try to fix a race in TTL thread shutdown (#9096)

CHANGELOG

@@ -1,7 +1,9 @@
 devel
 -----
 
-* updated ArangoDB Starter 0.14.4
+* fix a race in TTL thread deactivation/shutdown
+
+* updated ArangoDB starter to 0.14.4
 
 * fixed internal issue #3919: The web UI is now using precompiled ejs templates.
 

arangod/RestServer/TtlFeature.cpp

@@ -514,14 +514,31 @@ void TtlFeature::allowRunning(bool value) {
       _allowRunning = false;
     }
   }
-  
-  if (!value && _thread != nullptr) {
-    // wait until TTL operations have finished
-    _thread->wakeup();
 
-    while (_thread->isCurrentlyWorking()) {
-      std::this_thread::sleep_for(std::chrono::milliseconds(10));
+  if (value) {
+    return;
+  }
+     
+  waitForThreadWork();
+}
+
+void TtlFeature::waitForThreadWork() {
+  while (true) {
+    {
+      MUTEX_LOCKER(locker, _threadMutex);
+
+      if (_thread == nullptr) {
+        break;
+      }
+
+      _thread->wakeup();
+
+      if (!_thread->isCurrentlyWorking()) {
+        break;
+      }
     }
+
+    std::this_thread::sleep_for(std::chrono::milliseconds(5));
   }
 }
   
@@ -548,13 +565,7 @@ void TtlFeature::deactivate() {
     _active = false; 
   }
 
-  if (_thread != nullptr) {
-    _thread->wakeup();
-
-    while (_thread->isCurrentlyWorking()) {
-      std::this_thread::sleep_for(std::chrono::milliseconds(10));
-    }
-  }
+  waitForThreadWork();
   
   LOG_TOPIC("898a7", DEBUG, Logger::TTL) << "deactivated TTL background thread";
 }

arangod/RestServer/TtlFeature.h

@@ -83,6 +83,9 @@ class TtlFeature final : public application_features::ApplicationFeature {
   /// but the user has set _active to false.
   void allowRunning(bool value);
 
+  /// @brief wait until the TTL thread has successfully stopped working
+  void waitForThreadWork();
+
   /// @brief turn expiring/removing outdated documents on
   void activate();
   /// @brief turn expiring/removing outdated documents off, blocks until