From afa5a70ca825dc1051e150c7c23e465197d1a00b Mon Sep 17 00:00:00 2001
From: Steven Wu <stevenwu@apple.com>
Date: Thu, 26 Jun 2025 09:49:55 -0700
Subject: [PATCH] [clang][DependencyFile] Use atomic write for dependency file

Previously when switch to output backend, dependency file was relying
on non-atomic write + discard to remove the out of date dependency file
when there are missing headers. This makes the write non-atomic and can
cause races conditions that build system is trying to read the file
while the file is being updated by a different compiler instance.

rdar://154128578
---
 clang/lib/Frontend/DependencyFile.cpp | 22 +++++++---------------
 1 file changed, 7 insertions(+), 15 deletions(-)

diff --git a/clang/lib/Frontend/DependencyFile.cpp b/clang/lib/Frontend/DependencyFile.cpp
index 0c4812e764006..293eb332b1fc1 100644
--- a/clang/lib/Frontend/DependencyFile.cpp
+++ b/clang/lib/Frontend/DependencyFile.cpp
@@ -383,16 +383,13 @@ static void PrintFilename(raw_ostream &OS, StringRef Filename,
 }
 
 void DependencyFileGenerator::outputDependencyFile(DiagnosticsEngine &Diags) {
-  // The use of NoAtomicWrite and calling discard on SeenMissingHeader
-  // preserves the previous behaviour: no temporary files are used, and when
-  // SeenMissingHeader is true it deletes a previously-existing file.
-  // FIXME: switch to atomic-write based on FrontendOptions::UseTemporary and
-  // and not deleting the previous file, if possible.
-  Expected<llvm::vfs::OutputFile> O =
-      OutputBackend->createFile(OutputFile, llvm::vfs::OutputConfig()
-                                                .setTextWithCRLF()
-                                                .setNoAtomicWrite()
-                                                .setNoDiscardOnSignal());
+  if (SeenMissingHeader) {
+    llvm::sys::fs::remove(OutputFile);
+    return;
+  }
+
+  Expected<llvm::vfs::OutputFile> O = OutputBackend->createFile(
+      OutputFile, llvm::vfs::OutputConfig().setTextWithCRLF());
 
   if (!O) {
     Diags.Report(diag::err_fe_error_opening)
@@ -400,11 +397,6 @@ void DependencyFileGenerator::outputDependencyFile(DiagnosticsEngine &Diags) {
     return;
   }
 
-  if (SeenMissingHeader) {
-    consumeError(O->discard());
-    return;
-  }
-
   outputDependencyFile(O->getOS());
 
   if (auto Err = O->keep())