Fix incorrect assertion in CacheDataSource

#minor-release
#exofixit

PiperOrigin-RevId: 395233639

RELEASENOTES.md

@@ -14,6 +14,9 @@
     *   Remove `ExoPlayerLibraryInfo.GL_ASSERTIONS_ENABLED`. Use
         `GlUtil.glAssertionsEnabled` instead.
     *   Fix `FlagSet#equals` on API levels below 24.
+    *   Fix `NullPointerException` being thrown from `CacheDataSource` when
+        reading a fully cached resource with `DataSpec.position` equal to the
+        resource length.
 *   Extractors:
     *   Support TS packets without PTS flag
         ([#9294](https://github.com/google/ExoPlayer/issues/9294)).

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

@@ -597,14 +597,14 @@ public final class CacheDataSource implements DataSource {
 
   @Override
   public int read(byte[] buffer, int offset, int length) throws IOException {
-    DataSpec requestDataSpec = checkNotNull(this.requestDataSpec);
-    DataSpec currentDataSpec = checkNotNull(this.currentDataSpec);
     if (length == 0) {
       return 0;
     }
     if (bytesRemaining == 0) {
       return C.RESULT_END_OF_INPUT;
     }
+    DataSpec requestDataSpec = checkNotNull(this.requestDataSpec);
+    DataSpec currentDataSpec = checkNotNull(this.currentDataSpec);
     try {
       if (readPosition >= checkCachePosition) {
         openNextSource(requestDataSpec, true);

library/core/src/test/java/com/google/android/exoplayer2/upstream/cache/CacheDataSourceTest.java

@@ -124,6 +124,28 @@ public final class CacheDataSourceTest {
     assertCacheAndRead(boundedDataSpec, /* unknownLength= */ false);
   }
 
+  @Test
+  public void cacheAndReadFromLength_readsZeroBytes() throws Exception {
+    // Read and cache all data from upstream.
+    CacheDataSource cacheDataSource =
+        createCacheDataSource(
+            /* setReadException= */ false, /* unknownLength= */ false, /* cacheKeyFactory= */ null);
+    assertReadDataContentLength(
+        cacheDataSource,
+        unboundedDataSpec,
+        /* unknownLength= */ false,
+        /* customCacheKey= */ false);
+
+    // Read from position == length.
+    cacheDataSource =
+        createCacheDataSource(
+            /* setReadException= */ true, /* unknownLength= */ false, /* cacheKeyFactory= */ null);
+    assertReadData(
+        cacheDataSource,
+        unboundedDataSpec.buildUpon().setPosition(TEST_DATA.length).build(),
+        /* unknownLength= */ false);
+  }
+
   @Test
   public void propagatesHttpHeadersUpstream() throws Exception {
     CacheDataSource cacheDataSource =