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, 4 insertions, 27 deletions

diff --git a/aio-win32.c b/aio-win32.c
index 2ac38a897c..1ec434a83c 100644
--- a/aio-win32.c
+++ b/aio-win32.c
@@ -130,11 +130,12 @@ static bool aio_dispatch_handlers(AioContext *ctx, HANDLE event)
     return progress;
 }
 
-static bool aio_dispatch(AioContext *ctx)
+bool aio_dispatch(AioContext *ctx)
 {
     bool progress;
 
-    progress = aio_dispatch_handlers(ctx, INVALID_HANDLE_VALUE);
+    progress = aio_bh_poll(ctx);
+    progress |= aio_dispatch_handlers(ctx, INVALID_HANDLE_VALUE);
     progress |= timerlistgroup_run_timers(&ctx->tlg);
     return progress;
 }
@@ -149,23 +150,6 @@ bool aio_poll(AioContext *ctx, bool blocking)
 
     progress = false;
 
-    /*
-     * If there are callbacks left that have been queued, we need to call then.
-     * Do not call select in this case, because it is possible that the caller
-     * does not need a complete flush (as is the case for aio_poll loops).
-     */
-    if (aio_bh_poll(ctx)) {
-        blocking = false;
-        progress = true;
-    }
-
-    /* Dispatch any pending callbacks from the GSource.  */
-    progress |= aio_dispatch(ctx);
-
-    if (progress && !blocking) {
-        return true;
-    }
-
     ctx->walking_handlers++;
 
     /* fill fd sets */
@@ -205,14 +189,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
         events[ret - WAIT_OBJECT_0] = events[--count];
     }
 
-    if (blocking) {
-        /* Run the timers a second time. We do this because otherwise aio_wait
-         * will not note progress - and will stop a drain early - if we have
-         * a timer that was not ready to run entering g_poll but is ready
-         * after g_poll. This will only do anything if a timer has expired.
-         */
-        progress |= timerlistgroup_run_timers(&ctx->tlg);
-    }
+    progress |= timerlistgroup_run_timers(&ctx->tlg);
 
     return progress;
 }