From a6948da86ad7e78d66b26263c2681ef6385cc234 Mon Sep 17 00:00:00 2001
From: Saleem Abdulrasool <compnerd@compnerd.org>
Date: Mon, 14 Jun 2021 19:41:30 -0700
Subject: [PATCH] DirectoryWatcher: close a possible window of race on Windows

The initial scan occurring before the watcher is ready allows a race
condition where a change occurs before the initial scan completes.
Ensure that we wait for the watcher to begin executing the initial scan.

Addresses some feedback from Adrian McCarthy in post-commit review.
---
 .../lib/DirectoryWatcher/windows/DirectoryWatcher-windows.cpp  | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/clang/lib/DirectoryWatcher/windows/DirectoryWatcher-windows.cpp b/clang/lib/DirectoryWatcher/windows/DirectoryWatcher-windows.cpp
index 847d20bfbb6f..8a4f5a87d967 100644
--- a/clang/lib/DirectoryWatcher/windows/DirectoryWatcher-windows.cpp
+++ b/clang/lib/DirectoryWatcher/windows/DirectoryWatcher-windows.cpp
@@ -133,6 +133,9 @@ DirectoryWatcherWindows::~DirectoryWatcherWindows() {
 }
 
 void DirectoryWatcherWindows::InitialScan() {
+  std::unique_lock<std::mutex> lock(Mutex);
+  Ready.wait(lock, [this] { return this->WatcherActive; });
+
   Callback(getAsFileEvents(scanDirectory(Path.data())), /*IsInitial=*/true);
 }