Re: [libvirt] [PATCH] virNetSocketNewConnectUNIX: Use flocks when spawning a daemon

https://bugzilla.redhat.com/show_bug.cgi?id=1200149 Even though we have a mutex mechanism so that two clients don't spawn two daemons, it's not strong enough. It can happen that while one client is spawning the daemon, the other one fails to connect. Basically two possible errors can happen: error: Failed to connect socket to '/home/mprivozn/.cache/libvirt/libvirt-sock': Connection refused or: error: Failed to connect socket to '/home/mprivozn/.cache/libvirt/libvirt-sock': No such file or directory The problem in both cases is, the daemon is only starting up, while we are trying to connect (and fail). We should postpone the connecting phase until the daemon is started (by the other thread that is spawning it). In order to do that, create a file lock 'libvirt-lock' in the directory where session daemon would create its socket. So even when called from multiple processes, spawning a daemon will serialize on the file lock. So only the first to come will spawn the daemon. Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com&gt; --- diff to v1: -hopefully this one is race free src/rpc/virnetsocket.c | 148 ++++++++++++------------------------------------- 1 file changed, 36 insertions(+), 112 deletions(-)

diff --git a/src/rpc/virnetsocket.c b/src/rpc/virnetsocket.c index 6b019cc..2b891ae 100644 --- a/src/rpc/virnetsocket.c +++ b/src/rpc/virnetsocket.c @@ -543,10 +539,10 @@ int virNetSocketNewConnectUNIX(const char *path, const char *binary, virNetSocketPtr *retsock) { - char *binname = NULL; - char *pidpath = NULL; - int fd, passfd = -1; - int pidfd = -1; + char *lockpath = NULL; + int lockfd = -1; + int fd = -1; + int retries = 100; virSocketAddr localAddr; virSocketAddr remoteAddr; @@ -561,6 +557,22 @@ int virNetSocketNewConnectUNIX(const char *path, return -1; } + if (spawnDaemon) { + if (virPidFileConstructPath(false, NULL, "libvirt-lock", &lockpath) < 0) + goto error;