[libvirt] [PATCHv2] snapshot: detect when qemu lacks disk-snapshot support

[Eric Blake]

Noticed when testing new libvirt against old qemu that lacked the
snapshot_blkdev HMP command.  Libvirt was mistakenly treating the
command as successful, and re-writing the domain XML to use the
just-created 0-byte file, rendering the domain broken on restart.

* src/qemu/qemu_monitor_text.c (qemuMonitorTextDiskSnapshot):
Notice another possible error message.
* src/qemu/qemu_driver.c
(qemuDomainSnapshotCreateSingleDiskActive): Don't keep 0-byte file
on failure.
---

v2: clean up on failure

 src/qemu/qemu_driver.c       |    2 +-
 src/qemu/qemu_monitor_text.c |    3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index f833655..84ef4dd 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -9027,7 +9027,6 @@ qemuDomainSnapshotCreateSingleDiskActive(struct qemud_driver *driver,
             VIR_WARN("Unable to release lock on %s", source);
         goto cleanup;
     }
-    need_unlink = false;

     disk->src = origsrc;
     origsrc = NULL;
@@ -9041,6 +9040,7 @@ qemuDomainSnapshotCreateSingleDiskActive(struct qemud_driver *driver,
         goto cleanup;

     /* Update vm in place to match changes.  */
+    need_unlink = false;
     VIR_FREE(disk->src);
     disk->src = source;
     source = NULL;
diff --git a/src/qemu/qemu_monitor_text.c b/src/qemu/qemu_monitor_text.c
index 2f31d99..4774df9 100644
--- a/src/qemu/qemu_monitor_text.c
+++ b/src/qemu/qemu_monitor_text.c
@@ -3064,7 +3064,8 @@ qemuMonitorTextDiskSnapshot(qemuMonitorPtr mon, const char *device,
         goto cleanup;
     }

-    if (strstr(reply, "error while creating qcow2") != NULL) {
+    if (strstr(reply, "error while creating qcow2") != NULL ||
+        strstr(reply, "unknown command:") != NULL) {
         qemuReportError(VIR_ERR_OPERATION_FAILED,
                         _("Failed to take snapshot: %s"), reply);
         goto cleanup;
-- 
1.7.4.4