AioContext: export and use aio_dispatch

So far, aio_poll's scheme was dispatch/poll/dispatch, where
the first dispatch phase was used only in the GSource case in
order to avoid a blocking poll.  Earlier patches changed it to
dispatch/prepare/poll/dispatch, where prepare is aio_compute_timeout.

By making aio_dispatch public, we can remove the first dispatch
phase altogether, so that both aio_poll and the GSource use the same
prepare/poll/dispatch scheme.

This patch breaks the invariant that aio_poll(..., true) will not block
the first time it returns false.  This used to be fundamental for
qemu_aio_flush's implementation as "while (qemu_aio_wait()) {}" but
no code in QEMU relies on this invariant anymore.  The return value
of aio_poll() is now comparable with that of g_main_context_iteration.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/include/block/aio.h b/include/block/aio.h
index 05b531c..7ba3e96 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -211,6 +211,12 @@ void qemu_bh_delete(QEMUBH *bh);
  */
 bool aio_pending(AioContext *ctx);
 
+/* Dispatch any pending callbacks from the GSource attached to the AioContext.
+ *
+ * This is used internally in the implementation of the GSource.
+ */
+bool aio_dispatch(AioContext *ctx);
+
 /* Progress in completing AIO work to occur.  This can issue new pending
  * aio as a result of executing I/O completion or bh callbacks.
  *