r310618 - Fixed a race condition in PrecompiledPreamble.

[Ilya Biryukov via cfe-commits]

Author: ibiryukov
Date: Thu Aug 10 09:10:40 2017
New Revision: 310618

URL: http://llvm.org/viewvc/llvm-project?rev=310618&view=rev
Log:
Fixed a race condition in PrecompiledPreamble.

Summary:
Two PrecompiledPreambles, used in parallel on separate threads,
could be writing preamble to the same temporary file.

Reviewers: bkramer, krasimir, klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D36529

Modified:
    cfe/trunk/lib/Frontend/PrecompiledPreamble.cpp

Modified: cfe/trunk/lib/Frontend/PrecompiledPreamble.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Frontend/PrecompiledPreamble.cpp?rev=310618&r1=310617&r2=310618&view=diff
==============================================================================
--- cfe/trunk/lib/Frontend/PrecompiledPreamble.cpp (original)
+++ cfe/trunk/lib/Frontend/PrecompiledPreamble.cpp Thu Aug 10 09:10:40 2017
@@ -28,6 +28,7 @@
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Mutex.h"
 #include "llvm/Support/MutexGuard.h"
+#include "llvm/Support/Process.h"
 
 using namespace clang;
 
@@ -462,9 +463,16 @@ llvm::ErrorOr<PrecompiledPreamble::TempP
 PrecompiledPreamble::TempPCHFile::createInSystemTempDir(const Twine &Prefix,
                                                         StringRef Suffix) {
   llvm::SmallString<64> File;
-  auto EC = llvm::sys::fs::createTemporaryFile(Prefix, Suffix, /*ref*/ File);
+  // Using a version of createTemporaryFile with a file descriptor guarantees
+  // that we would never get a race condition in a multi-threaded setting (i.e.,
+  // multiple threads getting the same temporary path).
+  int FD;
+  auto EC = llvm::sys::fs::createTemporaryFile(Prefix, Suffix, /*ref*/ FD,
+                                               /*ref*/ File);
   if (EC)
     return EC;
+  // We only needed to make sure the file exists, close the file right away.
+  llvm::sys::Process::SafelyCloseFileDescriptor(FD);
   return TempPCHFile(std::move(File).str());
 }