[libvirt] [PATCH v2] Fix race condition reconnecting to vms & loading configs

[Daniel P. Berrange]

From: "Daniel P. Berrange" <berrange at redhat.com>

The following sequence

 1. Define a persistent QMEU guest
 2. Start the QEMU guest
 3. Stop libvirtd
 4. Kill the QEMU process
 5. Start libvirtd
 6. List persistent guets

At the last step, the previously running persistent guest
will be missing. This is because of a race condition in the
QEMU driver startup code. It does

 1. Load all VM state files
 2. Spawn thread to reconnect to each VM
 3. Load all VM config files

Only at the end of step 3, does the 'virDomainObjPtr' get
marked as "persistent". There is therefore a window where
the thread reconnecting to the VM will remove the persistent
VM from the list.

The easy fix is to simply switch the order of steps 2 & 3.

In addition to this though, we must only attempt to reconnect
to a VM which had a non-zero PID loaded from its state file.

Signed-off-by: Daniel P. Berrange <berrange at redhat.com>
---
 src/qemu/qemu_driver.c  | 3 +--
 src/qemu/qemu_process.c | 3 +++
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index c613967..9c3daad 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -816,8 +816,6 @@ qemuStateInitialize(bool privileged,
 
     conn = virConnectOpen(cfg->uri);
 
-    qemuProcessReconnectAll(conn, qemu_driver);
-
     /* Then inactive persistent configs */
     if (virDomainObjListLoadAllConfigs(qemu_driver->domains,
                                        cfg->configDir,
@@ -828,6 +826,7 @@ qemuStateInitialize(bool privileged,
                                        NULL, NULL) < 0)
         goto error;
 
+    qemuProcessReconnectAll(conn, qemu_driver);
 
     virDomainObjListForEach(qemu_driver->domains,
                             qemuDomainSnapshotLoad,
diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
index b278742..bdffdf8 100644
--- a/src/qemu/qemu_process.c
+++ b/src/qemu/qemu_process.c
@@ -3253,6 +3253,9 @@ qemuProcessReconnectHelper(virDomainObjPtr obj,
     struct qemuProcessReconnectData *src = opaque;
     struct qemuProcessReconnectData *data;
 
+    if (!obj->pid)
+        return 0;
+
     if (VIR_ALLOC(data) < 0)
         return -1;
 
-- 
1.8.3.1