Index: llvm/include/llvm/Support/FileCollector.h
===================================================================
--- llvm/include/llvm/Support/FileCollector.h
+++ llvm/include/llvm/Support/FileCollector.h
@@ -59,6 +59,7 @@
   }
 
 protected:
+  /// FIXME: Seems more like it synchronizes everything (...accessible via public interface).
   /// Synchronizes adding files.
   std::mutex Mutex;
 
Index: llvm/lib/Support/FileCollector.cpp
===================================================================
--- llvm/lib/Support/FileCollector.cpp
+++ llvm/lib/Support/FileCollector.cpp
@@ -123,6 +123,9 @@
 }
 
 std::error_code FileCollector::copyFiles(bool StopOnError) {
+  // TODO: We're reading from VFSWriter so we should lock the Mutex, right?
+  std::lock_guard<std::mutex> lock(Mutex);
+
   for (auto &entry : VFSWriter.getMappings()) {
     // Create directory tree.
     if (std::error_code EC =