fix: Fix race condition in MockSharedArbitrationTest.localArbitrationRunInParallelWithGlobalArbitration (#16731)

Summary:
Pull Request resolved: #16731

The test spawned `globalArbitrationTriggerThread` before installing the
`SCOPED_TESTVALUE_SET` callback for `runGlobalArbitration`. If the thread's
allocations triggered global arbitration before the callback was registered,
`globalArbitrationStarted` would never be set to true, causing the main thread
to deadlock on `globalArbitrationStartWait.await()`.

Fix by moving the `SCOPED_TESTVALUE_SET` and its synchronization variables
before the thread spawn, ensuring the callback is always registered before any
allocation can invoke `runGlobalArbitration`.

Fixes #15336

Reviewed By: xiaoxmeng

Differential Revision: D96181396

fbshipit-source-id: d4674a551411ddcc1eaf4b82ca5a060d7aeb11be

Author: tanjialiang

velox/common/memory/tests/MockSharedArbitratorTest.cpp

@@ -1900,6 +1900,26 @@ DEBUG_ONLY_TEST_F(
   auto* localArbitrationOp = localArbitrationTask->addMemoryOp(true);
   localArbitrationOp->allocate(memoryPoolCapacity);
 
+  // Install the test value callback BEFORE spawning the thread to avoid a race
+  // condition where the thread triggers global arbitration before the callback
+  // is registered, causing the main thread to deadlock. (GitHub issue #15336)
+  std::atomic_bool globalArbitrationStarted{false};
+  folly::EventCount globalArbitrationStartWait;
+  std::atomic_bool globalArbitrationWaitFlag{true};
+  folly::EventCount globalArbitrationWait;
+  SCOPED_TESTVALUE_SET(
+      "facebook::velox::memory::SharedArbitrator::runGlobalArbitration",
+      std::function<void(const SharedArbitrator*)>(
+          ([&](const SharedArbitrator* /*unused*/) {
+            if (globalArbitrationStarted.exchange(true)) {
+              return;
+            }
+            globalArbitrationStartWait.notifyAll();
+
+            globalArbitrationWait.await(
+                [&]() { return !globalArbitrationWaitFlag.load(); });
+          })));
+
   auto globalArbitrationTriggerThread = std::thread([&]() {
     std::unordered_map<std::string, RuntimeMetric> runtimeStats;
     auto statsWriter = std::make_unique<TestRuntimeStatWriter>(runtimeStats);
@@ -1952,23 +1972,6 @@ DEBUG_ONLY_TEST_F(
         1);
   });
 
-  std::atomic_bool globalArbitrationStarted{false};
-  folly::EventCount globalArbitrationStartWait;
-  std::atomic_bool globalArbitrationWaitFlag{true};
-  folly::EventCount globalArbitrationWait;
-  SCOPED_TESTVALUE_SET(
-      "facebook::velox::memory::SharedArbitrator::runGlobalArbitration",
-      std::function<void(const SharedArbitrator*)>(
-          ([&](const SharedArbitrator* /*unused*/) {
-            if (globalArbitrationStarted.exchange(true)) {
-              return;
-            }
-            globalArbitrationStartWait.notifyAll();
-
-            globalArbitrationWait.await(
-                [&]() { return !globalArbitrationWaitFlag.load(); });
-          })));
-
   globalArbitrationStartWait.await(
       [&]() { return globalArbitrationStarted.load(); });