[GStreamer] Get length from content-length header when available in chunked content encoding

https://bugs.webkit.org/show_bug.cgi?id=266691

Reviewed by Xabier Rodriguez-Calvar.

Currently, chunked content-encoding media content transfers are considered not to have a
defined length and are treated as live videos (no seek available, the whole content is
downloaded in a monolithic way). However, sometimes the web server specifies a
content-length that, even if not mandatory, gives a hint about the total length. If that
info was used, we could treat the download in a much more flexible way, downloading only the
required pieces on demand, as the player needs them, and also enabling seek.

See: #1257

This patch leverages the information supplied by the content-length header when
content-encoding is set as chunked.

* Source/WebCore/platform/graphics/gstreamer/WebKitWebSourceGStreamer.cpp:
(CachedResourceStreamingClient::responseReceived): Set length with the right information when the circumstances are appropriate.

Canonical link: https://commits.webkit.org/272355@main

Author: eocanha

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

@@ -1011,6 +1011,15 @@ void CachedResourceStreamingClient::responseReceived(PlatformMediaResource&, con
 
     // length will be zero (unknown) if no Content-Length is provided or the response is compressed with Content-Encoding.
     uint64_t length = !response.httpHeaderFields().contains(HTTPHeaderName::ContentEncoding) ? response.expectedContentLength() : 0;
+
+    // But in some cases, Content-Encoding: chunked can specify the total length. Use it.
+    std::optional<uint64_t> contentLength;
+    // There's a contentLength assignment (=) on purpose, plus an implicit comparison on the variable having a value.
+    if (!length && response.httpHeaderFields().contains(HTTPHeaderName::TransferEncoding)
+        && equalLettersIgnoringASCIICase(response.httpHeaderField(HTTPHeaderName::TransferEncoding), "chunked"_s)
+        && (contentLength = parseInteger<uint64_t>(response.httpHeaderField(HTTPHeaderName::ContentLength))))
+        length = contentLength.value();
+
     if (length > 0 && members->requestedPosition && response.httpStatusCode() == 206)
         length += members->requestedPosition;