EME: Improve promise lifetime

third_party/WebKit/Source/modules/encryptedmedia/ContentDecryptionModuleResultPromise.h

Index: third_party/WebKit/Source/modules/encryptedmedia/ContentDecryptionModuleResultPromise.h
diff --git a/third_party/WebKit/Source/modules/encryptedmedia/ContentDecryptionModuleResultPromise.h b/third_party/WebKit/Source/modules/encryptedmedia/ContentDecryptionModuleResultPromise.h
index 104d15b4004d1686527843e6945961a750445c07..51ace41b38bfe1adef8e16cee88706af2d78f475 100644
--- a/third_party/WebKit/Source/modules/encryptedmedia/ContentDecryptionModuleResultPromise.h
+++ b/third_party/WebKit/Source/modules/encryptedmedia/ContentDecryptionModuleResultPromise.h
@@ -19,6 +19,15 @@ ExceptionCode WebCdmExceptionToExceptionCode(
 // complete(), completeWithSession(), etc. methods will reject the promise
 // with an error. It needs to be subclassed and the appropriate complete()
 // method overridden to resolve the promise as needed.
+//
+// Subclasses need to keep a Member<> to the object that created them so
+// that the creator remains around as long as this promise is pending. This
+// promise is not referenced by the object that created it (e.g MediaKeys,
+// MediaKeySession, Navigator.requestMediaKeySystemAccess), so this promise
+// may be cleaned up before or after it's creator once both become unreachable.
+// If it is after, the destruction of the creator may trigger this promise,
+// so use isValidToFulfillPromise() to verify that it is safe to fulfill
+// the promise.
 class ContentDecryptionModuleResultPromise
     : public ContentDecryptionModuleResult {
  public:
@@ -46,20 +55,21 @@ class ContentDecryptionModuleResultPromise
   // promise.
   template <typename... T>
   void resolve(T... value) {
+    DCHECK(isValidToFulfillPromise());
+
     m_resolver->resolve(value...);
     m_resolver.clear();
   }
 
-  // Rejects the promise with a DOMException. This will post a task to
-  // actually reject the promise later on.
+  // Rejects the promise with a DOMException.
   void reject(ExceptionCode, const String& errorMessage);
 
   ExecutionContext* getExecutionContext() const;
 
- private:
-  // Rejects the promise with a DOMException.
-  void rejectInternal(ExceptionCode, const String& errorMessage);
+  // Determine if it's OK to resolve/reject this promise.
+  bool isValidToFulfillPromise();
 
+ private:
   Member<ScriptPromiseResolver> m_resolver;
 };