core: add a FlagResetRule test fixture

Author: jdcormie

core/src/testFixtures/java/io/grpc/internal/FlagResetRule.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2026 The gRPC Authors
+ *
+ * Licensed 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 io.grpc.internal;
+
+import java.util.ArrayDeque;
+import java.util.Deque;
+import org.junit.rules.ExternalResource;
+
+/**
+ * A {@link org.junit.rules.TestRule} that lets you set one or more feature flags just for
+ * the duration of the current test case.
+ *
+ * <p>Flags and other global variables must be reset to ensure no state leaks across tests.
+ */
+public final class FlagResetRule extends ExternalResource {
+
+  /** A functional interface representing a standard gRPC feature flag setter. */
+  public interface SetterMethod<T> {
+    /** Sets a flag for testing and returns its previous value. */
+    T set(T val);
+  }
+
+  private final Deque<Runnable> toRunAfter = new ArrayDeque<>();
+
+  /**
+   * Sets a global feature flag to 'value' using 'setter' and arranges for its previous value to be
+   * unconditionally restored when the test completes.
+   */
+  public <T> void setFlagForTest(SetterMethod<T> setter, T value) {
+    final T oldValue = setter.set(value);
+    toRunAfter.push(() -> setter.set(oldValue));
+  }
+
+  @Override
+  protected void after() {
+    RuntimeException toThrow = null;
+    while (!toRunAfter.isEmpty()) {
+      try {
+        toRunAfter.pop().run();
+      } catch (RuntimeException e) {
+        if (toThrow == null) {
+          toThrow = e;
+        } else {
+          toThrow.addSuppressed(e);
+        }
+      }
+    }
+    if (toThrow != null) {
+      throw toThrow;
+    }
+  }
+}