[GStreamer] Use an explicit allowlist of AAC AOTs

https://bugs.webkit.org/show_bug.cgi?id=311347

Reviewed by NOBODY (OOPS!).

Currently the GStreamer ports of WebKit report support to any codec
string starting with "mp4a" if there is a decoder that supports
audio/mpeg, mpegversion=4. This is too optimistic and can lead to
applications choosing to use newer MPEG-4 Audio codecs not yet
supported by the user's system.

This patch makes WebKit play it safer and instead only report support
for AOTs with widespread support.

This includes reporting AOT 42 (typically used with xHE-AAC) as not
supported. Ideally we would query support for it somehow and reply
accordingly, but that would necessitate decoders to provide a uniform
API for querying such support, which unfortunately does not exist (and
would be tricky in practice as support is not always binary). An
environment variable is provided to declare explicit support or lack
thereof in environments where this is well-known, as well as for
debugging and experimentation.

See: WebPlatformForEmbedded/WPEWebKit#1641 (but
note that this patch takes a different approach than the original PR).

* LayoutTests/platform/glib/imported/w3c/web-platform-tests/media-source/mediasource-is-type-supported-expected.txt:
* Source/WebCore/platform/graphics/gstreamer/GStreamerRegistryScanner.cpp:
(WebCore::GStreamerRegistryScanner::initializeDecoders):

Author: ntrrgc

LayoutTests/platform/glib/imported/w3c/web-platform-tests/media-source/mediasource-is-type-supported-expected.txt

@@ -22,10 +22,10 @@ FAIL Test invalid mismatch between MIME type and codec ID "audio/webm;codecs="mp
 FAIL Test invalid mismatch between MIME type and codec ID "video/mp4;codecs="vp8"" assert_equals: supported expected false but got true
 FAIL Test invalid mismatch between MIME type and codec ID "video/mp4;codecs="vorbis"" assert_equals: supported expected false but got true
 FAIL Test invalid mismatch between MIME type and codec ID "video/webm;codecs="mp4a.40.2"" assert_equals: supported expected false but got true
-FAIL Test invalid codec ID "audio/mp4;codecs="mp4a"" assert_equals: supported expected false but got true
-FAIL Test invalid codec ID "audio/mp4;codecs="mp4a.40"" assert_equals: supported expected false but got true
-FAIL Test invalid codec ID "audio/mp4;codecs="mp4a.40."" assert_equals: supported expected false but got true
-FAIL Test invalid codec ID "audio/mp4;codecs="mp4a.67.3"" assert_equals: supported expected false but got true
+PASS Test invalid codec ID "audio/mp4;codecs="mp4a""
+PASS Test invalid codec ID "audio/mp4;codecs="mp4a.40""
+PASS Test invalid codec ID "audio/mp4;codecs="mp4a.40.""
+PASS Test invalid codec ID "audio/mp4;codecs="mp4a.67.3""
 PASS Test valid WebM type "video/webm;codecs="vp8""
 PASS Test valid WebM type "video/webm;codecs="vorbis""
 PASS Test valid WebM type "video/webm;codecs="vp8,vorbis""

Source/WebCore/platform/graphics/gstreamer/GStreamerRegistryScanner.cpp

@@ -468,7 +468,29 @@ void GStreamerRegistryScanner::initializeDecoders(const GStreamerRegistryScanner
         m_decoderMimeTypeSet.add("audio/mpeg"_s);
         m_decoderMimeTypeSet.add("audio/x-mpeg"_s);
         m_decoderCodecMap.add("mpeg"_s, result);
-        m_decoderCodecMap.add("mp4a*"_s, result);
+        // AAC has accumulated lots of extensions over the years.
+        // Unfortunately, decoders don't generally provide an API for querying support level, and support is not necessarily
+        // binary as features may be incomplete.
+        // We'll assume support for the extensions that as of 2026 are somewhat mainstream to avoid websites serving
+        // audio incompatible with the user decoder, which would cause errors or quality degradation depending on signalling.
+        //
+        // RFC 6381 3.3. ISO Base Media File Format Name Space
+        // Syntax: "mp4a." oti [ "." aud-oti ]
+        // oti is the ObjectTypeIndication from MPEG-4 Systems (ISO 14996-1).
+        // For oti=40 (MPEG-4 Audio), aud-oti represents the AOT.
+        m_decoderCodecMap.add("mp4a.67"_s, result); // MPEG-2 AAC LC
+        m_decoderCodecMap.add("mp4a.40.2"_s, result); // MPEG-4 AAC LC
+        m_decoderCodecMap.add("mp4a.40.02"_s, result); // MPEG-4 AAC LC
+        m_decoderCodecMap.add("mp4a.40.5"_s, result); // MPEG-4 HE-AAC v1 (AAC LC + SBR)
+        m_decoderCodecMap.add("mp4a.40.05"_s, result); // MPEG-4 HE-AAC v1 (AAC LC + SBR)
+        m_decoderCodecMap.add("mp4a.40.29"_s, result); // MPEG-4 HE-AAC v2 (AAC LC + SBR + PS)
+        // As of writing, support for Extended HE-AAC (MPEG-D USAC) and xHE-AAC (MPEG-D USAC + MPEG-D DRC) -- which uses the
+        // USAC AOT, is not yet widely available enough to be enabled by default.
+        auto value = CStringView::unsafeFromUTF8(g_getenv("WEBKIT_GST_CAN_PLAY_USAC"));
+        bool canPlayUsac = value.isEmpty() ? false : (WTF::equalLettersIgnoringASCIICase(value.span(), "true"_s)
+            || WTF::equalLettersIgnoringASCIICase(value.span(), "1"_s));
+        if (canPlayUsac)
+            m_decoderCodecMap.add("mp4a.40.42"_s, result); // MPEG-4 Extended HE-AAC and xHE-AAC (USAC AOT)
     }
 
     auto opusSupported = factories.hasElementForMediaType(ElementFactories::Type::AudioDecoder, "audio/x-opus"_s);