[WGPUFuture] Use map over unordered_map for JS event ordering.

Bug: dawn:2066
Change-Id: Ie8e720c5e48f3e471a8eb5fe1019bcd0441583ad
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/153000
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/dawn/wire/client/EventManager.cpp

diff --git a/src/dawn/wire/client/EventManager.cpp b/src/dawn/wire/client/EventManager.cpp
index f567078..1e0f1e0 100644
--- a/src/dawn/wire/client/EventManager.cpp
+++ b/src/dawn/wire/client/EventManager.cpp
@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include <unordered_map>
+#include <map>
 #include <utility>
 #include <vector>
 
@@ -47,13 +47,14 @@
 void EventManager::ShutDown() {
     // Call any outstanding callbacks before destruction.
     while (true) {
-        std::unordered_map<FutureID, TrackedEvent> movedEvents;
+        std::map<FutureID, TrackedEvent> movedEvents;
         mTrackedEvents.Use([&](auto trackedEvents) { movedEvents = std::move(*trackedEvents); });
 
         if (movedEvents.empty()) {
             break;
         }
 
+        // Ordering guaranteed because we are using a sorted map.
         for (auto& [futureID, trackedEvent] : movedEvents) {
             // Event should be already marked Ready since events are actually driven by
             // RequestTrackers (at the time of this writing), which all shut down before this.
@@ -74,6 +75,7 @@
 }
 
 void EventManager::ProcessPollEvents() {
+    // Since events are already stored in an ordered map, this list must already be ordered.
     std::vector<TrackedEvent> eventsToCompleteNow;
 
     // TODO(crbug.com/dawn/2060): EventManager shouldn't bother to track ProcessEvents-type events
@@ -120,7 +122,9 @@
         return WGPUWaitStatus_Success;
     }
 
-    std::vector<TrackedEvent> eventsToCompleteNow;
+    // Since the user can specify the FutureIDs in any order, we need to use another ordered map
+    // here to ensure that the result is ordered for JS event ordering.
+    std::map<FutureID, TrackedEvent> eventsToCompleteNow;
     bool anyCompleted = false;
     const FutureID firstInvalidFutureID = mNextFutureID;
     mTrackedEvents.Use([&](auto trackedEvents) {
@@ -141,7 +145,7 @@
             if (event.mReady) {
                 anyCompleted = true;
                 if (event.mCallback) {
-                    eventsToCompleteNow.emplace_back(std::move(event));
+                    eventsToCompleteNow.emplace(it->first, std::move(event));
                 }
                 trackedEvents->erase(it);
             }
@@ -149,7 +153,7 @@
     });
 
     // TODO(crbug.com/dawn/2066): Guarantee the event ordering from the JS spec.
-    for (TrackedEvent& event : eventsToCompleteNow) {
+    for (auto& [_, event] : eventsToCompleteNow) {
         DAWN_ASSERT(event.mReady && event.mCallback);
         // .completed has already been set to true (before the callback, per API contract).
         event.mCallback(EventCompletionType::Ready);

src/dawn/wire/client/EventManager.h

diff --git a/src/dawn/wire/client/EventManager.h b/src/dawn/wire/client/EventManager.h
index 2260b73..2c599e2 100644
--- a/src/dawn/wire/client/EventManager.h
+++ b/src/dawn/wire/client/EventManager.h
@@ -17,7 +17,7 @@
 
 #include <cstddef>
 #include <functional>
-#include <unordered_map>
+#include <map>
 #include <utility>
 
 #include "dawn/common/FutureUtils.h"
@@ -70,8 +70,10 @@
 
     Client* mClient;
 
-    // Tracks all kinds of events (for both WaitAny and ProcessEvents).
-    MutexProtected<std::unordered_map<FutureID, TrackedEvent>> mTrackedEvents;
+    // Tracks all kinds of events (for both WaitAny and ProcessEvents). We use an ordered map so
+    // that in most cases, event ordering is already implicit when we iterate the map. (Not true for
+    // WaitAny though because the user could specify the FutureIDs out of order.)
+    MutexProtected<std::map<FutureID, TrackedEvent>> mTrackedEvents;
     std::atomic<FutureID> mNextFutureID = 1;
 };