[fuchsia] avoid masking bits passed to FDIO

We can do our masking later, avoiding violating FDIO's assumptions.

Change-Id: Ia28b74b9d7fadae6512af51df18e39935e3e90d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1697140Reviewed-by: James Robinson <jamesr@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676612}

base/message_loop/message_pump_fuchsia.cc

@@ -92,14 +92,6 @@ void MessagePumpFuchsia::ZxHandleWatchController::HandleSignal(
 
   controller->handler = nullptr;
 
-  // |signal| can include other spurious things, in particular, that an fd
-  // is writable, when we only asked to know when it was readable. In that
-  // case, we don't want to call both the CanWrite and CanRead callback,
-  // when the caller asked for only, for example, readable callbacks. So,
-  // mask with the events that we actually wanted to know about.
-  zx_signals_t signals = signal->trigger & signal->observed;
-  DCHECK_NE(0u, signals);
-
   // In the case of a persistent Watch, the Watch may be stopped and
   // potentially deleted by the caller within the callback, in which case
   // |controller| should not be accessed again, and we mustn't continue the
@@ -108,7 +100,7 @@ void MessagePumpFuchsia::ZxHandleWatchController::HandleSignal(
   bool was_stopped = false;
   controller->was_stopped_ = &was_stopped;
 
-  controller->watcher_->OnZxHandleSignalled(wait->object, signals);
+  controller->watcher_->OnZxHandleSignalled(wait->object, signal->observed);
 
   if (was_stopped)
     return;
@@ -125,6 +117,14 @@ void MessagePumpFuchsia::FdWatchController::OnZxHandleSignalled(
   uint32_t events;
   fdio_unsafe_wait_end(io_, signals, &events);
 
+  // |events| can include other spurious things, in particular, that an fd
+  // is writable, when we only asked to know when it was readable. In that
+  // case, we don't want to call both the CanWrite and CanRead callback,
+  // when the caller asked for only, for example, readable callbacks. So,
+  // mask with the events that we actually wanted to know about.
+  events &= desired_events_;
+  DCHECK_NE(0u, events);
+
   // Each |watcher_| callback we invoke may stop or delete |this|. The pump has
   // set |was_stopped_| to point to a safe location on the calling stack, so we
   // can use that to detect being stopped mid-callback and avoid doing further