client::Buffer: In debug mode, clobber mMappedData when it is freed

This will help detect cases where the mapped data is used after it is
freed, in particular in WebGPU tests around the interaction of mapping
and GC.

Bug: chromium:971949
Change-Id: I820d9885d39379fbc95c6504b9a4151053768d93
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/48382
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_wire/client/Buffer.cpp

@@ -119,6 +119,8 @@ namespace dawn_wire { namespace client {
             }
         }
         mRequests.clear();
+
+        FreeMappedData(true);
     }
 
     void Buffer::CancelCallbacksForDisconnect() {
@@ -360,15 +362,9 @@ namespace dawn_wire { namespace client {
                     mWriteHandle->SerializeFlush(writeHandleBuffer);
                     return WireResult::Success;
                 });
-            mWriteHandle = nullptr;
-
-        } else if (mReadHandle) {
-            mReadHandle = nullptr;
         }
 
-        mMappedData = nullptr;
-        mMapOffset = 0;
-        mMapSize = 0;
+        FreeMappedData(false);
 
         // Tag all mapping requests still in flight as unmapped before callback.
         for (auto& it : mRequests) {
@@ -384,9 +380,7 @@ namespace dawn_wire { namespace client {
 
     void Buffer::Destroy() {
         // Remove the current mapping.
-        mWriteHandle = nullptr;
-        mReadHandle = nullptr;
-        mMappedData = nullptr;
+        FreeMappedData(true);
 
         // Tag all mapping requests still in flight as destroyed before callback.
         for (auto& it : mRequests) {
@@ -420,4 +414,22 @@ namespace dawn_wire { namespace client {
         size_t offsetInMappedRange = offset - mMapOffset;
         return offsetInMappedRange <= mMapSize - size;
     }
+
+    void Buffer::FreeMappedData(bool destruction) {
+#if defined(DAWN_ENABLE_ASSERTS)
+        // When in "debug" mode, 0xCA-out the mapped data when we free it so that in we can detect
+        // use-after-free of the mapped data. This is particularly useful for WebGPU test about the
+        // interaction of mapping and GC.
+        if (mMappedData && destruction) {
+            memset(mMappedData, 0xCA, mMapSize);
+        }
+#endif  // defined(DAWN_ENABLE_ASSERTS)
+
+        mMapOffset = 0;
+        mMapSize = 0;
+        mWriteHandle = nullptr;
+        mReadHandle = nullptr;
+        mMappedData = nullptr;
+    }
+
 }}  // namespace dawn_wire::client

src/dawn_wire/client/Buffer.h

@@ -57,6 +57,8 @@ namespace dawn_wire { namespace client {
         bool IsMappedForWriting() const;
         bool CheckGetMappedRangeOffsetSize(size_t offset, size_t size) const;
 
+        void FreeMappedData(bool destruction);
+
         Device* mDevice;
 
         // We want to defer all the validation to the server, which means we could have multiple