On 11/18/2017 12:54 PM, Eric Blake wrote:
On 11/17/2017 12:28 PM, Eric Blake wrote:
There's nothing wrong with this patch, but it might be easier to use an attribute((cleanup)) handler to deal with the unlocking. See these links for how we do it in libguestfs:
Oh cool! Yes, that looks nicer. Although the diffstat for doing so is larger, because it requires adding to new sub-function {} scopes and reindenting (we have two different locks in play here).
Interestingly, it doesn't compile if you lack attribute((cleanup)). Libguestfs has:
common/utils/cleanups.h:/* XXX no safe equivalent to CLEANUP_GL_RECURSIVE_LOCK_UNLOCK */
and will fail to compile if the compiler lacks the attribute - arguably, since no one has reported that compilation failure, it seems no one cares about libguestfs that is not also in possession of modern gcc or clang. And we already have a configure-time check whether attribute((cleanup)) is detected. Is it okay to tighten that check to now require a modern compiler for nbdkit, just to get nicer code?
Another argument in favor of making it mandatory - it is VERY easy to write a client that would make nbdkit run out of memory if nbdkit is compiled without cleanup support. connections.c uses CLEANUP_FREE char *buf, which can be allocated at up to 64M per NBD_CMD_WRITE; all a client has to do is send the NBD_CMD_WRITE header, drop the connection, and reconnect, and if buf is not auto-freed, nbdkit will leak quite rapidly. So that's the approach I'll submit in my next round of patch postings. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org