Re: [libvirt] [PATCH] Introduce an internal API for handling file based lockspaces

From: "Daniel P. Berrange" <berrange(a)redhat.com&gt; The previously introduced virFile{Lock,Unlock} APIs provide a way to acquire/release fcntl() locks on individual files. For unknown reason though, the POSIX spec says that fcntl() locks are released when *any* file handle refering to the same path is closed. In the following sequence threadA: fd1 = open("foo") threadB: fd2 = open("foo") threadA: virFileLock(fd1) threadB: virFileLock(fd2) threadB: close(fd2) you'd expect threadA to come out holding a lock on 'foo', and indeed it does hold a lock for a very short time. Unfortunately when threadB does close(fd2) this releases the lock associated with fd1. For the current libvirt use case for virFileLock - pidfiles - this doesn't matter since the lock is acquired at startup while single threaded an never released until exit. To provide a more generally useful API though, it is neccessary to introduce a slightly higher level abstraction, which is to be referred to as a "lockspace". This is to be provided by a virLockSpacePtr object in src/util/virlockspace.{c,h}. The core idea is that the lockspace keeps track of what files are already open+locked. This means that when a 2nd thread comes along and tries to acquire a lock, it doesn't end up opening and closing a new FD. The lockspace just checks the current list of held locks and immediately returns VIR_ERR_RESOURCE_BUSY. NB, the API as it stands is designed on the basis that the files being locked are not being otherwise opened and used by the application code. ie the files which are used to maintain the locks are not the files associated with the resources themselves. One approach to using this API is to acquire locks based on a hash of the filepath. eg to lock /var/lib/libvirt/images/foo.img the application might do virLockSpacePtr lockspace = virLockSpaceNew("/var/lib/libvirt/imagelocks"); lockname = md5sum("/var/lib/libvirt/images/foo.img") virLockSpaceAcquireLock(lockspace, lockname) This patch is the basis for the soon to be re-submitted patch series providing a virtlockd daemon