[MSE][GStreamer] Advertise AC4 in systems supporting it

https://bugs.webkit.org/show_bug.cgi?id=310845

Reviewed by NOBODY (OOPS!).

This patch enables Dolby AC-4 support in systems that have a GStreamer
decoder that supports it.  As of writing AC-4 support is still not mature
in desktop but it is present in some set top boxes.

The changes involve adding support for the MIME type and codec strings
in MSE and whitelisting the caps in WebKitThunderDecryptorGStreamer. The
codec string is also checked for format correctness and presentation
version support.

Unfortunately, as of writing the existing GStreamer decoders don't have
a established way of querying AC-4 support levels in a fine grained
manner.

This patch will accept codec strings for presentation version 1.
Presentation version 1 is the minimum version supported by bitstream
version 2, which is the only bitstream version that is part of the
current Dolby AC-4 Kit (1.5) test signals. This combination is also the
only one supported in CMAF according to the current version of the AC-4
spec (ETSI TS 103 190-2 V1.3.1, 2025-07).

This is understood to be a reasonable baseline for what is commonly
supported for a contemporary AC-4 decoder. It is possible however that
in the future we need to add support for e.g. environment variables
to further customize support level, assuming that by then there is still
no way of querying the support level from WebKit.

Original author: Andrzej Surdej <Andrzej_Surdej@comcast.com>
See: WebPlatformForEmbedded/WPEWebKit#1641

* Source/WebCore/platform/graphics/gstreamer/GStreamerRegistryScanner.cpp:
(WebCore::GStreamerRegistryScanner::initializeDecoders):
(WebCore::GStreamerRegistryScanner::isCodecSupported const):
(WebCore::parseAC4LevelAndProfile):
* Source/WebCore/platform/graphics/gstreamer/eme/GStreamerEMEUtilities.h:
* Source/WebCore/platform/graphics/gstreamer/eme/WebKitThunderDecryptorGStreamer.cpp:
* Source/WebCore/platform/graphics/gstreamer/eme/WebKitThunderParser.cpp:

Author: ntrrgc

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

@@ -49,6 +49,8 @@ namespace WebCore {
 GST_DEBUG_CATEGORY_STATIC(webkit_media_gst_registry_scanner_debug);
 #define GST_CAT_DEFAULT webkit_media_gst_registry_scanner_debug
 
+static bool parseAC4LevelAndProfile(const String& codec);
+
 struct VideoDecodingLimits {
     unsigned mediaMaxWidth = 0;
     unsigned mediaMaxHeight = 0;
@@ -552,6 +554,7 @@ void GStreamerRegistryScanner::initializeDecoders(const GStreamerRegistryScanner
     Vector<GstCapsWebKitMapping> mseCompatibleMapping = {
         { ElementFactories::Type::AudioDecoder, "audio/x-ac3"_s, { }, { "x-ac3"_s, "ac-3"_s, "ac3"_s } },
         { ElementFactories::Type::AudioDecoder, "audio/x-eac3"_s, { "audio/x-ac3"_s }, { "x-eac3"_s, "ec3"_s, "ec-3"_s, "eac3"_s } },
+        { ElementFactories::Type::AudioDecoder, "audio/x-ac4"_s, { }, { "x-ac4"_s, "ac-4*"_s, "ac4"_s } },
         { ElementFactories::Type::AudioDecoder, "audio/x-flac"_s, { "audio/x-flac"_s, "audio/flac"_s }, { "x-flac"_s, "flac"_s, "fLaC"_s } },
     };
     fillMimeTypeSetFromCapsMapping(factories, mseCompatibleMapping);
@@ -800,6 +803,8 @@ GStreamerRegistryScanner::CodecLookupResult GStreamerRegistryScanner::isCodecSup
         result = isAVC1CodecSupported(configuration, codecName, shouldCheckForHardwareUse);
     else if (codecName.startsWith("hev1"_s) || codecName.startsWith("hvc1"_s))
         result = isHEVCCodecSupported(configuration, codecName, shouldCheckForHardwareUse);
+    else if (codecName.startsWith("ac-4"_s) && !parseAC4LevelAndProfile(codecName))
+        result = { false, nullptr };
     else {
         auto& codecMap = configuration == Configuration::Decoding ? m_decoderCodecMap : m_encoderCodecMap;
         for (const auto& [codecId, lookupResult] : codecMap) {
@@ -1028,6 +1033,38 @@ ASCIILiteral GStreamerRegistryScanner::configurationNameForLogging(Configuration
     return ""_s;
 }
 
+static bool parseAC4LevelAndProfile(const String& codec)
+{
+    auto parts = codec.split('.');
+    // "ac-4" with no dots is valid (generic, unconstrained).
+    if (parts.size() == 1 && equalIgnoringASCIICase(parts[0], "ac-4"_s))
+        return true;
+    // Full format requires exactly 4 components: ["ac-4", bitstream_version, presentation_version, mdcompat].
+    // See ETSI TS 103 190-2 v1.3.1 Appendix E.13.
+    if (parts.size() != 4) {
+        GST_WARNING("AC-4 codec string has wrong number of components: %s", codec.utf8().data());
+        return false;
+    }
+
+    // The current available AC-4 decoders don't have an agreed upon API to query support in a fine-grained manner,
+    // so instead we make some conservative assumptions, matching the subset of AC-4 supported in CMAF.
+
+    // presentation_version must be 1 (stereo/5.1); value 2 denotes IMS which is assumed not supported.
+    auto presentationVersion = parseInteger<unsigned>(parts[2]);
+    if (!presentationVersion || *presentationVersion != 1) {
+        GST_DEBUG("AC-4 codec string has unsupported presentation_version: %s", codec.utf8().data());
+        return false;
+    }
+    // md_compat (level): only levels 0-3 are assumed supported.
+    // Levels 4-6 are reserved by the AC-4 spec. Level 7 (unlimited number of tracks) is assumed unsupported.
+    auto mdcompat = parseInteger<unsigned>(parts[3]);
+    if (!mdcompat || *mdcompat > 3) {
+        GST_DEBUG("AC-4 codec string has unsupported mdcompat level: %s", codec.utf8().data());
+        return false;
+    }
+    return true;
+}
+
 GStreamerRegistryScanner::RegistryLookupResult GStreamerRegistryScanner::isConfigurationSupported(Configuration configuration, const PlatformMediaConfiguration& mediaConfiguration) const
 {
 #ifndef GST_DISABLE_GST_DEBUG

Source/WebCore/platform/graphics/gstreamer/eme/GStreamerEMEUtilities.h

@@ -98,8 +98,8 @@ class GStreamerEMEUtilities {
     static constexpr auto s_unspecifiedUUID = GST_PROTECTION_UNSPECIFIED_SYSTEM_ID ""_s;
     static constexpr auto s_unspecifiedKeySystem = GST_PROTECTION_UNSPECIFIED_SYSTEM_ID ""_s;
 
-    static constexpr std::array<ASCIILiteral, 11> s_cencEncryptionMediaTypes = { "video/mp4"_s, "audio/mp4"_s, "video/x-h264"_s, "video/x-h265"_s, "audio/mpeg"_s,
-        "audio/x-eac3"_s, "audio/x-ac3"_s, "audio/x-flac"_s, "audio/x-opus"_s, "video/x-vp9"_s, "video/x-av1"_s };
+    static constexpr std::array<ASCIILiteral, 12> s_cencEncryptionMediaTypes = { "video/mp4"_s, "audio/mp4"_s, "video/x-h264"_s, "video/x-h265"_s, "audio/mpeg"_s,
+        "audio/x-eac3"_s, "audio/x-ac3"_s, "audio/x-ac4"_s, "audio/x-flac"_s, "audio/x-opus"_s, "video/x-vp9"_s, "video/x-av1"_s };
     static constexpr std::array<ASCIILiteral, 7> s_webmEncryptionMediaTypes = { "video/webm"_s, "audio/webm"_s, "video/x-vp9"_s, "video/x-av1"_s, "audio/x-opus"_s, "audio/x-vorbis"_s, "video/x-vp8"_s };
 
     static bool isClearKeyKeySystem(const String& keySystem)

Source/WebCore/platform/graphics/gstreamer/eme/WebKitThunderDecryptorGStreamer.cpp

@@ -56,6 +56,7 @@ static GstStaticPadTemplate thunderSrcTemplate = GST_STATIC_PAD_TEMPLATE("src",
         "audio/x-flac; "
         "audio/x-eac3; "
         "audio/x-ac3; "
+        "audio/x-ac4; "
         "video/x-h264; "
         "video/x-h265; "
         "video/x-vp9; video/x-vp8; "

Source/WebCore/platform/graphics/gstreamer/eme/WebKitThunderParser.cpp

@@ -67,6 +67,7 @@ static GstStaticPadTemplate thunderParseSrcTemplate = GST_STATIC_PAD_TEMPLATE("s
         "audio/x-flac; "
         "audio/x-eac3; "
         "audio/x-ac3; "
+        "audio/x-ac4; "
         "video/x-h264; "
         "video/x-h265; "
         "video/x-vp9; video/x-vp8; "