Bug 1570701: Part 1 - Add IsClassThreadAwareInprocServer utility function to MSCOM; r=Jamie

We want to ensure that the code being added in part 2 is only used when it is
reasonable and safe to do so. One way to ensure this is to add a check that the
desired CLSID is an in-process, thread-aware server.

Differential Revision: https://phabricator.services.mozilla.com/D44518

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

ipc/mscom/Utils.cpp

@@ -414,5 +414,43 @@ bool IsInterfaceEqualToOrInheritedFrom(REFIID aInterface, REFIID aFrom,
 
 #endif  // defined(ACCESSIBILITY)
 
+#if defined(MOZILLA_INTERNAL_API)
+bool IsClassThreadAwareInprocServer(REFCLSID aClsid) {
+  nsAutoString strClsid;
+  GUIDToString(aClsid, strClsid);
+
+  nsAutoString inprocServerSubkey(NS_LITERAL_STRING("CLSID\\"));
+  inprocServerSubkey.Append(strClsid);
+  inprocServerSubkey.Append(NS_LITERAL_STRING("\\InprocServer32"));
+
+  // Of the possible values, "Apartment" is the longest, so we'll make this
+  // buffer large enough to hold that one.
+  wchar_t threadingModelBuf[ArrayLength(L"Apartment")] = {};
+
+  DWORD numBytes = sizeof(threadingModelBuf);
+  LONG result = ::RegGetValueW(HKEY_CLASSES_ROOT, inprocServerSubkey.get(),
+                               L"ThreadingModel", RRF_RT_REG_SZ, nullptr,
+                               threadingModelBuf, &numBytes);
+  if (result != ERROR_SUCCESS) {
+    // This will also handle the case where the CLSID is not an inproc server.
+    return false;
+  }
+
+  DWORD numChars = numBytes / sizeof(wchar_t);
+  // numChars includes the null terminator
+  if (numChars <= 1) {
+    return false;
+  }
+
+  nsDependentString threadingModel(threadingModelBuf, numChars - 1);
+
+  // Ensure that the threading model is one of the known values that indicates
+  // that the class can operate natively (ie, no proxying) inside a MTA.
+  return threadingModel.EqualsLiteral("Both") ||
+         threadingModel.EqualsLiteral("Free") ||
+         threadingModel.EqualsLiteral("Neutral");
+}
+#endif  // defined(MOZILLA_INTERNAL_API)
+
 }  // namespace mscom
 }  // namespace mozilla

ipc/mscom/Utils.h

@@ -51,6 +51,29 @@ uint32_t CreateStream(const uint8_t* aBuf, const uint32_t aBufLen,
 uint32_t CopySerializedProxy(IStream* aInStream, IStream** aOutStream);
 
 #if defined(MOZILLA_INTERNAL_API)
+/**
+ * Checks the registry to see if |aClsid| is a thread-aware in-process server.
+ *
+ * In DCOM, an in-process server is a server that is implemented inside a DLL
+ * that is loaded into the client's process for execution. If |aClsid| declares
+ * itself to be a local server (that is, a server that resides in another
+ * process), this function returns false.
+ *
+ * For the server to be thread-aware, its registry entry must declare a
+ * ThreadingModel that is one of "Free", "Both", or "Neutral". If the threading
+ * model is "Apartment" or some other, invalid value, the class is treated as
+ * being single-threaded.
+ *
+ * NB: This function cannot check CLSIDs that were registered via manifests,
+ * as unfortunately there is not a documented API available to query for those.
+ * This should not be an issue for most CLSIDs that Gecko is interested in, as
+ * we typically instantiate system CLSIDs which are available in the registry.
+ *
+ * @param aClsid The CLSID of the COM class to be checked.
+ * @return true if the class meets the above criteria, otherwise false.
+ */
+bool IsClassThreadAwareInprocServer(REFCLSID aClsid);
+
 void GUIDToString(REFGUID aGuid, nsAString& aOutString);
 #endif  // defined(MOZILLA_INTERNAL_API)