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Re: [libvirt] cont command failing via JSON monitor on restore

On 01/13/2011 04:29 PM, Jim Fehlig wrote:
Daniel P. Berrange wrote:
>  Yep, it wasn't really intended as a fix. It was intended to make
>  the error scenario clearly detectable, which has succeeded as per
>  Jim's report. The fact that QMP returned an error in this way,
>  means we can now reliably detect, usleep(1000), and then retry
>  the 'cont' again at which point we should succeed.
Something like the attached patch?  I'm not quite sure about the retry
bounds (currently 1 second).  In my testing, it always succeeds on the
second attempt - even with large memory vms.

In my tests, 250ms was more than enough, so I'm guessing it's okay, although there's probably no hurt in making it a bit larger - it's not like this is something that repeats all day every day :-)

Would it be possible for you to add the same thing into the text monitor version of the code, so both fixes would travel together as the patch gets cherry-picked around?


> From ec9109b40a4b2c45035495e0e4f65824a92dcf3d Mon Sep 17 00:00:00 2001
From: Jim Fehlig<jfehlig novell com>
Date: Thu, 13 Jan 2011 12:52:23 -0700
Subject: [PATCH] qemu: Retry JSON monitor cont cmd on MigrationExpected error

When restoring a saved qemu instance via JSON monitor, the vm is
left in a paused state.  Turns out the 'cont' cmd was failing with
"MigrationExpected" error class and "An incoming migration is
expected before this command can be executed" error description
due to migration (restore) not yet complete.

Detect if 'cont' cmd fails with "MigrationExpecte" error class and
retry 'cont' cmd.
  src/qemu/qemu_monitor_json.c |   20 +++++++++++++++++---
  1 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/src/qemu/qemu_monitor_json.c b/src/qemu/qemu_monitor_json.c
index 7877731..63b736e 100644
--- a/src/qemu/qemu_monitor_json.c
+++ b/src/qemu/qemu_monitor_json.c
@@ -702,13 +702,27 @@ qemuMonitorJSONStartCPUs(qemuMonitorPtr mon,
      int ret;
      virJSONValuePtr cmd = qemuMonitorJSONMakeCommand("cont", NULL);
      virJSONValuePtr reply = NULL;
+    int i = 0, timeout = 3;
      if (!cmd)
          return -1;

-    ret = qemuMonitorJSONCommand(mon, cmd,&reply);
+    do {
+        ret = qemuMonitorJSONCommand(mon, cmd,&reply);

-    if (ret == 0)
-        ret = qemuMonitorJSONCheckError(cmd, reply);
+        if (ret != 0)
+            break;
+        /* If no error, we're done */
+        if ((ret = qemuMonitorJSONCheckError(cmd, reply)) == 0)
+            break;
+        /* If error class is not MigrationExpected, we're done.
+         * Otherwise try 'cont' cmd again */
+        if (!qemuMonitorJSONHasError(reply, "MigrationExpected"))
+            break;
+        virJSONValueFree(reply);
+    } while ((++i<= timeout)&&  (usleep(250000)<=0));


Doesn't this end up doing a double-free of reply if it times out? virJSONValueFree doesn't update the pointer that's free'd like VIR_FREE does (it can't, since it's a function call rather than a macro).

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