[Index] Use a single global serial queue for registration of units

lib/Index/IndexDatastore.cpp

@@ -61,6 +61,18 @@ static sys::TimePoint<> toTimePoint(timespec ts) {
 
 const static dispatch_qos_class_t unitChangesQOS = QOS_CLASS_UTILITY;
 
+/// Returns a global serial queue for unit processing.
+/// This is useful to avoid doing a lot of parallel CPU and I/O work when opening multiple workspaces.
+static dispatch_queue_t getGlobalQueueForUnitChanges() {
+  static dispatch_queue_t queueForUnitChanges;
+  static dispatch_once_t onceToken = 0;
+  dispatch_once(&onceToken, ^{
+    dispatch_queue_attr_t qosAttribute = dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL, unitChangesQOS, 0);
+    queueForUnitChanges = dispatch_queue_create("IndexStoreDB.store.unit.processing", qosAttribute);
+  });
+  return queueForUnitChanges;
+}
+
 namespace {
 
   class UnitMonitor;
@@ -86,8 +98,6 @@ class StoreUnitRepo : public std::enable_shared_from_this<StoreUnitRepo> {
   std::shared_ptr<DoneInitState> DoneInitializingPtr;
   dispatch_semaphore_t InitSemaphore;
 
-  dispatch_queue_t queueForUnitChanges;
-
   mutable llvm::sys::Mutex StateMtx;
   std::unordered_map<IDCode, std::shared_ptr<UnitMonitor>> UnitMonitorsByCode;
 
@@ -102,16 +112,11 @@ class StoreUnitRepo : public std::enable_shared_from_this<StoreUnitRepo> {
 
     DoneInitializingPtr = std::make_shared<DoneInitState>();
     InitSemaphore = dispatch_semaphore_create(0);
-    dispatch_queue_attr_t qosAttribute = dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL, unitChangesQOS, 0);
-    queueForUnitChanges = dispatch_queue_create("IndexStoreDB.store.unit.processing", qosAttribute);
   }
   ~StoreUnitRepo() {
     dispatch_release(InitSemaphore);
-    dispatch_release(queueForUnitChanges);
   }
 
-  dispatch_queue_t getQueueForUnitChanges() const { return queueForUnitChanges; }
-
   void onFilesChange(std::vector<UnitEventInfo> evts,
                      function_ref<void(unsigned)> ReportCompleted,
                      function_ref<void()> DirectoryDeleted);
@@ -691,6 +696,56 @@ sys::TimePoint<> UnitMonitor::getModTimeForOutOfDateCheck(StringRef filePath) {
 // IndexDatastoreImpl
 //===----------------------------------------------------------------------===//
 
+namespace {
+/// A thread-safe deque object for UnitEventInfo objects.
+class UnitEventInfoDeque {
+  std::deque<UnitEventInfo> EventsDequeue;
+  mutable llvm::sys::Mutex StateMtx;
+
+public:
+  void addEvents(ArrayRef<UnitEventInfo> evts) {
+    sys::ScopedLock L(StateMtx);
+    EventsDequeue.insert(EventsDequeue.end(), evts.begin(), evts.end());
+  }
+
+  Optional<UnitEventInfo> popFront() {
+    sys::ScopedLock L(StateMtx);
+    if (EventsDequeue.empty())
+      return None;
+    UnitEventInfo evt = EventsDequeue.front();
+    EventsDequeue.pop_front();
+    return evt;
+  }
+};
+}
+
+/// Enqueues asynchronous processing of the unit events in an incremental fashion.
+/// Events are queued-up individually and the next event is enqueued only after
+/// the current one has been processed.
+static void processUnitEventsIncrementally(std::shared_ptr<UnitEventInfoDeque> evts,
+                                           std::weak_ptr<StoreUnitRepo> weakUnitRepo,
+                                           std::shared_ptr<IndexSystemDelegate> delegate,
+                                           dispatch_queue_t queue) {
+  Optional<UnitEventInfo> evtOpt = evts->popFront();
+  if (!evtOpt.hasValue())
+    return;
+  auto UnitRepo = weakUnitRepo.lock();
+  if (!UnitRepo)
+    return;
+
+  UnitEventInfo evt = evtOpt.getValue();
+  UnitRepo->onFilesChange({evt}, [&](unsigned NumCompleted){
+    delegate->processingCompleted(NumCompleted);
+  }, [&](){
+    // FIXME: the database should recover.
+  });
+
+  // Enqueue processing the rest of the events.
+  dispatch_async(queue, ^{
+    processUnitEventsIncrementally(evts, weakUnitRepo, delegate, queue);
+  });
+}
+
 bool IndexDatastoreImpl::init(IndexStoreRef idxStore,
                               SymbolIndexRef SymIndex,
                               std::shared_ptr<IndexSystemDelegate> Delegate,
@@ -706,28 +761,24 @@ bool IndexDatastoreImpl::init(IndexStoreRef idxStore,
 
   auto UnitRepo = std::make_shared<StoreUnitRepo>(this->IdxStore, SymIndex, Delegate, CanonPathCache);
   std::weak_ptr<StoreUnitRepo> WeakUnitRepo = UnitRepo;
-  auto OnUnitsChange = [WeakUnitRepo, Delegate](IndexStore::UnitEventNotification EventNote) {
-    auto UnitRepo = WeakUnitRepo.lock();
-    if (!UnitRepo)
-      return;
-
+  auto eventsDeque = std::make_shared<UnitEventInfoDeque>();
+  auto OnUnitsChange = [WeakUnitRepo, Delegate, eventsDeque](IndexStore::UnitEventNotification EventNote) {
     std::vector<UnitEventInfo> evts;
     for (size_t i = 0, e = EventNote.getEventsCount(); i != e; ++i) {
       auto evt = EventNote.getEvent(i);
       evts.push_back(UnitEventInfo{evt.getKind(), evt.getUnitName()});
     }
 
+    Delegate->processingAddedPending(evts.size());
+    eventsDeque->addEvents(evts);
+
     // Create the block with QoS explicitly to ensure that the QoS from the indexstore callback can't affect the onFilesChange priority. This call may do a lot of I/O and we don't want to wedge the system by running at elevated priority.
     dispatch_block_t onUnitChangeBlock = dispatch_block_create_with_qos_class(DISPATCH_BLOCK_INHERIT_QOS_CLASS, unitChangesQOS, 0, ^{
-      Delegate->processingAddedPending(evts.size());
-      UnitRepo->onFilesChange(std::move(evts), [&](unsigned NumCompleted){
-        Delegate->processingCompleted(NumCompleted);
-      }, [&](){
-        // FIXME: the database should recover.
-      });
+      // Pass registration events to be processed incrementally by the global serial queue.
+      // This allows intermixing processing of registration events from multiple workspaces.
+      processUnitEventsIncrementally(eventsDeque, WeakUnitRepo, Delegate, getGlobalQueueForUnitChanges());
     });
-
-    dispatch_async(UnitRepo->getQueueForUnitChanges(), onUnitChangeBlock);
+    dispatch_async(getGlobalQueueForUnitChanges(), onUnitChangeBlock);
     Block_release(onUnitChangeBlock);
   };