fix: ResultSet#close() should not throw exceptions from session creat…

…ion (#487)

* fix: ResultSet#close() should not throw exceptions from session creation

If a client application requested a session from the session pool, and that request
required a new BatchCreateSessions request that would fail, then the error from the
BatchCreateSessions RPC would be propagated to both the ReadContext#executeQuery(..)
method as well as the ResultSet#close() method. The latter should not happen, as the
close method should silently ignore any errors from the session creation.

* docs: clear up comment

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

@@ -61,7 +61,15 @@ public Struct getCurrentRowAsStruct() {
 
   @Override
   public void close() {
-    delegate.get().close();
+    ResultSet rs;
+    try {
+      rs = delegate.get();
+    } catch (Exception e) {
+      // Ignore any exceptions when getting the underlying result set, as that means that there is
+      // nothing that needs to be closed.
+      return;
+    }
+    rs.close();
   }
 
   @Override

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

@@ -1255,7 +1255,10 @@ public void close() {
         leakedException = null;
         checkedOutSessions.remove(this);
       }
-      get().close();
+      PooledSession delegate = getOrNull();
+      if (delegate != null) {
+        delegate.close();
+      }
     }
 
     @Override
@@ -1264,7 +1267,19 @@ public ApiFuture<Empty> asyncClose() {
         leakedException = null;
         checkedOutSessions.remove(this);
       }
-      return get().asyncClose();
+      PooledSession delegate = getOrNull();
+      if (delegate != null) {
+        return delegate.asyncClose();
+      }
+      return ApiFutures.immediateFuture(Empty.getDefaultInstance());
+    }
+
+    private PooledSession getOrNull() {
+      try {
+        return get();
+      } catch (Throwable t) {
+        return null;
+      }
     }
 
     @Override

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

@@ -1599,4 +1599,31 @@ public Long run(TransactionContext transaction) throws Exception {
               }
             });
   }
+
+  @Test
+  public void testBatchCreateSessionsFailure_shouldNotPropagateToCloseMethod() {
+    try {
+      // Simulate session creation failures on the backend.
+      mockSpanner.setBatchCreateSessionsExecutionTime(
+          SimulatedExecutionTime.ofStickyException(Status.RESOURCE_EXHAUSTED.asRuntimeException()));
+      DatabaseClient client =
+          spannerWithEmptySessionPool.getDatabaseClient(
+              DatabaseId.of(TEST_PROJECT, TEST_INSTANCE, TEST_DATABASE));
+      // This will not cause any failure as getting a session from the pool is guaranteed to be
+      // non-blocking, and any exceptions will be delayed until actual query execution.
+      ResultSet rs = client.singleUse().executeQuery(SELECT1);
+      try {
+        while (rs.next()) {
+          fail("Missing expected exception");
+        }
+      } catch (SpannerException e) {
+        assertThat(e.getErrorCode()).isEqualTo(ErrorCode.RESOURCE_EXHAUSTED);
+      } finally {
+        // This should not cause any failures.
+        rs.close();
+      }
+    } finally {
+      mockSpanner.setBatchCreateSessionsExecutionTime(SimulatedExecutionTime.none());
+    }
+  }
 }