From db9a70b159a107da29f88865d63ba14dd127556f Mon Sep 17 00:00:00 2001 From: John Keeping Date: Tue, 3 Mar 2015 19:22:31 +0000 Subject: 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 '^
' \ -e '^Last-Modified: ' \ -e ^'Expires: ' >expect $ seq 50000 | dd bs=8192 | parallel -j200 "diff -u expect <(./cgit | grep -v -e '^
' \ -e '^Last-Modified: ' \ -e ^'Expires: ') || echo BAD" This printed the fail message several times without ever printing "BAD". Signed-off-by: John Keeping --- cache.c | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/cache.c b/cache.c index 801e63f..900b161 100644 --- a/cache.c +++ b/cache.c @@ -161,10 +161,23 @@ static int close_lock(struct cache_slot *slot) */ 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; -- cgit 1.4.1-21-gabe81