weiqingy commented on code in PR #28247:
URL: https://github.com/apache/flink/pull/28247#discussion_r3449776048


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flink-table/flink-table-planner/src/test/java/org/apache/flink/table/planner/calcite/FlinkRelOptClusterFactoryTest.java:
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@@ -0,0 +1,68 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.table.planner.calcite;
+
+import org.apache.flink.table.planner.delegation.PlannerContext;
+import org.apache.flink.table.planner.utils.PlannerMocks;
+
+import org.apache.calcite.plan.RelOptCluster;
+import org.apache.calcite.rel.metadata.RelMetadataQuery;
+import org.apache.calcite.rel.metadata.RelMetadataQueryBase;
+import org.junit.jupiter.api.Test;
+
+import java.lang.reflect.Field;
+import java.util.concurrent.atomic.AtomicReference;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/** Test for {@link FlinkRelOptClusterFactory}. */
+class FlinkRelOptClusterFactoryTest {
+
+    @Test
+    void testMetadataQuerySupplierUsableOnFreshThread() throws Throwable {
+        PlannerContext plannerContext = 
PlannerMocks.create().getPlannerContext();
+        RelOptCluster cluster = plannerContext.createRelBuilder().getCluster();
+
+        AtomicReference<Throwable> failure = new AtomicReference<>();
+        Thread freshThread =
+                new Thread(
+                        () -> {
+                            try {
+                                // Simulate a worker thread that has never had 
THREAD_PROVIDERS set.
+                                RelMetadataQueryBase.THREAD_PROVIDERS.remove();
+
+                                RelMetadataQuery mq = 
cluster.getMetadataQuerySupplier().get();
+
+                                Field providerField =
+                                        
RelMetadataQueryBase.class.getDeclaredField(
+                                                "metadataHandlerProvider");
+                                providerField.setAccessible(true);
+                                assertThat(providerField.get(mq)).isNotNull();

Review Comment:
   The fix is verified by reflecting into 
`RelMetadataQueryBase.metadataHandlerProvider` — a `private final` field — 
through `getDeclaredField` + `setAccessible`. That ties the test to a 
Calcite-internal field name, so a Calcite upgrade that renames it would break 
this test rather than flag a real regression (a few Calcite-bump PRs are in 
flight right now). `RelMetadataQueryBase` also exposes a `public final 
JaninoRelMetadataProvider metadataProvider`, populated from that same 
`THREAD_PROVIDERS` value — so once the fix sets the thread-local it's non-null, 
and null on a fresh thread without it, the same condition the private field 
tracks. Would asserting on that let the reflection and `setAccessible` go away?
   
   ```java
   RelMetadataQuery mq = cluster.getMetadataQuerySupplier().get();
   assertThat(mq.metadataProvider).isNotNull();
   ```



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