Some boilerplate for DASH EMSG support

- Parse EMSG leaf atoms during FMP4 extraction (although
  currently they're just discarded).
- Add readNullTerminatedString to ParsableByteArray.

Issue: #2176

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=143555965

library/src/androidTest/java/com/google/android/exoplayer2/util/ParsableByteArrayTest.java

@@ -371,6 +371,73 @@ public void testReadEmptyString() {
     assertNull(parser.readLine());
   }
 
+  public void testReadNullTerminatedStringWithLengths() {
+    byte[] bytes = new byte[] {
+        'f', 'o', 'o', 0, 'b', 'a', 'r', 0
+    };
+    // Test with lengths that match NUL byte positions.
+    ParsableByteArray parser = new ParsableByteArray(bytes);
+    assertEquals("foo", parser.readNullTerminatedString(4));
+    assertEquals(4, parser.getPosition());
+    assertEquals("bar", parser.readNullTerminatedString(4));
+    assertEquals(8, parser.getPosition());
+    assertNull(parser.readNullTerminatedString());
+    // Test with lengths that do not match NUL byte positions.
+    parser = new ParsableByteArray(bytes);
+    assertEquals("fo", parser.readNullTerminatedString(2));
+    assertEquals(2, parser.getPosition());
+    assertEquals("o", parser.readNullTerminatedString(2));
+    assertEquals(4, parser.getPosition());
+    assertEquals("bar", parser.readNullTerminatedString(3));
+    assertEquals(7, parser.getPosition());
+    assertEquals("", parser.readNullTerminatedString(1));
+    assertEquals(8, parser.getPosition());
+    assertNull(parser.readNullTerminatedString());
+    // Test with limit at NUL
+    parser = new ParsableByteArray(bytes, 4);
+    assertEquals("foo", parser.readNullTerminatedString(4));
+    assertEquals(4, parser.getPosition());
+    assertNull(parser.readNullTerminatedString());
+    // Test with limit before NUL
+    parser = new ParsableByteArray(bytes, 3);
+    assertEquals("foo", parser.readNullTerminatedString(3));
+    assertEquals(3, parser.getPosition());
+    assertNull(parser.readNullTerminatedString());
+  }
+
+  public void testReadNullTerminatedString() {
+    byte[] bytes = new byte[] {
+        'f', 'o', 'o', 0, 'b', 'a', 'r', 0
+    };
+    // Test normal case.
+    ParsableByteArray parser = new ParsableByteArray(bytes);
+    assertEquals("foo", parser.readNullTerminatedString());
+    assertEquals(4, parser.getPosition());
+    assertEquals("bar", parser.readNullTerminatedString());
+    assertEquals(8, parser.getPosition());
+    assertNull(parser.readNullTerminatedString());
+    // Test with limit at NUL.
+    parser = new ParsableByteArray(bytes, 4);
+    assertEquals("foo", parser.readNullTerminatedString());
+    assertEquals(4, parser.getPosition());
+    assertNull(parser.readNullTerminatedString());
+    // Test with limit before NUL.
+    parser = new ParsableByteArray(bytes, 3);
+    assertEquals("foo", parser.readNullTerminatedString());
+    assertEquals(3, parser.getPosition());
+    assertNull(parser.readNullTerminatedString());
+  }
+
+  public void testReadNullTerminatedStringWithoutEndingNull() {
+    byte[] bytes = new byte[] {
+        'f', 'o', 'o', 0, 'b', 'a', 'r'
+    };
+    ParsableByteArray parser = new ParsableByteArray(bytes);
+    assertEquals("foo", parser.readNullTerminatedString());
+    assertEquals("bar", parser.readNullTerminatedString());
+    assertNull(parser.readNullTerminatedString());
+  }
+
   public void testReadSingleLineWithoutEndingTrail() {
     byte[] bytes = new byte[] {
       'f', 'o', 'o'

library/src/main/java/com/google/android/exoplayer2/extractor/mp4/Atom.java

@@ -127,6 +127,7 @@
   public static final int TYPE_mean = Util.getIntegerCodeForString("mean");
   public static final int TYPE_name = Util.getIntegerCodeForString("name");
   public static final int TYPE_data = Util.getIntegerCodeForString("data");
+  public static final int TYPE_emsg = Util.getIntegerCodeForString("emsg");
   public static final int TYPE_st3d = Util.getIntegerCodeForString("st3d");
   public static final int TYPE_sv3d = Util.getIntegerCodeForString("sv3d");
   public static final int TYPE_proj = Util.getIntegerCodeForString("proj");

library/src/main/java/com/google/android/exoplayer2/extractor/mp4/FragmentedMp4Extractor.java

@@ -339,6 +339,8 @@ private void onLeafAtomRead(LeafAtom leaf, long inputPosition) throws ParserExce
       ChunkIndex segmentIndex = parseSidx(leaf.data, inputPosition);
       extractorOutput.seekMap(segmentIndex);
       haveOutputSeekMap = true;
+    } else if (leaf.type == Atom.TYPE_emsg) {
+      onEmsgLeafAtomRead(leaf);
     }
   }
 
@@ -417,6 +419,10 @@ private void onMoofContainerAtomRead(ContainerAtom moof) throws ParserException
     }
   }
 
+  private void onEmsgLeafAtomRead(LeafAtom emsg) {
+    // Do nothing.
+  }
+
   /**
    * Parses a trex atom (defined in 14496-12).
    */
@@ -1134,7 +1140,7 @@ private static boolean shouldParseLeafAtom(int atom) {
         || atom == Atom.TYPE_trun || atom == Atom.TYPE_pssh || atom == Atom.TYPE_saiz
         || atom == Atom.TYPE_saio || atom == Atom.TYPE_senc || atom == Atom.TYPE_uuid
         || atom == Atom.TYPE_sbgp || atom == Atom.TYPE_sgpd || atom == Atom.TYPE_elst
-        || atom == Atom.TYPE_mehd;
+        || atom == Atom.TYPE_mehd || atom == Atom.TYPE_emsg;
   }
 
   /** Returns whether the extractor should decode a container atom with type {@code atom}. */

library/src/main/java/com/google/android/exoplayer2/util/ParsableByteArray.java

@@ -424,11 +424,24 @@ public String readString(int length) {
   }
 
   /**
-   * Reads the next {@code length} bytes as UTF-8 characters. A terminating NUL byte is ignored,
+   * Reads the next {@code length} bytes as characters in the specified {@link Charset}.
+   *
+   * @param length The number of bytes to read.
+   * @param charset The character set of the encoded characters.
+   * @return The string encoded by the bytes in the specified character set.
+   */
+  public String readString(int length, Charset charset) {
+    String result = new String(data, position, length, charset);
+    position += length;
+    return result;
+  }
+
+  /**
+   * Reads the next {@code length} bytes as UTF-8 characters. A terminating NUL byte is discarded,
    * if present.
    *
    * @param length The number of bytes to read.
-   * @return The string encoded by the bytes.
+   * @return The string, not including any terminating NUL byte.
    */
   public String readNullTerminatedString(int length) {
     if (length == 0) {
@@ -439,22 +452,31 @@ public String readNullTerminatedString(int length) {
     if (lastIndex < limit && data[lastIndex] == 0) {
       stringLength--;
     }
-    String result = new String(data, position, stringLength, Charset.defaultCharset());
+    String result = new String(data, position, stringLength);
     position += length;
     return result;
   }
 
   /**
-   * Reads the next {@code length} bytes as characters in the specified {@link Charset}.
+   * Reads up to the next NUL byte (or the limit) as UTF-8 characters.
    *
-   * @param length The number of bytes to read.
-   * @param charset The character set of the encoded characters.
-   * @return The string encoded by the bytes in the specified character set.
+   * @return The string not including any terminating NUL byte, or null if the end of the data has
+   *     already been reached.
    */
-  public String readString(int length, Charset charset) {
-    String result = new String(data, position, length, charset);
-    position += length;
-    return result;
+  public String readNullTerminatedString() {
+    if (bytesLeft() == 0) {
+      return null;
+    }
+    int stringLimit = position;
+    while (stringLimit < limit && data[stringLimit] != 0) {
+      stringLimit++;
+    }
+    String string = new String(data, position, stringLimit - position);
+    position = stringLimit;
+    if (position < limit) {
+      position++;
+    }
+    return string;
   }
 
   /**
@@ -464,8 +486,8 @@ public String readString(int length, Charset charset) {
    * ('\n'), or a carriage return followed immediately by a line feed ('\r\n'). The system's default
    * charset (UTF-8) is used.
    *
-   * @return A String containing the contents of the line, not including any line-termination
-   *     characters, or null if the end of the stream has been reached.
+   * @return The line not including any line-termination characters, or null if the end of the data
+   *     has already been reached.
    */
   public String readLine() {
     if (bytesLeft() == 0) {