aio-win32: reorganize polling loop

Preparatory bugfixes and tweaks to the loop before the next patch:

- disable dispatch optimization during aio_prepare.  This fixes a bug.

- do not modify "blocking" until after the first WaitForMultipleObjects
call.  This is needed in the next patch.

- change the loop to do...while.  This makes it obvious that the loop
is always entered at least once.  In the next patch this is important
because the first iteration undoes the ctx->notify_me increment that
happened before entering the loop.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Message-id: 1437487673-23740-4-git-send-email-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

aio-win32.c

@@ -284,11 +284,6 @@ bool aio_poll(AioContext *ctx, bool blocking)
     int timeout;
 
     aio_context_acquire(ctx);
-    have_select_revents = aio_prepare(ctx);
-    if (have_select_revents) {
-        blocking = false;
-    }
-
     was_dispatching = ctx->dispatching;
     progress = false;
 
@@ -304,6 +299,8 @@ bool aio_poll(AioContext *ctx, bool blocking)
      */
     aio_set_dispatching(ctx, !blocking);
 
+    have_select_revents = aio_prepare(ctx);
+
     ctx->walking_handlers++;
 
     /* fill fd sets */
@@ -317,12 +314,18 @@ bool aio_poll(AioContext *ctx, bool blocking)
     ctx->walking_handlers--;
     first = true;
 
-    /* wait until next event */
-    while (count > 0) {
+    /* ctx->notifier is always registered.  */
+    assert(count > 0);
+
+    /* Multiple iterations, all of them non-blocking except the first,
+     * may be necessary to process all pending events.  After the first
+     * WaitForMultipleObjects call ctx->notify_me will be decremented.
+     */
+    do {
         HANDLE event;
         int ret;
 
-        timeout = blocking
+        timeout = blocking && !have_select_revents
             ? qemu_timeout_ns_to_ms(aio_compute_timeout(ctx)) : 0;
         if (timeout) {
             aio_context_release(ctx);
@@ -351,7 +354,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
         blocking = false;
 
         progress |= aio_dispatch_handlers(ctx, event);
-    }
+    } while (count > 0);
 
     progress |= timerlistgroup_run_timers(&ctx->tlg);