fix: getCommitResponse() should return error if tx has not committed (googleapis#4021)

getCommitResponse() returned an error if it was called before the transaction
had been committed when using regular sessions. This did not happen when using
multiplexed sessions. This change makes sure that the behavior is the same for
both regular and multiplexed sessions, by adding a check to the
getCommitResponse() method directly.

Author: olavloite

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

@@ -203,7 +203,10 @@ interface AsyncTransactionFunction<I, O> {
   /** Returns the state of the transaction. */
   TransactionState getState();
 
-  /** Returns the {@link CommitResponse} of this transaction. */
+  /**
+   * Returns the {@link CommitResponse} of this transaction. This method may only be called after
+   * committing the transaction.
+   */
   ApiFuture<CommitResponse> getCommitResponse();
 
   /**

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

@@ -221,6 +221,9 @@ public TransactionState getState() {
 
   @Override
   public ApiFuture<CommitResponse> getCommitResponse() {
+    Preconditions.checkState(
+        txnState == TransactionState.COMMITTED,
+        "getCommitResponse can only be invoked if the transaction was successfully committed");
     return commitResponse;
   }
 

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

@@ -200,6 +200,14 @@ public void asyncTransactionManager_shouldRollbackOnCloseAsync() throws Exceptio
         0L);
   }
 
+  @Test
+  public void testAsyncTransactionManager_getCommitResponseReturnsErrorBeforeCommit() {
+    try (AsyncTransactionManager manager = client().transactionManagerAsync()) {
+      TransactionContextFuture transactionContextFuture = manager.beginAsync();
+      assertThrows(IllegalStateException.class, manager::getCommitResponse);
+    }
+  }
+
   @Test
   public void testAsyncTransactionManager_returnsCommitStats() throws Exception {
     try (AsyncTransactionManager manager =