Add Matroska resync logic for malformed files, add RVE sponsor

src/matroska/ebml.ts

@@ -122,9 +122,8 @@ export const LEVEL_0_EBML_IDS: EBMLId[] = [
 	EBMLId.Segment,
 ];
 
+// All the stuff that can appear in a segment, basically
 export const LEVEL_1_EBML_IDS: EBMLId[] = [
-	EBMLId.EBMLMaxIDLength,
-	EBMLId.EBMLMaxSizeLength,
 	EBMLId.SeekHead,
 	EBMLId.Info,
 	EBMLId.Cluster,
@@ -585,6 +584,28 @@ export class EBMLReader {
 
 		return null;
 	}
+
+	/** Searches for the next occurrence of an element ID using a naive byte-wise search. */
+	async resync(ids: EBMLId[], until: number) {
+		const loadChunkSize = 2 ** 20; // 1 MiB
+		const idsSet = new Set(ids);
+
+		while (this.pos <= until - MIN_HEADER_SIZE) {
+			if (!this.reader.rangeIsLoaded(this.pos, Math.min(this.pos + MAX_HEADER_SIZE, until))) {
+				await this.reader.loadRange(this.pos, Math.min(this.pos + loadChunkSize, until));
+			}
+
+			const elementStartPos = this.pos;
+			const elementId = this.readElementId();
+			if (elementId !== null && idsSet.has(elementId)) {
+				return elementStartPos;
+			}
+
+			this.pos = elementStartPos + 1;
+		}
+
+		return null;
+	}
 }
 
 export const CODEC_STRING_MAP: Partial<Record<MediaCodec, string>> = {

src/matroska/matroska-demuxer.ts

@@ -56,6 +56,7 @@ import {
 	EBMLId,
 	EBMLReader,
 	LEVEL_0_AND_1_EBML_IDS,
+	LEVEL_1_EBML_IDS,
 	MAX_HEADER_SIZE,
 	MIN_HEADER_SIZE,
 	readVarInt,
@@ -175,6 +176,7 @@ const METADATA_ELEMENTS = [
 	{ id: EBMLId.Tracks, flag: 'tracksSeen' },
 	{ id: EBMLId.Cues, flag: 'cuesSeen' },
 ] as const;
+const MAX_RESYNC_LENGTH = 10 * 2 ** 20; // 10 MiB
 
 export class MatroskaDemuxer extends Demuxer {
 	metadataReader: EBMLReader;
@@ -330,8 +332,22 @@ export class MatroskaDemuxer extends Demuxer {
 
 			const elementStartPos = this.metadataReader.pos;
 			const header = this.metadataReader.readElementHeader();
-			if (!header) {
-				break;
+
+			if (!header || !LEVEL_1_EBML_IDS.includes(header.id)) {
+				// Potential junk. Let's try to resync
+
+				this.metadataReader.pos = elementStartPos;
+				const nextPos = await this.metadataReader.resync(
+					LEVEL_1_EBML_IDS,
+					Math.min(this.currentSegment.elementEndPos, this.metadataReader.pos + MAX_RESYNC_LENGTH),
+				);
+
+				if (nextPos) {
+					this.metadataReader.pos = nextPos;
+					continue;
+				} else {
+					break; // Resync failed
+				}
 			}
 
 			const { id, size } = header;
@@ -352,6 +368,10 @@ export class MatroskaDemuxer extends Demuxer {
 				}
 			}
 
+			if (size !== null) {
+				this.metadataReader.pos = dataStartPos + size;
+			}
+
 			if (this.currentSegment.infoSeen && this.currentSegment.tracksSeen && this.currentSegment.cuesSeen) {
 				// No need to search anymore, we have everything
 				break;
@@ -381,10 +401,15 @@ export class MatroskaDemuxer extends Demuxer {
 			if (size === null) {
 				break;
 			}
+		}
 
-			this.metadataReader.pos = dataStartPos + size;
+		if (!clusterEncountered) {
+			const seekEntry = this.currentSegment.seekEntries.find(entry => entry.id === EBMLId.Cluster);
 
-			if (!clusterEncountered) {
+			if (seekEntry) {
+				// The seek head points us to the first cluster, nice
+				this.currentSegment.clusterSeekStartPos = segmentDataStart + seekEntry.segmentPosition;
+			} else {
 				this.currentSegment.clusterSeekStartPos = this.metadataReader.pos;
 			}
 		}
@@ -1609,8 +1634,24 @@ abstract class MatroskaTrackBacking implements InputTrackBacking {
 				await metadataReader.reader.loadRange(metadataReader.pos, metadataReader.pos + MAX_HEADER_SIZE);
 				const elementStartPos = metadataReader.pos;
 				const elementHeader = metadataReader.readElementHeader();
-				if (!elementHeader) {
-					break;
+
+				if (!elementHeader || !LEVEL_1_EBML_IDS.includes(elementHeader.id)) {
+					// There's an element here that shouldn't be here (or Void). Might be garbage. In this case, let's
+					// try and resync to the next valid element.
+
+					metadataReader.pos = elementStartPos;
+
+					const nextPos = await metadataReader.resync(
+						LEVEL_1_EBML_IDS,
+						Math.min(segment.elementEndPos, metadataReader.pos + MAX_RESYNC_LENGTH),
+					);
+
+					if (nextPos) {
+						metadataReader.pos = nextPos;
+						continue;
+					} else {
+						break; // Resync failed
+					}
 				}
 
 				const id = elementHeader.id;