[libvirt] [RFC] BlockJob: Support sync/async virDomainBlockJobAbort

[Stefan Hajnoczi]

On Thu, Dec 8, 2011 at 10:33 PM, Adam Litke <agl at us.ibm.com> wrote:
> Qemu has changed the semantics of the "block_job_cancel" API.  Originally, the
> operation was synchronous (ie. upon command completion, the operation was
> guaranteed to be completely stopped).  With the new semantics, a
> "block_job_cancel" merely requests that the operation be cancelled and an event
> is triggered once the cancellation request has been honored.
>
> To adopt the new semantics while preserving compatibility I propose the
> following updates to the virDomainBlockJob API:
>
> A new block job event type VIR_DOMAIN_BLOCK_JOB_CANCELLED will be recognized by
> libvirt.  Regardless of the flags used with virDomainBlockJobAbort, this event
> will be raised whenever it is received from qemu.  This event indicates that a
> block job has been successfully cancelled.
>
> A new extension flag VIR_DOMAIN_BLOCK_JOB_ABORT_ASYNC will be added to the
> virDomainBlockJobAbort API.  When enabled, this function will operate
> asynchronously (ie, it can return before the job has actually been cancelled).
> When the API is used in this mode, it is the responsibility of the caller to
> wait for a VIR_DOMAIN_BLOCK_JOB_CANCELLED event or poll via the
> virDomainGetBlockJobInfo API to check the cancellation status.
>
> Without the VIR_DOMAIN_BLOCK_JOB_ABORT_ASYNC flag, libvirt will internally poll
> using qemu's "query-block-jobs" API and will not return until the operation has
> been completed.  API users are advised that this operation is unbounded and
> further interaction with the domain during this period may block.
>
> This patch implements the new event type, the API flag, and the polling.  The
> main outstanding issue is whether we should bound the amount of time we will
> wait for cancellation and return an error.
>
> Comments on this proposal?
>
> Signed-off-by: Adam Litke <agl at us.ibm.com>
> Cc: Stefan Hajnoczi <stefanha at gmail.com>
> Cc: Eric Blake <eblake at redhat.com>
> ---
>  include/libvirt/libvirt.h.in |   10 ++++++++++
>  src/libvirt.c                |    9 ++++++++-
>  src/qemu/qemu_driver.c       |   24 +++++++++++++++++-------
>  src/qemu/qemu_monitor.c      |   24 ++++++++++++++++++++++++
>  src/qemu/qemu_monitor.h      |    2 ++
>  src/qemu/qemu_monitor_json.c |   36 +++++++++++++++++++++++++++++-------
>  6 files changed, 90 insertions(+), 15 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha at linux.vnet.ibm.com>