Trim zero padding from EBML string values

Issue #4010

-------------
Created by MOE: https://github.com/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://github.com/google/ExoPlayer/issues/4010)).
 * Fix ANR issue on Redmi 4X and Redmi Note 4
   ([#4006](https://github.com/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 @@ import java.util.Stack;
   }
 
   /**
-   * 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 @@ import java.util.Stack;
     }
     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 class DefaultEbmlReaderTest {
     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);