neocgit

a more 'modern' version of cgit
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commit db9a70b159a107da29f88865d63ba14dd127556f
parent 2e4a41e84029cbfa70c654fe5dfb35747f2dfbba
Author: John Keeping <john@keeping.me.uk>
Date:   Tue,  3 Mar 2015 19:22:31 +0000

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>

Diffstat:
Mcache.c | 15++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)

diff --git 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;