Fix ContentDataSource bytesRemaining calculation

The bytesRemaining didn't always take into account any skipped bytes, which
meant that reaching the end of the file was not correctly detected in read().

Issue: #3216

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=167016672

library/core/src/androidTest/java/com/google/android/exoplayer2/upstream/ContentDataSourceTest.java

@@ -23,9 +23,12 @@ import android.database.Cursor;
 import android.net.Uri;
 import android.support.annotation.NonNull;
 import android.test.InstrumentationTestCase;
+import android.test.MoreAsserts;
+import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.testutil.TestUtil;
 import java.io.FileNotFoundException;
 import java.io.IOException;
+import java.util.Arrays;
 
 /**
  * Unit tests for {@link ContentDataSource}.
@@ -35,6 +38,9 @@ public final class ContentDataSourceTest extends InstrumentationTestCase {
   private static final String AUTHORITY = "com.google.android.exoplayer2.core.test";
   private static final String DATA_PATH = "binary/1024_incrementing_bytes.mp3";
 
+  private static final int TEST_DATA_OFFSET = 1;
+  private static final int TEST_DATA_LENGTH = 1023;
+
   public void testReadValidUri() throws Exception {
     ContentDataSource dataSource = new ContentDataSource(getInstrumentation().getContext());
     Uri contentUri = new Uri.Builder()
@@ -64,6 +70,27 @@ public final class ContentDataSourceTest extends InstrumentationTestCase {
     }
   }
 
+  public void testReadFromOffsetToEndOfInput() throws Exception {
+    ContentDataSource dataSource = new ContentDataSource(getInstrumentation().getContext());
+    Uri contentUri = new Uri.Builder()
+        .scheme(ContentResolver.SCHEME_CONTENT)
+        .authority(AUTHORITY)
+        .path(DATA_PATH).build();
+    try {
+      DataSpec dataSpec = new DataSpec(contentUri, TEST_DATA_OFFSET, C.LENGTH_UNSET, null);
+      long length = dataSource.open(dataSpec);
+      assertEquals(TEST_DATA_LENGTH, length);
+      byte[] expectedData = Arrays.copyOfRange(
+          TestUtil.getByteArray(getInstrumentation(), DATA_PATH), TEST_DATA_OFFSET,
+          TEST_DATA_OFFSET + TEST_DATA_LENGTH);
+      byte[] readData = TestUtil.readToEnd(dataSource);
+      MoreAsserts.assertEquals(expectedData, readData);
+      assertEquals(C.RESULT_END_OF_INPUT, dataSource.read(new byte[1], 0, 1));
+    } finally {
+      dataSource.close();
+    }
+  }
+
   /**
    * A {@link ContentProvider} for the test.
    */

library/core/src/main/java/com/google/android/exoplayer2/upstream/ContentDataSource.java

@@ -76,8 +76,8 @@ public final class ContentDataSource implements DataSource {
         throw new FileNotFoundException("Could not open file descriptor for: " + uri);
       }
       inputStream = new FileInputStream(assetFileDescriptor.getFileDescriptor());
-      long assertStartOffset = assetFileDescriptor.getStartOffset();
-      long skipped = inputStream.skip(assertStartOffset + dataSpec.position) - assertStartOffset;
+      long assetStartOffset = assetFileDescriptor.getStartOffset();
+      long skipped = inputStream.skip(assetStartOffset + dataSpec.position) - assetStartOffset;
       if (skipped != dataSpec.position) {
         // We expect the skip to be satisfied in full. If it isn't then we're probably trying to
         // skip beyond the end of the data.
@@ -86,8 +86,8 @@ public final class ContentDataSource implements DataSource {
       if (dataSpec.length != C.LENGTH_UNSET) {
         bytesRemaining = dataSpec.length;
       } else {
-        bytesRemaining = assetFileDescriptor.getLength();
-        if (bytesRemaining == AssetFileDescriptor.UNKNOWN_LENGTH) {
+        long assetFileDescriptorLength = assetFileDescriptor.getLength();
+        if (assetFileDescriptorLength == AssetFileDescriptor.UNKNOWN_LENGTH) {
           // The asset must extend to the end of the file.
           bytesRemaining = inputStream.available();
           if (bytesRemaining == 0) {
@@ -96,6 +96,8 @@ public final class ContentDataSource implements DataSource {
             // case, so treat as unbounded.
             bytesRemaining = C.LENGTH_UNSET;
           }
+        } else {
+          bytesRemaining = assetFileDescriptorLength - skipped;
         }
       }
     } catch (IOException e) {

testutils/src/main/java/com/google/android/exoplayer2/testutil/TestUtil.java

@@ -181,7 +181,7 @@ public class TestUtil {
       byte[] expectedData) throws IOException {
     try {
       long length = dataSource.open(dataSpec);
-      Assert.assertEquals(length, expectedData.length);
+      Assert.assertEquals(expectedData.length, length);
       byte[] readData = TestUtil.readToEnd(dataSource);
       MoreAsserts.assertEquals(expectedData, readData);
     } finally {