Author: John Keeping <john@keeping.me.uk>
cache: use F_SETLK to avoid stale lock files If CGit is killed while it holds a lock on a cache slot (for example because it is taking too long to generate a page), the lock file will be left in place. This prevents any future attempt to use the same slot since it will fail to exclusively create the lock file. Since CGit is the only program that should be manipulating lock files, we can use advisory locking to detect whether another process is actually using the lock file or if it is now stale. I have confirmed that this works on Linux by setting a short TTL in a custom cgitrc and running the following with CGit patched to print a message to stderr if the fcntl(2) fails: $ export CGIT_CONFIG=$PWD/cgitrc $ export QUERY_STRING=url=cgit/tree/ui-shared.c $ ./cgit | grep -v -e '^<div class=.footer.>' \ -e '^Last-Modified: ' \ -e ^'Expires: ' >expect $ seq 50000 | dd bs=8192 | parallel -j200 "diff -u expect <(./cgit | grep -v -e '^<div class=.footer.>' \ -e '^Last-Modified: ' \ -e ^'Expires: ') || echo BAD" This printed the fail message several times without ever printing "BAD". Signed-off-by: John Keeping <john@keeping.me.uk>
cache.c | 15 ++++++++++++++-
diff --git a/cache.c b/cache.c index 801e63fd9d40c1525de5c18ca5adf61b5adbd220..900b1614df8e1bb01ed7f203519a2fd4abd1cd9e 100644 --- a/cache.c +++ b/cache.c @@ -161,10 +161,23 @@ * Returns 0 on success and errno otherwise. */ static int lock_slot(struct cache_slot *slot) { - slot->lock_fd = open(slot->lock_name, O_RDWR | O_CREAT | O_EXCL, + struct flock lock = { + .l_type = F_WRLCK, + .l_whence = SEEK_SET, + .l_start = 0, + .l_len = 0, + }; + + slot->lock_fd = open(slot->lock_name, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR); if (slot->lock_fd == -1) return errno; + if (fcntl(slot->lock_fd, F_SETLK, &lock) < 0) { + int saved_errno = errno; + close(slot->lock_fd); + slot->lock_fd = -1; + return saved_errno; + } if (xwrite(slot->lock_fd, slot->key, slot->keylen + 1) < 0) return errno; return 0;