Correctly pass screenshot fences to transaction callbacks
In some instances, a screenshot may be captured before a layer has a
release callback registered. This can happen when a new buffer has
not yet been transacted before a screenshot is captured. This causes the
screenshot fence to be dropped and the possibility of tearing when
capturing a screenshot while continuously rendering.
To resolve this, buffer screenshot fences into a list of future fences
when there is no callback registered, and merge those fences when
dispatching the release callback.
Bug: 302703346
Test: SurfaceViewTests#testMovingWhiteSurfaceView 100 times
Change-Id: I91aec3cdb0973092d48cd77e59dd3999e9d9e847
diff --git a/services/surfaceflinger/Utils/FenceUtils.h b/services/surfaceflinger/Utils/FenceUtils.h
new file mode 100644
index 0000000..f6f7006
--- /dev/null
+++ b/services/surfaceflinger/Utils/FenceUtils.h
@@ -0,0 +1,51 @@
+/**
+ * Copyright (C) 2024 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <ui/Fence.h>
+
+namespace android {
+
+// TODO: measure if Fence::merge is cheaper
+inline void mergeFence(const char* debugName, sp<Fence>&& incomingFence, sp<Fence>& prevFence) {
+ if (prevFence == nullptr && incomingFence->getStatus() != Fence::Status::Invalid) {
+ prevFence = std::move(incomingFence);
+ } else if (prevFence != nullptr) {
+ // If both fences are signaled or both are unsignaled, we need to merge
+ // them to get an accurate timestamp.
+ if (prevFence->getStatus() != Fence::Status::Invalid &&
+ prevFence->getStatus() == incomingFence->getStatus()) {
+ char fenceName[32] = {};
+ snprintf(fenceName, 32, "%.28s", debugName);
+ sp<Fence> mergedFence = Fence::merge(fenceName, prevFence, incomingFence);
+ if (mergedFence->isValid()) {
+ prevFence = std::move(mergedFence);
+ }
+ } else if (incomingFence->getStatus() == Fence::Status::Unsignaled) {
+ // If one fence has signaled and the other hasn't, the unsignaled
+ // fence will approximately correspond with the correct timestamp.
+ // There's a small race if both fences signal at about the same time
+ // and their statuses are retrieved with unfortunate timing. However,
+ // by this point, they will have both signaled and only the timestamp
+ // will be slightly off; any dependencies after this point will
+ // already have been met.
+ prevFence = std::move(incomingFence);
+ }
+ }
+}
+
+} // namespace android