test: added getSnapshotTimestamp() method (googleapis#4000)

The snapshot timestamp is the timestamp at which all the reads in a
transaction ran when using repeatable read isolation. The method is
only intended to be used for internal testing purposes, so the snapshot
timestamp will be `null` if attempted to be used externally.

Author: skuruppu

google-cloud-spanner/src/main/java/com/google/cloud/spanner/CommitResponse.java

@@ -19,6 +19,7 @@
 import com.google.cloud.Timestamp;
 import com.google.common.base.Preconditions;
 import java.util.Objects;
+import javax.annotation.Nullable;
 
 /** Represents a response from a commit operation. */
 public class CommitResponse {
@@ -41,6 +42,18 @@ public Timestamp getCommitTimestamp() {
     return Timestamp.fromProto(proto.getCommitTimestamp());
   }
 
+  /**
+   * Returns a {@link Timestamp} representing the timestamp at which all reads in the transaction
+   * ran at, if the transaction ran at repeatable read isolation in internal test environments, and
+   * otherwise returns null.
+   */
+  public @Nullable Timestamp getSnapshotTimestamp() {
+    if (proto.getSnapshotTimestamp() == com.google.protobuf.Timestamp.getDefaultInstance()) {
+      return null;
+    }
+    return Timestamp.fromProto(proto.getSnapshotTimestamp());
+  }
+
   /**
    * @return true if the {@link CommitResponse} includes {@link CommitStats}
    */

google-cloud-spanner/src/test/java/com/google/cloud/spanner/CommitResponseTest.java

@@ -101,4 +101,22 @@ public void testHasCommitStats() {
     CommitResponse responseWithCommitStats = new CommitResponse(protoWithCommitStats);
     assertTrue(responseWithCommitStats.hasCommitStats());
   }
+
+  @Test
+  public void testGetSnapshotTimestamp() {
+    com.google.spanner.v1.CommitResponse protoWithoutSnapshotTimestamp =
+        com.google.spanner.v1.CommitResponse.getDefaultInstance();
+    CommitResponse responseWithoutSnapshotTimestamp =
+        new CommitResponse(protoWithoutSnapshotTimestamp);
+    assertEquals(null, responseWithoutSnapshotTimestamp.getSnapshotTimestamp());
+
+    com.google.protobuf.Timestamp timestamp =
+        com.google.protobuf.Timestamp.newBuilder().setSeconds(123L).setNanos(456).build();
+    com.google.spanner.v1.CommitResponse protoWithSnapshotTimestamp =
+        com.google.spanner.v1.CommitResponse.newBuilder().setSnapshotTimestamp(timestamp).build();
+    CommitResponse responseWithSnapshotTimestamp = new CommitResponse(protoWithSnapshotTimestamp);
+    assertEquals(
+        Timestamp.ofTimeSecondsAndNanos(123L, 456),
+        responseWithSnapshotTimestamp.getSnapshotTimestamp());
+  }
 }