Trim zero padding from EBML string values

Issue #4010

-------------
Created by MOE: https:/google/moe
MOE_MIGRATED_REVID=190442962

RELEASENOTES.md

@@ -34,6 +34,8 @@
  * Prevent multiple instances of SimpleCache in the same folder.
  Previous instance must be released.
 * DRM: Allow multiple listeners for `DefaultDrmSessionManager`.
+* Fix handling of zero padded strings when parsing Matroska streams
+ ([#4010](https:/google/ExoPlayer/issues/4010)).
 * Fix ANR issue on Redmi 4X and Redmi Note 4
  ([#4006](https:/google/ExoPlayer/issues/4006)).
 * Removed default renderer time offset of 60000000 from internal player. The

library/core/src/main/java/com/google/android/exoplayer2/extractor/mkv/DefaultEbmlReader.java

@@ -209,10 +209,11 @@ private double readFloat(ExtractorInput input, int byteLength)
  }
 
  /**
- * Reads and returns a string of length {@code byteLength} from the {@link ExtractorInput}.
+ * Reads a string of length {@code byteLength} from the {@link ExtractorInput}. Zero padding is
+ * removed, so the returned string may be shorter than {@code byteLength}.
  *
  * @param input The {@link ExtractorInput} from which to read.
- * @param byteLength The length of the float being read.
+ * @param byteLength The length of the string being read, including zero padding.
  * @return The read string value.
  * @throws IOException If an error occurs reading from the input.
  * @throws InterruptedException If the thread is interrupted.
@@ -224,7 +225,12 @@ private String readString(ExtractorInput input, int byteLength)
  }
  byte[] stringBytes = new byte[byteLength];
  input.readFully(stringBytes, 0, byteLength);
- return new String(stringBytes);
+ // Remove zero padding.
+ int trimmedLength = byteLength;
+ while (trimmedLength > 0 && stringBytes[trimmedLength - 1] == 0) {
+ trimmedLength--;
+ }
+ return new String(stringBytes, 0, trimmedLength);
  }
 
  /**

library/core/src/test/java/com/google/android/exoplayer2/extractor/mkv/DefaultEbmlReaderTest.java

@@ -89,6 +89,14 @@ public void testStringElement() throws IOException, InterruptedException {
  assertEvents(input, expected.events);
  }
 
+ @Test
+ public void testStringElementWithZeroPadding() throws IOException, InterruptedException {
+ ExtractorInput input = createTestInput(0x42, 0x82, 0x86, 0x41, 0x62, 0x63, 0x00, 0x00, 0x00);
+ TestOutput expected = new TestOutput();
+ expected.stringElement(TestOutput.ID_DOC_TYPE, "Abc");
+ assertEvents(input, expected.events);
+ }
+
  @Test
  public void testStringElementEmpty() throws IOException, InterruptedException {
  ExtractorInput input = createTestInput(0x42, 0x82, 0x80);