Fix potential events deadlock when unref'ing virConnectPtr

When the last reference to a virConnectPtr is released by
libvirtd, it was possible for a deadlock to occur in the
virDomainEventState functions. The virDomainEventStatePtr
holds a reference on virConnectPtr for each registered
callback. When removing a callback, the virUnrefConnect
function is run. If this causes the last reference on the
virConnectPtr to be released, then virReleaseConnect can
be run, which in turns calls qemudClose. This function has
a call to virDomainEventStateDeregisterConn which is intended
to remove all callbacks associated with the virConnectPtr
instance. This will try to grab a lock on virDomainEventState
but this lock is already held. Deadlock ensues

Thread 1 (Thread 0x7fcbb526a840 (LWP 23185)):

Since each callback associated with a virConnectPtr holds a
reference on virConnectPtr, it is impossible for the qemudClose
method to be invoked while any callbacks are still registered.
Thus the call to virDomainEventStateDeregisterConn must in fact
be a no-op. Thus it is possible to just remove all trace of
virDomainEventStateDeregisterConn and avoid the deadlock.

* src/conf/domain_event.c, src/conf/domain_event.h,
  src/libvirt_private.syms: Delete virDomainEventStateDeregisterConn
* src/libxl/libxl_driver.c, src/lxc/lxc_driver.c,
  src/qemu/qemu_driver.c, src/uml/uml_driver.c: Remove
  calls to virDomainEventStateDeregisterConn

1 files changed, 0 insertions, 4 deletions

diff --git a/src/libxl/libxl_driver.c b/src/libxl/libxl_driver.c
index 45bf1f84c..fc90d1650 100644
--- a/src/libxl/libxl_driver.c
+++ b/src/libxl/libxl_driver.c
@@ -1081,10 +1081,6 @@ libxlClose(virConnectPtr conn ATTRIBUTE_UNUSED)
 {
     libxlDriverPrivatePtr driver = conn->privateData;
 
-    libxlDriverLock(driver);
-    virDomainEventStateDeregisterConn(conn,
-                                      driver->domainEventState);
-    libxlDriverUnlock(driver);
     conn->privateData = NULL;
     return 0;
 }