fix: Use middle segment when guessing MIME type on HLS (shaka-project…

…#4269) (shaka-project#4270)

I have been working with a server which is too aggressive at expiring
segments at the left edge of its DVR. As a result of this, when Shaka's
HLS manifest parser puts out a HEAD request on the first segment to
guess its MIME type, sometimes the segment has already been removed
(404). I have filed a bug against the server team and I hope they relax
the expiration, but truth be told if you wait long enough, any request
against the left edge of DVR will eventually fail. It seems like a
simple workaround for now to simply request the middle segment rather
than the first segment.

Closes shaka-project#4269

Co-authored-by: Raymond Cheng <raycheng100@gmail.com>

lib/hls/hls_parser.js

@@ -2267,12 +2267,16 @@ shaka.hls.HlsParser = class {
     const ContentType = shaka.util.ManifestParserUtils.ContentType;
     const requestType = shaka.net.NetworkingEngine.RequestType.SEGMENT;
 
+    // If you wait long enough, requesting the first segment can fail
+    // because it has fallen off the left edge of DVR, so to be safer,
+    // let's request the middle segment.
     goog.asserts.assert(playlist.segments.length,
         'Playlist should have segments!');
-    const firstSegmentUri = this.variableSubstitution_(
-        playlist.segments[0].absoluteUri, variables);
+    const middleSegmentIdx = Math.trunc((playlist.segments.length - 1) / 2);
+    const middleSegmentUri = this.variableSubstitution_(
+        playlist.segments[middleSegmentIdx].absoluteUri, variables);
 
-    const parsedUri = new goog.Uri(firstSegmentUri);
+    const parsedUri = new goog.Uri(middleSegmentUri);
     const extension = parsedUri.getPath().split('.').pop();
     const map = HlsParser.EXTENSION_MAP_BY_CONTENT_TYPE_[contentType];
 
@@ -2302,7 +2306,7 @@ shaka.hls.HlsParser = class {
     // If unable to guess mime type, request a segment and try getting it
     // from the response.
     const headRequest = shaka.net.NetworkingEngine.makeRequest(
-        [firstSegmentUri], this.config_.retryParameters);
+        [middleSegmentUri], this.config_.retryParameters);
     headRequest.method = 'HEAD';
 
     const response = await this.makeNetworkRequest_(