diff options
Diffstat (limited to 'lockfile.c')
| -rw-r--r-- | lockfile.c | 79 |
1 files changed, 77 insertions, 2 deletions
@@ -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; |