[PATCH 1/2] lockfile: allow file locking to be retried with a timeout
- From
Michael Haggerty <mhagger@alum.mit.edu>
- Date
- May 1, 2015, 14:52 UTC
- Message-ID
- <1430491977-25817-2-git-send-email-mhagger@alum.mit.edu>
- In-Reply-To
- <1430491977-25817-1-git-send-email-mhagger@alum.mit.edu>
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> ---
Notes (discussion):
It would be easy to also add a hold_lock_file_for_append_timeout(),
but I can't yet think of an application for it.lockfile.c | 79 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-- lockfile.h | 16 +++++++++++-- 2 files changed, 91 insertions(+), 4 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; diff --git a/lockfile.h b/lockfile.h index cd2ec95..b4abc61 100644 --- a/lockfile.h +++ b/lockfile.h @@ -74,8 +74,20 @@ struct lock_file { extern void unable_to_lock_message(const char *path, int err, struct strbuf *buf); extern NORETURN void unable_to_lock_die(const char *path, int err); -extern int hold_lock_file_for_update(struct lock_file *, const char *path, int); -extern int hold_lock_file_for_append(struct lock_file *, const char *path, int); +extern int hold_lock_file_for_update_timeout( + struct lock_file *lk, const char *path, + int flags, long timeout_ms); + +static inline int hold_lock_file_for_update( + struct lock_file *lk, const char *path, + int flags) +{ + return hold_lock_file_for_update_timeout(lk, path, flags, 0); +} + +extern int hold_lock_file_for_append(struct lock_file *lk, const char *path, + int flags); + extern FILE *fdopen_lock_file(struct lock_file *, const char *mode); extern char *get_locked_file_path(struct lock_file *); extern int commit_lock_file_to(struct lock_file *, const char *path);
-- 2.1.4