REGRESSION (274873@main): Audio is not anchored to WKWebView's spatia…

…l location

https://bugs.webkit.org/show_bug.cgi?id=270919
rdar://123800596

Reviewed by Jer Noble.

In 274873@main we added the infrastructure for passing a spatial tracking label from the UI process
to the GPU process. Because a null WTF::String is implicitly converted to @"", this change had the
unintended side effect of setting AVSampleBufferVidoRenderer and AVPlayer's spatial tracking label
to the empty string rather than the default value of nil, breaking spatial tracking for the audio
emitted from WKWebView.

Resolved this by introducing nsStringNilIfNull -- returning nil if the WTF::String is null otherwise
converting it as-is to NSString -- and using it to set spatial tracking labels in MediaPlayerPrivate.

* Source/WTF/wtf/text/WTFString.h:
(WTF::nsStringNilIfNull):
* Source/WebCore/platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.mm:
(WebCore::MediaPlayerPrivateAVFoundationObjC::createAVPlayer):
(WebCore::MediaPlayerPrivateAVFoundationObjC::setSpatialTrackingLabel):
* Source/WebCore/platform/graphics/avfoundation/objc/MediaPlayerPrivateMediaSourceAVFObjC.mm:
(WebCore::MediaPlayerPrivateMediaSourceAVFObjC::updateSpatialTrackingLabel):

Canonical link: https://commits.webkit.org/276074@main

Source/WTF/wtf/text/WTFString.h

@@ -341,6 +341,9 @@ inline void swap(String& a, String& b) { a.swap(b); }
 // Used in a small number of places where the long standing behavior has been "nil if empty".
 NSString * nsStringNilIfEmpty(const String&);
 
+// Used in a small number of places where null strings should be converted to nil but empty strings should be maintained.
+NSString * nsStringNilIfNull(const String&);
+
 #endif
 
 WTF_EXPORT_PRIVATE int codePointCompare(const String&, const String&);
@@ -517,6 +520,13 @@ inline NSString * nsStringNilIfEmpty(const String& string)
     return *string.impl();
 }
 
+inline NSString * nsStringNilIfNull(const String& string)
+{
+    if (string.isNull())
+        return nil;
+    return *string.impl();
+}
+
 #endif
 
 inline bool codePointCompareLessThan(const String& a, const String& b)

Source/WebCore/platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.mm

@@ -1154,7 +1154,7 @@ static URL conformFragmentIdentifierForURL(const URL& url)
     }
 
 #if HAVE(SPATIAL_TRACKING_LABEL)
-    [m_avPlayer _setSTSLabel:m_spatialTrackingLabel];
+    [m_avPlayer _setSTSLabel:nsStringNilIfNull(m_spatialTrackingLabel)];
 #endif
 
     if (m_avPlayerItem)
@@ -4011,9 +4011,9 @@ void determineChangedTracksFromNewTracksAndOldItems(MediaSelectionGroupAVFObjC*
 {
     if (m_spatialTrackingLabel == spatialTrackingLabel)
         return;
-    m_spatialTrackingLabel = spatialTrackingLabel;
+    m_spatialTrackingLabel = WTFMove(spatialTrackingLabel);
     if (m_avPlayer)
-        [m_avPlayer _setSTSLabel:spatialTrackingLabel];
+        [m_avPlayer _setSTSLabel:nsStringNilIfNull(m_spatialTrackingLabel)];
 }
 #endif
 

Source/WebCore/platform/graphics/avfoundation/objc/MediaPlayerPrivateMediaSourceAVFObjC.mm

@@ -1677,7 +1677,7 @@ void getSupportedTypes(HashSet<String>& types) const final
     if (!m_visible)
         [m_sampleBufferDisplayLayer sampleBufferRenderer].STSLabel = session.spatialTrackingLabel;
     else
-        [m_sampleBufferDisplayLayer sampleBufferRenderer].STSLabel = m_spatialTrackingLabel;
+        [m_sampleBufferDisplayLayer sampleBufferRenderer].STSLabel = nsStringNilIfNull(m_spatialTrackingLabel);
 }
 #endif