Add DataSource contract tests to verify offset and position

These tests addresses two identified gaps in the contract:
 - Ensures that the output buffer offset passed to the `DataSource.read` method is correctly applied.
 - Verifies that the position within the input stream is properly incremented when reading in two parts.

PiperOrigin-RevId: 656358935

RELEASENOTES.md

@@ -95,6 +95,9 @@
 *   Leanback extension:
 *   Cast Extension:
 *   Test Utilities:
+    *   `DataSourceContractTest` now includes tests to verify:
+        *   Input stream `read position` is updated.
+        *   Output buffer `offset` is applied correctly.
 *   Demo app:
 *   Remove deprecated symbols:
     *   Remove deprecated `Player.hasPrevious`, `Player.hasPreviousWindow()`.

libraries/datasource/src/test/java/androidx/media3/datasource/UdpDataSourceContractTest.java

@@ -83,6 +83,11 @@ public class UdpDataSourceContractTest extends DataSourceContractTest {
   @Override
   public void dataSpecWithLength_readExpectedRange() {}
 
+  @Test
+  @Ignore("UdpDataSource doesn't support DataSpec's position or length [internal: b/175856954]")
+  @Override
+  public void dataSpecWithLength_readUntilEndInTwoParts() {}
+
   @Test
   @Ignore("UdpDataSource doesn't support DataSpec's position or length [internal: b/175856954]")
   @Override

libraries/test_utils/src/main/java/androidx/media3/test/utils/DataSourceContractTest.java

@@ -151,6 +151,41 @@ public abstract class DataSourceContractTest {
         });
   }
 
+  @Test
+  public void unboundedDataSpec_readExpectedBytesWithOffset() throws Exception {
+    forAllTestResourcesAndDataSources(
+        (resource, dataSource) -> {
+          try {
+            dataSource.open(new DataSpec.Builder().setUri(resource.getUri()).build());
+            int offset = 2;
+            byte[] data = new byte[resource.getExpectedBytes().length + offset];
+            // Initialize the first two bytes with non-zero values.
+            data[0] = (byte) 0xA5;
+            data[1] = (byte) 0x5A;
+
+            while (offset != data.length) {
+              int bytesRead = dataSource.read(data, offset, resource.getExpectedBytes().length);
+
+              if (bytesRead == C.RESULT_END_OF_INPUT) {
+                break;
+              }
+
+              offset += bytesRead;
+            }
+
+            // Assert that the first two bytes have not been modified.
+            assertThat(data[0]).isEqualTo((byte) 0xA5);
+            assertThat(data[1]).isEqualTo((byte) 0x5A);
+
+            assertThat(offset).isEqualTo(data.length);
+            byte[] actualData = Arrays.copyOfRange(data, 2, data.length);
+            assertThat(actualData).isEqualTo(resource.getExpectedBytes());
+          } finally {
+            dataSource.close();
+          }
+        });
+  }
+
   @Test
   public void dataSpecWithPosition_readUntilEnd() throws Exception {
     forAllTestResourcesAndDataSources(
@@ -196,6 +231,29 @@ public abstract class DataSourceContractTest {
         });
   }
 
+  @Test
+  public void dataSpecWithLength_readUntilEndInTwoParts() throws Exception {
+    forAllTestResourcesAndDataSources(
+        (resource, dataSource) -> {
+          try {
+            int length = resource.getExpectedBytes().length;
+            dataSource.open(
+                new DataSpec.Builder().setUri(resource.getUri()).setLength(length).build());
+
+            byte[] firstPartData = DataSourceUtil.readExactly(dataSource, length / 2);
+            byte[] secondPartData = DataSourceUtil.readToEnd(dataSource);
+
+            byte[] expectedFirstPartData = Arrays.copyOf(resource.getExpectedBytes(), length / 2);
+            byte[] expectedSecondPartData =
+                Arrays.copyOfRange(resource.getExpectedBytes(), length / 2, length);
+            assertThat(firstPartData).isEqualTo(expectedFirstPartData);
+            assertThat(secondPartData).isEqualTo(expectedSecondPartData);
+          } finally {
+            dataSource.close();
+          }
+        });
+  }
+
   @Test
   public void dataSpecWithPositionAndLength_readExpectedRange() throws Exception {
     forAllTestResourcesAndDataSources(