From 0a827eb82193cb2d2bb62974414327288dde0c26 Mon Sep 17 00:00:00 2001
From: Jan Svoboda <jan_svoboda@apple.com>
Date: Thu, 24 Apr 2025 11:30:08 -0700
Subject: [PATCH] [clang] Enable making `CompilerInstance` output backend
 thread-safe

---
 clang/lib/Frontend/CompilerInstance.cpp | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
index 93086494b8f6d..887e9321b7f73 100644
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -1418,10 +1418,15 @@ std::unique_ptr<CompilerInstance> CompilerInstance::cloneForModuleCompileImpl(
   auto WrapGenModuleAction = getGenModuleActionWrapper();
   Instance.setGenModuleActionWrapper(WrapGenModuleAction);
 
-  // Share an output manager.
   assert(hasOutputBackend() &&
          "Expected an output manager to already be set up");
-  Instance.setOutputBackend(&getOutputBackend());
+  if (ThreadSafeConfig) {
+    // Create a clone of the existing output (pointing to the same destination).
+    Instance.setOutputBackend(getOutputBackend().clone());
+  } else {
+    // Share the existing output manager.
+    Instance.setOutputBackend(&getOutputBackend());
+  }
 
   if (ThreadSafeConfig) {
     Instance.setModuleDepCollector(ThreadSafeConfig->getModuleDepCollector());