[libvirt] [PATCH v2] virsh: improve waiting for block job readiness

[Michael Chapman]

After a block job hits 100%, we only need to apply a timeout waiting for
a block job event if exactly one of the BLOCK_JOB or BLOCK_JOB_2
callbacks were able to be registered.

If neither callback could be registered, there's clearly no need for a
timeout.

If both callbacks were registered, then we're guaranteed to eventually
get one of the events. The path being used by virsh must be exactly the
source path or target device in the domain's disk definition, and these
are the respective strings sent back in these two events.

Signed-off-by: Michael Chapman <mike at very.puzzling.org>
---

v1 discussion at:
http://www.redhat.com/archives/libvir-list/2016-January/msg00031.html

Changes since v1:
- Fixed bugs in cb_id/cb_id2 conditionals
- Consistently used break to exit loop, dropped cleanup label
- Clarified the logic and behaviour in comments
- Improved commit message

 tools/virsh-domain.c | 60 ++++++++++++++++++++++++++++------------------------
 1 file changed, 32 insertions(+), 28 deletions(-)

diff --git a/tools/virsh-domain.c b/tools/virsh-domain.c
index edbbc34..853416c 100644
--- a/tools/virsh-domain.c
+++ b/tools/virsh-domain.c
@@ -1875,14 +1875,17 @@ virshBlockJobWaitFree(virshBlockJobWaitDataPtr data)
  * virshBlockJobWait:
  * @data: private data initialized by virshBlockJobWaitInit
  *
- * Waits for the block job to complete. This function prefers to get an event
- * from libvirt but still has fallback means if the device name can't be matched
+ * Waits for the block job to complete. This function prefers to wait for a
+ * matching VIR_DOMAIN_EVENT_ID_BLOCK_JOB or VIR_DOMAIN_EVENT_ID_BLOCK_JOB_2
+ * event from libvirt, however it has a fallback mode should either of these
+ * events not be available.
  *
- * This function returns values from the virConnectDomainEventBlockJobStatus enum
- * or -1 in case of a internal error. Fallback states if a block job vanishes
- * without triggering the event is VIR_DOMAIN_BLOCK_JOB_COMPLETED. For two phase
- * jobs after the retry count for waiting for the event expires is
- * VIR_DOMAIN_BLOCK_JOB_READY.
+ * This function returns values from the virConnectDomainEventBlockJobStatus
+ * enum or -1 in case of a internal error.
+ *
+ * If the fallback mode is activated the returned event is
+ * VIR_DOMAIN_BLOCK_JOB_COMPLETED if the block job vanishes, or
+ * VIR_DOMAIN_BLOCK_JOB_READY if the block job reaches 100%.
  */
 static int
 virshBlockJobWait(virshBlockJobWaitDataPtr data)
@@ -1932,28 +1935,32 @@ virshBlockJobWait(virshBlockJobWaitDataPtr data)
 
         if (result < 0) {
             vshError(data->ctl, _("failed to query job for disk %s"), data->dev);
-            goto cleanup;
+            break;
         }
 
-        /* if we've got an event for the device we are waiting for we can end
-         * the waiting loop */
+        /* If either callback could be registered and we've got an event, we can
+         * can end the waiting loop */
         if ((data->cb_id >= 0 || data->cb_id2 >= 0) && data->status != -1) {
             ret = data->status;
-            goto cleanup;
+            break;
         }
 
-        /* since virsh can't guarantee that the path provided by the user will
-         * later be matched in the event we will need to keep the fallback
-         * approach and claim success if the block job finishes or vanishes. */
-        if (result == 0)
-            break;
+        /* Fallback behaviour is only needed if one or both callbacks could not
+         * be registered */
+        if (data->cb_id < 0 || data->cb_id2 < 0) {
+            /* If the block job vanishes, synthesize a COMPLETED event */
+            if (result == 0) {
+                ret = VIR_DOMAIN_BLOCK_JOB_COMPLETED;
+                break;
+            }
 
-        /* for two-phase jobs we will try to wait in the synchronized phase
-         * for event arrival since 100% completion doesn't necessarily mean that
-         * the block job has finished and can be terminated with success */
-        if (info.end == info.cur && --retries == 0) {
-            ret = VIR_DOMAIN_BLOCK_JOB_READY;
-            goto cleanup;
+            /* If the block job hits 100%, wait a little while for a possible
+             * event from libvirt, else synthesize our own READY event */
+            if (info.end == info.cur &&
+                ((data->cb_id < 0 && data->cb_id2 < 0) || --retries == 0)) {
+                ret = VIR_DOMAIN_BLOCK_JOB_READY;
+                break;
+            }
         }
 
         if (data->verbose)
@@ -1962,26 +1969,23 @@ virshBlockJobWait(virshBlockJobWaitDataPtr data)
 
         if (data->timeout && virTimeMillisNow(&curr) < 0) {
             vshSaveLibvirtError();
-            goto cleanup;
+            break;
         }
 
         if (intCaught || (data->timeout && (curr - start > data->timeout))) {
             if (virDomainBlockJobAbort(data->dom, data->dev, abort_flags) < 0) {
                 vshError(data->ctl, _("failed to abort job for disk '%s'"),
                          data->dev);
-                goto cleanup;
+                break;
             }
 
             ret = VIR_DOMAIN_BLOCK_JOB_CANCELED;
-            goto cleanup;
+            break;
         }
 
         usleep(500 * 1000);
     }
 
-    ret = VIR_DOMAIN_BLOCK_JOB_COMPLETED;
-
- cleanup:
     /* print 100% completed */
     if (data->verbose &&
         (ret == VIR_DOMAIN_BLOCK_JOB_COMPLETED ||
-- 
2.4.3