Re: [Qemu-devel] [PATCH v6 05/10] block: add transactional callbacks feature

[Max Reitz <mreitz@redhat.com>]

On 05.06.2015 23:53, John Snow wrote:
> The goal here is to add a new method to transactions that allows
> developers to specify a callback that will get invoked only once
> all jobs spawned by a transaction are completed, allowing developers
> the chance to perform actions conditionally pending complete success,
> partial failure, or complete failure.
>
> In order to register the new callback to be invoked, a user must request
> a callback pointer and closure by calling new_action_cb_wrapper, which
> creates a wrapper around an opaque pointer and callback that would have
> originally been passed to e.g. backup_start().
>
> The function will return a function pointer and a new opaque pointer to
> be passed instead. The transaction system will effectively intercept the
> original callbacks and perform book-keeping on the transaction after it
> has delivered the original enveloped callback.
>
> This means that Transaction Action callback methods will be called after
> all callbacks triggered by all Actions in the Transactional group have
> been received.
>
> This feature has no knowledge of any jobs spawned by Actions that do not
> inform the system via new_action_cb_wrapper().
>
> For an example of how to use the feature, please skip ahead to:
> 'block: drive_backup transaction callback support' which serves as an example
> for how to hook up a post-transaction callback to the Drive Backup action.
>
>
> Note 1: Defining a callback method alone is not sufficient to have the new
>          method invoked. You must call new_action_cb_wrapper() AND ensure the
>          callback it returns is the one used as the callback for the job
>          launched by the action.
>
> Note 2: You can use this feature for any system that registers completions of
>          an asynchronous task via a callback of the form
>          (void *opaque, int ret), not just block job callbacks.
>
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>   blockdev.c | 181 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
>   1 file changed, 177 insertions(+), 4 deletions(-)

Reviewed-by: Max Reitz <mreitz@redhat.com>