[clangd] Use AsyncTaskRunner in BackgroundIndex instead of std::thread

[ilya-biryukov]

Summary

To unify the way we create threads in clangd.
This should simplify landing D50993.

[kadircet]

LGTM

clang-tools-extra/clangd/index/Background.cpp

152 ↗

(On Diff #198788)

NIT: why not count from zero :P

164 ↗

(On Diff #198788)

Destructor of AsyncTaskRunner already does that

[ilya-biryukov]

• Count from 0.

clang-tools-extra/clangd/index/Background.cpp

152 ↗

(On Diff #198788)

Done. The worker numbers still start with 1, sorry about that :-)

164 ↗

(On Diff #198788)

Yeah, but then we'll have to ensure the AsyncTaskRunner is the last member (I've been bitten by this before).
Having an explicit call to wait() in the destructor is bulletproof.

[• umanwizard]

clang-tools-extra/trunk/clangd/index/Background.cpp

164

This is already called in ~AsyncTaskRunner.

[• umanwizard]

Ignore me. I didn't read the previous discussion before commenting.

clang-tools-extra/trunk/clangd/index/Background.h

private:
   void enqueueTask(Task T, llvm::ThreadPriority Prioirty);
   void enqueueLocked(tooling::CompileCommand Cmd,
                      BackgroundIndexStorage *IndexStorage);
   std::mutex QueueMu;
   unsigned NumActiveTasks = 0; // Only idle when queue is empty *and* no tasks.
   std::condition_variable QueueCV;
   bool ShouldStop = false;
   std::deque<std::pair<Task, llvm::ThreadPriority>> Queue;
-  std::vector<std::thread> ThreadPool; // FIXME: Abstract this away.
+  AsyncTaskRunner ThreadPool;
   GlobalCompilationDatabase::CommandChanged::Subscription CommandsChanged;
 };
 
 } // namespace clangd
 } // namespace clang
 
 #endif

clang-tools-extra/trunk/clangd/index/Background.cpp

: SwapIndex(llvm::make_unique<MemIndex>()), FSProvider(FSProvider),
       SymbolsUpdatedSinceLastIndex(false),
       IndexStorageFactory(std::move(IndexStorageFactory)),
       CommandsChanged(
           CDB.watch([&](const std::vector<std::string> &ChangedFiles) {
             enqueue(ChangedFiles);
           })) {
   assert(ThreadPoolSize > 0 && "Thread pool size can't be zero.");
   assert(this->IndexStorageFactory && "Storage factory can not be null!");
-  while (ThreadPoolSize--)
-    ThreadPool.emplace_back([this] { run(); });
+  for (unsigned I = 0; I < ThreadPoolSize; ++I) {
+    ThreadPool.runAsync("background-worker-" + llvm::Twine(I + 1),
+                        [this] { run(); });
+  }
   if (BuildIndexPeriodMs > 0) {
     log("BackgroundIndex: build symbol index periodically every {0} ms.",
         BuildIndexPeriodMs);
-    ThreadPool.emplace_back([this] { buildIndex(); });
+    ThreadPool.runAsync("background-index-builder", [this] { buildIndex(); });
   }
 }
 
 BackgroundIndex::~BackgroundIndex() {
   stop();
-  for (auto &Thread : ThreadPool)
-    Thread.join();
+  ThreadPool.wait();
 }
 
 void BackgroundIndex::stop() {
   {
     std::lock_guard<std::mutex> QueueLock(QueueMu);
     std::lock_guard<std::mutex> IndexLock(IndexMu);
     ShouldStop = true;
   }