[clangd] Fix threading bugs in (not-yet-used) BackgroundIndex, re-ena…

…ble test.

Summary:
One relatively boring bug: forgot to notify the CV after enqueue.

One much more fun bug: the thread member could access instance variables before
they were initialized. Although the thread was last in the init list, QueueCV
etc were listed after Thread in the class, so their default constructors raced
with the thread itself.
We have to get very unlucky to lose this race, I saw it 0.02% of the time.

Reviewers: ioeric

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D53313

llvm-svn: 344595

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

@@ -75,8 +75,11 @@ void BackgroundIndex::blockUntilIdleForTest() {
 
 void BackgroundIndex::enqueue(StringRef Directory,
                               tooling::CompileCommand Cmd) {
-  std::lock_guard<std::mutex> Lock(QueueMu);
-  enqueueLocked(std::move(Cmd));
+  {
+    std::lock_guard<std::mutex> Lock(QueueMu);
+    enqueueLocked(std::move(Cmd));
+  }
+  QueueCV.notify_all();
 }
 
 void BackgroundIndex::enqueueAll(StringRef Directory,

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

@@ -65,12 +65,12 @@ class BackgroundIndex : public SwapIndex {
   using Task = std::function<void()>; // FIXME: use multiple worker threads.
   void run(); // Main loop executed by Thread. Runs tasks from Queue.
   void enqueueLocked(tooling::CompileCommand Cmd);
-  std::thread Thread;
   std::mutex QueueMu;
   unsigned NumActiveTasks = 0; // Only idle when queue is empty *and* no tasks.
   std::condition_variable QueueCV;
   bool ShouldStop = false;
   std::deque<Task> Queue;
+  std::thread Thread; // Must be last, spawned thread reads instance vars.
 };
 
 } // namespace clangd

clang-tools-extra/unittests/clangd/BackgroundIndexTests.cpp

@@ -11,8 +11,6 @@ namespace clangd {
 
 MATCHER_P(Named, N, "") { return arg.Name == N; }
 
-// Temporarily disabled: test timing out on buildbots.
-#if 0
 TEST(BackgroundIndexTest, IndexTwoFiles) {
   MockFSProvider FS;
   // a.h yields different symbols when included by A.cc vs B.cc.
@@ -34,7 +32,6 @@ TEST(BackgroundIndexTest, IndexTwoFiles) {
   EXPECT_THAT(runFuzzyFind(Idx, ""),
               UnorderedElementsAre(Named("a_h"), Named("foo"), Named("bar")));
 }
-#endif
 
 } // namespace clangd
 } // namespace clang