fix(android): render double-NUL ASS subtitles

close #1681

android/libass/src/main/java/com/edde746/plezy/libass/media/parser/AssHeaderParser.kt

@@ -9,22 +9,36 @@ object AssHeaderParser {
 
   private const val ASS_EVENTS = "[Events]\n" +
     "Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text"
+  private val assEventsSuffix = ("\n" + ASS_EVENTS).toByteArray(Charsets.UTF_8)
 
   /**
-   * Fix some ass header with error end.
+   * Matroska CodecPrivate data may omit the events section and end with one or more NUL
+   * terminators. Remove every terminator before checking or appending the section so an
+   * embedded NUL cannot hide the event format from libass.
+   *
    * https://github.com/jellyfin/jellyfin-ffmpeg/issues/506
    */
-  private fun fixAssHeaderIfNeed(buffer: ByteArray): ByteArray = if (buffer[buffer.size - 1] != 0.toByte()) {
-    // validate ass header
-    buffer
-  } else {
-    // remote the last null character and append the events tag
-    (String(buffer, 0, buffer.size - 1) + "\n" + ASS_EVENTS).toByteArray()
+  private fun normalizeHeader(buffer: ByteArray): ByteArray {
+    var contentLength = buffer.size
+    while (contentLength > 0 && buffer[contentLength - 1] == 0.toByte()) {
+      contentLength--
+    }
+
+    val header = String(buffer, 0, contentLength, Charsets.UTF_8)
+    val hasEventsSection = header.lineSequence().any {
+      it.trim().equals("[Events]", ignoreCase = true)
+    }
+    if (hasEventsSection) {
+      return if (contentLength == buffer.size) buffer else buffer.copyOf(contentLength)
+    }
+
+    return buffer.copyOf(contentLength + assEventsSuffix.size).also {
+      assEventsSuffix.copyInto(it, destinationOffset = contentLength)
+    }
   }
 
   /**
-   * Parses the headers from the initialization data of the given [format]. The original
-   * headers are preserved (duplication checks are handled by libass).
+   * Parses and normalizes the ASS header from the initialization data of the given [format].
    */
-  fun parse(format: Format): ByteArray = fixAssHeaderIfNeed(format.initializationData[1])
+  fun parse(format: Format): ByteArray = normalizeHeader(format.initializationData[1])
 }

android/libass/src/test/java/com/edde746/plezy/libass/media/parser/AssHeaderParserTest.kt

@@ -0,0 +1,60 @@
+package com.edde746.plezy.libass.media.parser
+
+import androidx.annotation.OptIn
+import androidx.media3.common.Format
+import androidx.media3.common.util.UnstableApi
+import org.junit.Assert.assertArrayEquals
+import org.junit.Test
+
+@OptIn(UnstableApi::class)
+class AssHeaderParserTest {
+
+  @Test
+  fun doubleNulTerminatedHeaderAppendsEventsAfterContent() {
+    val input = SCRIPT_HEADER.toByteArray() + byteArrayOf(0, 0)
+
+    assertArrayEquals(expectedHeader(), parse(input))
+  }
+
+  @Test
+  fun unterminatedHeaderWithoutEventsIsRepaired() {
+    assertArrayEquals(expectedHeader(), parse(SCRIPT_HEADER.toByteArray()))
+  }
+
+  @Test
+  fun repairPreservesNonUtf8HeaderBytes() {
+    val content = SCRIPT_HEADER.toByteArray() + byteArrayOf(0xE9.toByte())
+    val input = content + byteArrayOf(0)
+    val expected = content + ("\n" + EVENTS_SECTION).toByteArray()
+
+    assertArrayEquals(expected, parse(input))
+  }
+
+  @Test
+  fun existingEventsSectionIsPreserved() {
+    val input = expectedHeader()
+
+    assertArrayEquals(input, parse(input))
+  }
+
+  @Test
+  fun trailingNulsAreRemovedWithoutDuplicatingExistingEvents() {
+    val input = expectedHeader() + byteArrayOf(0, 0)
+
+    assertArrayEquals(expectedHeader(), parse(input))
+  }
+
+  private fun parse(header: ByteArray): ByteArray = AssHeaderParser.parse(
+    Format.Builder()
+      .setInitializationData(listOf(byteArrayOf(), header))
+      .build()
+  )
+
+  private fun expectedHeader(): ByteArray = "$SCRIPT_HEADER\n$EVENTS_SECTION".toByteArray()
+
+  private companion object {
+    const val SCRIPT_HEADER = "[Script Info]\r\nScriptType: v4.00+\r\n"
+    const val EVENTS_SECTION = "[Events]\n" +
+      "Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text"
+  }
+}