[libvirt] [PATCH v3] Close the source fd if the destination qemu exits during tunnelled migration

[Jiri Denemark]

On Thu, Oct 08, 2015 at 17:59:07 +0530, Shivaprasad G Bhat wrote:
> Tunnelled migration can hang if the destination qemu exits despite all the
> ABI checks. This happens whenever the destination qemu exits before the
> complete transfer is noticed by source qemu. The savevm state checks at
> runtime can fail at destination and cause qemu to error out.
> The source qemu cant notice it as the EPIPE is not propogated to it.
> The qemuMigrationIOFunc() notices the stream being broken from virStreamSend()
> and it cleans up the stream alone. The qemuMigrationWaitForCompletion() would
> never get to 100% transfer completion.
> The qemuMigrationWaitForCompletion() never breaks out as well since
> the ssh connection to destination is healthy, and the source qemu also thinks
> the migration is ongoing as the Fd to which it transfers, is never
> closed or broken. So, the migration will hang forever. Even Ctrl-C on the
> virsh migrate wouldn't be honoured. Close the source side FD when there is
> an error in the stream. That way, the source qemu updates itself and
> qemuMigrationWaitForCompletion() notices the failure.
> 
> Close the FD for all kinds of errors to be sure. The error message is not
> copied for EPIPE so that the destination error is copied instead later.
> 
> Note:
> Reproducible with repeated migrations between Power hosts running in different
> subcores-per-core modes.
> 
> Changes from v2 -> v3:
>    VIR_FORCE_CLOSE() needn't be called on the qemu source fd as that is closed
>    by qemuMigrationIOFunc() for tunnelled migration. Set the fd to -1.
> 
> Signed-off-by: Shivaprasad G Bhat <sbhat at linux.vnet.ibm.com>
> ---
>  src/qemu/qemu_migration.c |   18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/src/qemu/qemu_migration.c b/src/qemu/qemu_migration.c
> index 7440108..c982838 100644
> --- a/src/qemu/qemu_migration.c
> +++ b/src/qemu/qemu_migration.c
> @@ -4019,6 +4019,7 @@ static void qemuMigrationIOFunc(void *arg)
>      if (virStreamFinish(data->st) < 0)
>          goto error;
>  
> +    VIR_FORCE_CLOSE(data->sock);
>      VIR_FREE(buffer);
>  
>      return;
> @@ -4036,7 +4037,11 @@ static void qemuMigrationIOFunc(void *arg)
>      }
>  
>   error:
> -    virCopyLastError(&data->err);
> +    /* Let the source qemu know that the transfer cant continue anymore.
> +     * Don't copy the error for EPIPE as destination has the actual error. */
> +    VIR_FORCE_CLOSE(data->sock);
> +    if (!virLastErrorIsSystemErrno(EPIPE))
> +        virCopyLastError(&data->err);
>      virResetLastError();
>      VIR_FREE(buffer);
>  }

Hmm, I don't think we need to filter out any error here. Were you
actually able to get the EPIPE error except for seeing it in the logs?
What I'm trying to say is: if we get an EPIPE in qemuMigrationIOFunc, we
close data->sock which will propagate to the source QEMU and
qemuMigrationWaitForCompletion should return a "migration unexpectedly
failed" error, which will get stored in orig_err and even though
qemuMigrationStopTunnel will set the thread local error to the one from
iothread->data, we will reset it back to the orig_err one. Only in case
qemuMigrationWaitForCompletion returned success but we still got error
on the tunnel, we'd actually see the error from iothread->err.

Otherwise the patch looks fine.

Jirka