Use ExclusiveStreamReader to lock Body.

Source/modules/fetch/Body.cpp

Index: Source/modules/fetch/Body.cpp
diff --git a/Source/modules/fetch/Body.cpp b/Source/modules/fetch/Body.cpp
index 892a13b10b8e8f415f58ccd49d951061b2143ab3..97e750314e5112c7ee716805fc2165dade5a56fb 100644
--- a/Source/modules/fetch/Body.cpp
+++ b/Source/modules/fetch/Body.cpp
@@ -15,6 +15,7 @@
 #include "core/fileapi/FileReaderLoader.h"
 #include "core/fileapi/FileReaderLoaderClient.h"
 #include "core/frame/UseCounter.h"
+#include "core/streams/ExclusiveStreamReader.h"
 #include "core/streams/UnderlyingSource.h"
 #include "modules/fetch/BodyStreamBuffer.h"
 
@@ -233,7 +234,7 @@ private:
 
 ScriptPromise Body::readAsync(ScriptState* scriptState, ResponseType type)
 {
-    if (m_bodyUsed)
+    if (bodyUsed())
         return ScriptPromise::reject(scriptState, V8ThrowException::createTypeError(scriptState->isolate(), "Already read"));
 
     // When the main thread sends a V8::TerminateExecution() signal to a worker
@@ -246,7 +247,7 @@ ScriptPromise Body::readAsync(ScriptState* scriptState, ResponseType type)
     if (!executionContext)
         return ScriptPromise();
 
-    m_bodyUsed = true;
+    setBodyUsed();
     m_responseType = type;
 
     ASSERT(!m_resolver);
@@ -346,11 +347,20 @@ ReadableStream* Body::body()
 
 bool Body::bodyUsed() const
 {
-    return m_bodyUsed;
+    return m_bodyUsed || (m_stream && m_stream->isLocked());
 }
 
 void Body::setBodyUsed()
 {
+    ASSERT(!m_bodyUsed);
+    ASSERT(!m_stream || !m_stream->isLocked());
+    // Note that technically we can set BodyUsed even when the stream is
+    // closed or errored, but getReader doesn't work then.
+    if (m_stream && m_stream->stateInternal() != ReadableStream::Closed && m_stream->stateInternal() != ReadableStream::Errored) {
+        TrackExceptionState exceptionState;
+        m_streamReader = m_stream->getReader(exceptionState);
+        ASSERT(!exceptionState.hadException());
+    }
     m_bodyUsed = true;
 }
 
@@ -391,6 +401,7 @@ DEFINE_TRACE(Body)
     visitor->trace(m_resolver);
     visitor->trace(m_stream);
     visitor->trace(m_streamSource);
+    visitor->trace(m_streamReader);
     ActiveDOMObject::trace(visitor);
 }