Bug 1528042 - Add media.devices.enumerate.legacy.enabled pref. r=karlt

For now legacy is enabled.

Differential Revision: https://phabricator.services.mozilla.com/D154139

dom/media/MediaDevices.cpp

@@ -238,6 +238,7 @@ RefPtr<MediaDeviceSetRefCnt> MediaDevices::FilterExposedDevices(
     // they are exposed only when explicitly and individually allowed by the
     // user.
   }
+  bool legacy = StaticPrefs::media_devices_enumerate_legacy_enabled();
   bool outputIsDefault = true;  // First output is the default.
   bool haveDefaultOutput = false;
   nsTHashSet<nsString> exposedMicrophoneGroupIds;
@@ -250,7 +251,7 @@ RefPtr<MediaDeviceSetRefCnt> MediaDevices::FilterExposedDevices(
         if (mCanExposeMicrophoneInfo) {
           exposedMicrophoneGroupIds.Insert(device->mRawGroupID);
         }
-        if (!DeviceInformationCanBeExposed()) {
+        if (!DeviceInformationCanBeExposed() && !legacy) {
           dropMics = true;
         }
         break;
@@ -258,7 +259,7 @@ RefPtr<MediaDeviceSetRefCnt> MediaDevices::FilterExposedDevices(
         if (dropCams) {
           continue;
         }
-        if (!DeviceInformationCanBeExposed()) {
+        if (!DeviceInformationCanBeExposed() && !legacy) {
           dropCams = true;
         }
         break;
@@ -406,15 +407,15 @@ void MediaDevices::ResumeEnumerateDevices(
 void MediaDevices::ResolveEnumerateDevicesPromise(
     Promise* aPromise, const LocalMediaDeviceSet& aDevices) const {
   nsTArray<RefPtr<MediaDeviceInfo>> infos;
+  bool legacy = StaticPrefs::media_devices_enumerate_legacy_enabled();
 
   for (const RefPtr<LocalMediaDevice>& device : aDevices) {
-    MOZ_ASSERT(device->Kind() < MediaDeviceKind::EndGuard_);
-    bool canExposeInfo = CanExposeInfo(device->Kind());
-
+    bool exposeInfo = CanExposeInfo(device->Kind()) || legacy;
+    bool exposeLabel = legacy ? DeviceInformationCanBeExposed() : exposeInfo;
     infos.AppendElement(MakeRefPtr<MediaDeviceInfo>(
-        canExposeInfo ? device->mID : u""_ns, device->Kind(),
-        canExposeInfo ? device->mName : u""_ns,
-        canExposeInfo ? device->mGroupID : u""_ns));
+        exposeInfo ? device->mID : u""_ns, device->Kind(),
+        exposeLabel ? device->mName : u""_ns,
+        exposeInfo ? device->mGroupID : u""_ns));
   }
   aPromise->MaybeResolve(std::move(infos));
 }

modules/libpref/init/StaticPrefList.yaml

@@ -10770,6 +10770,23 @@
   value: true
   mirror: once
 
+# This pref turns on legacy (non-spec) exposure of camera and microphone
+# information from enumerateDevices and devicechange ahead of successful
+# getUserMedia calls. Should only be turned on to resolve web compat issues,
+# since doing so reveals more user fingerprinting information to trackers.
+#
+# This mode is marginally more permissive than the legacy behavior it attempts
+# to emulate in that device labels do not disappear when tracks are stopped and
+# temporary permission expires. That behavior isn't emulated due to low relative
+# privacy value  compared to spec and it not being being conducive to spec
+# convergence.
+#
+# Planned next steps: true → @IS_NOT_NIGHTLY_BUILD@ → false
+- name: media.devices.enumerate.legacy.enabled
+  type: bool
+  value: true
+  mirror: always
+
 # WebRTC prefs follow
 
 # Enables RTCPeerConnection support. Note that, when true, this pref enables

testing/web-platform/meta/audio-output/__dir__.ini

@@ -1 +1 @@
-prefs: [media.navigator.permission.disabled:true, media.setsinkid.enabled:true]
+prefs: [media.navigator.permission.disabled:true, media.setsinkid.enabled:true, media.devices.enumerate.legacy.enabled:false]

testing/web-platform/meta/mediacapture-streams/__dir__.ini

@@ -1,2 +1,2 @@
-prefs: [media.navigator.permission.disabled:true, media.navigator.streams.fake:true, dom.security.featurePolicy.header.enabled:true, dom.security.featurePolicy.webidl.enabled:true]
+prefs: [media.navigator.permission.disabled:true, media.navigator.streams.fake:true, dom.security.featurePolicy.header.enabled:true, dom.security.featurePolicy.webidl.enabled:true,media.devices.enumerate.legacy.enabled:false]
 lsan-allowed: [NewSegment, mozilla::layers::BufferTextureData::CreateInternal]

testing/web-platform/tests/audio-output/enumerateDevices-with-selectAudioOutput.https.html

@@ -40,9 +40,9 @@ promise_test(async () => {
   const outputDevices = devices.filter(info => info.kind == "audiooutput");
   assert_equals(outputDevices.length, 1, "number of audiooutput devices.");
   assert_not_equals(selected, undefined);
-  const info = outputDevices[0];
-  assert_equals(info.deviceId, selected.deviceId);
-  assert_equals(info.groupId, selected.groupId);
-  assert_equals(info.label, selected.label);
+  const [info] = outputDevices;
+  assert_equals(info.deviceId, selected.deviceId, "deviceId exposed");
+  assert_equals(info.groupId, selected.groupId, "groupId exposed");
+  assert_equals(info.label, selected.label, "label exposed");
 }, "enumerateDevices() after selectAudioOutput()");
 </script>