Bug 1626802 part 1: mscom: Provide a way for Interceptors to avoid unnecessary cross-thread QueryInterface calls. r=aklotz

When marshaling a11y calls from the content process, there are quite a lot of cross-thread QueryInterface calls (ipc::mscom::Interceptor::QueryInterfaceTarget).
Some of these are for special COM interfaces like IAgileObject and IFastRundown, which we could just special case in Interceptor::QueryInterface like we do for INoMarshal.
However, it seems there are a lot of other interfaces being queried and it's not clear why.
This patch adds a new HandlerProvider method: IsInterfaceMaybeSupported.
This allows implementations to indicate when there are interfaces which they definitely don't support, allowing the call to be answered without a cross-thread call.

Differential Revision: https://phabricator.services.mozilla.com/D69285

--HG--
extra : moz-landing-system : lando

ipc/mscom/IHandlerProvider.h

@@ -25,6 +25,16 @@ struct HandlerProvider {
                                            NotNull<IStream*> aStream) = 0;
   virtual STDMETHODIMP_(REFIID) MarshalAs(REFIID aIid) = 0;
   virtual STDMETHODIMP DisconnectHandlerRemotes() = 0;
+
+  /**
+   * Determine whether this interface might be supported by objects using
+   * this HandlerProvider.
+   * This is used to avoid unnecessary cross-thread QueryInterface calls for
+   * interfaces known to be unsupported.
+   * Return S_OK if the interface might be supported, E_NOINTERFACE if it
+   * definitely isn't supported.
+   */
+  virtual STDMETHODIMP IsInterfaceMaybeSupported(REFIID aIid) { return S_OK; }
 };
 
 struct IHandlerProvider : public IUnknown, public HandlerProvider {

ipc/mscom/Interceptor.cpp

@@ -707,6 +707,10 @@ Interceptor::QueryInterfaceTarget(REFIID aIid, void** aOutput,
     return E_NOINTERFACE;
   }
 
+  if (mEventSink->IsInterfaceMaybeSupported(aIid) == E_NOINTERFACE) {
+    return E_NOINTERFACE;
+  }
+
   MainThreadInvoker invoker;
   HRESULT hr;
   auto runOnMainThread = [&]() -> void {

ipc/mscom/MainThreadHandoff.cpp

@@ -550,6 +550,14 @@ MainThreadHandoff::DisconnectHandlerRemotes() {
   return mHandlerProvider->DisconnectHandlerRemotes();
 }
 
+HRESULT
+MainThreadHandoff::IsInterfaceMaybeSupported(REFIID aIid) {
+  if (!mHandlerProvider) {
+    return S_OK;
+  }
+  return mHandlerProvider->IsInterfaceMaybeSupported(aIid);
+}
+
 HRESULT
 MainThreadHandoff::OnWalkInterface(REFIID aIid, PVOID* aInterface,
                                    BOOL aIsInParam, BOOL aIsOutParam) {

ipc/mscom/MainThreadHandoff.h

@@ -66,6 +66,7 @@ class MainThreadHandoff final : public IInterceptorSink,
                                    NotNull<IStream*> aStream) override;
   STDMETHODIMP_(REFIID) MarshalAs(REFIID aIid) override;
   STDMETHODIMP DisconnectHandlerRemotes() override;
+  STDMETHODIMP IsInterfaceMaybeSupported(REFIID aIid) override;
 
   // ICallFrameWalker
   STDMETHODIMP OnWalkInterface(REFIID aIid, PVOID* aInterface, BOOL aIsInParam,