation fails. But it might sometimes be advantageous to try acquiring a file lock a few times before giving up. So add a new function, hold_lock_file_for_update_timeout(), that allows a timeout to be specified. Make hold_lock_file_for_update() a thin wrapper around the new function. If timeout_ms is positive, then retry for at least that many milliseconds to acquire the lock. On each failed attempt, use select() to wait for a backoff time that increases quadratically (capped at 1 second) and has a random component to prevent two processes from getting synchronized. If timeout_ms is negative, retry indefinitely. In a moment we will switch to using the new function when locking packed-refs. Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'lockfile.c')
-rw-r--r--lockfile.c79
1 files changed, 77 insertions, 2 deletions
diff --git a/lockfile.c b/lockfile.c
index 9889277..30e65e9 100644
--- a/lockfile.c
+++ b/lockfile.c
@@ -157,6 +157,80 @@ static int lock_file(struct lock_file *lk, const char *path, int flags)
return lk->fd;
}
+static int sleep_microseconds(long us)
+{
+ struct timeval tv;
+ tv.tv_sec = 0;
+ tv.tv_usec = us;
+ return select(0, NULL, NULL, NULL, &tv);
+}
+
+/*
+ * 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_us = 0;
+ static int random_initialized = 0;
+
+ if (timeout_ms == 0)
+ return lock_file(lk, path, flags);
+
+ if (!random_initialized) {
+ srandom((unsigned int)getpid());
+ random_initialized = 1;
+ }
+
+ if (timeout_ms > 0) {
+ /* avoid overflow */
+ if (timeout_ms <= LONG_MAX / 1000)
+ remaining_us = timeout_ms * 1000;
+ else
+ remaining_us = LONG_MAX;
+ }
+
+ while (1) {
+ long backoff_ms, wait_us;
+ 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_us <= 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_us = (750 + random() % 500) * backoff_ms;
+ sleep_microseconds(wait_us);
+ remaining_us -= wait_us;
+
+ /* 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 +253,10 @@ 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;