Bug 1811442 - Automatically reject requests for MIDI access when no implementation is present r=bholley

Differential Revision: https://phabricator.services.mozilla.com/D167452

dom/midi/MIDIAccessManager.cpp

@@ -16,7 +16,7 @@
 #include "mozilla/ipc/Endpoint.h"
 #include "mozilla/ipc/PBackgroundChild.h"
 #include "mozilla/ipc/BackgroundChild.h"
-#include "mozilla/Preferences.h"
+#include "mozilla/StaticPrefs_midi.h"
 
 using namespace mozilla::ipc;
 
@@ -59,6 +59,18 @@ already_AddRefed<Promise> MIDIAccessManager::RequestMIDIAccess(
     return nullptr;
   }
 
+#ifndef MOZ_WEBMIDI_MIDIR_IMPL
+  if (!StaticPrefs::midi_testing()) {
+    // If we don't have a MIDI implementation and testing is disabled we can't
+    // allow accessing WebMIDI. However we don't want to return something
+    // different from a normal rejection because we don't want websites to use
+    // the error as a way to fingerprint users, so we throw a security error
+    // as if the request had been rejected by the user.
+    aRv.ThrowSecurityError("Access not allowed");
+    return nullptr;
+  }
+#endif
+
   if (!FeaturePolicyUtils::IsFeatureAllowed(doc, u"midi"_ns)) {
     aRv.Throw(NS_ERROR_DOM_SECURITY_ERR);
     return nullptr;