Use TempFile in the implementation of LockFileManager

[• espindola]

Summary

This move some of the complexity over to the lower level TempFile.

It also makes it a bit more explicit where errors are ignored since we now have a call to consumeError.

[benlangmuir]

Spotted one issue, but otherwise seems okay to me. I'd like @bruno to take a look at this, because he's thought about this code much more recently than me.

lib/Support/LockFileManager.cpp

128

The constructor is being called with a temporary lambda, so this is going to create a use-after-free.

[• rafael]

Use std::function instead of function_ref.

[• rafael]

Ping. This is now the last use of this sys::fs::createUniqueFile variant in llvm proper.

[rnk]

lgtm, let's give it a go.

lib/Support/LockFileManager.cpp

175

This is total bikeshedding, but I like to write these RAII helpers inline, like:

struct TempFileRemover {
  TempFile &TF;
  LockFileManager &LFM;
  bool Cancelled = false;
  TempFileRemover(TempFile &TF, LockFileManager &LFM) : TF(TF), LFM(LFM) {}
  ~TempFileRemover() {
    if (Cancelled)
      return;
    if (Error E = TF.discard())
      LFM.setError(errorToErrorCode(std::move(E)));
} RemoveTempFile(*TempFile, *this);

It saves a std::function, but you have to write out the capture members and a constructor to initialize them. Maybe this is just what happens when your brain has been damaged from working on a C++ compiler too long.

[• rafael]

r318550

I kept the RAII class outside the function since a cancelable cleanup seems a general concept and we might want to move it to a header in the future.

[bruno]

LGTM as well!

include/llvm/Support/LockFileManager.h

 //===--- LockFileManager.h - File-level locking utility ---------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 #ifndef LLVM_SUPPORT_LOCKFILEMANAGER_H
 #define LLVM_SUPPORT_LOCKFILEMANAGER_H
 
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/Support/FileSystem.h"
 #include <system_error>
 #include <utility> // for std::pair
 
 namespace llvm {
 class StringRef;
 
 /// \brief Class that manages the creation of a lock file to aid
 /// implicit coordination between different processes.
enum WaitForUnlockResult {
     Res_OwnerDied,
     /// \brief Reached timeout while waiting for the owner to release the lock.
     Res_Timeout
   };
 
 private:
   SmallString<128> FileName;
   SmallString<128> LockFileName;
-  SmallString<128> UniqueLockFileName;
+  Optional<sys::fs::TempFile> UniqueLockFile;
 
   Optional<std::pair<std::string, int> > Owner;
   std::error_code ErrorCode;
   std::string ErrorDiagMsg;
 
   LockFileManager(const LockFileManager &) = delete;
   LockFileManager &operator=(const LockFileManager &) = delete;
 

lib/Support/LockFileManager.cpp

if (StoredHostID == HostID && getsid(PID) == -1 && errno == ESRCH)
     return false;
 #endif
 
   return true;
 }
 
 namespace {
 
-/// An RAII helper object ensure that the unique lock file is removed.
-///
-/// Ensures that if there is an error or a signal before we finish acquiring the
-/// lock, the unique file will be removed. And if we successfully take the lock,
-/// the signal handler is left in place so that signals while the lock is held
-/// will remove the unique lock file. The caller should ensure there is a
-/// matching call to sys::DontRemoveFileOnSignal when the lock is released.
-class RemoveUniqueLockFileOnSignal {
-  StringRef Filename;
-  bool RemoveImmediately;
+/// An RAII helper object for cleanups.
+class RAIICleanup {
+  std::function<void()> Fn;
+  bool Canceled = false;
+
 public:
-  RemoveUniqueLockFileOnSignal(StringRef Name)
-  : Filename(Name), RemoveImmediately(true) {
-    sys::RemoveFileOnSignal(Filename, nullptr);
-  }
+  RAIICleanup(std::function<void()> Fn) : Fn(Fn) {}
 
-  ~RemoveUniqueLockFileOnSignal() {
-    if (!RemoveImmediately) {
-      // Leave the signal handler enabled. It will be removed when the lock is
-      // released.
+  ~RAIICleanup() {
+    if (Canceled)
       return;
-    }
-    sys::fs::remove(Filename);
-    sys::DontRemoveFileOnSignal(Filename);
+    Fn();
   }
 
-  void lockAcquired() { RemoveImmediately = false; }
+  void cancel() { Canceled = true; }
 };
 
 } // end anonymous namespace
 
 LockFileManager::LockFileManager(StringRef FileName)
 {
   this->FileName = FileName;
   if (std::error_code EC = sys::fs::make_absolute(this->FileName)) {
     std::string S("failed to obtain absolute path for ");
     S.append(this->FileName.str());
     setError(EC, S);
     return;
   }
   LockFileName = this->FileName;
   LockFileName += ".lock";
 
   // If the lock file already exists, don't bother to try to create our own
   // lock file; it won't work anyway. Just figure out who owns this lock file.
   if ((Owner = readLockFile(LockFileName)))
     return;
 
   // Create a lock file that is unique to this instance.
-  UniqueLockFileName = LockFileName;
-  UniqueLockFileName += "-%%%%%%%%";
-  int UniqueLockFileID;
-  if (std::error_code EC = sys::fs::createUniqueFile(
-          UniqueLockFileName, UniqueLockFileID, UniqueLockFileName)) {
-    std::string S("failed to create unique file ");
-    S.append(UniqueLockFileName.str());
+  Expected<sys::fs::TempFile> Temp =
+      sys::fs::TempFile::create(LockFileName + "-%%%%%%%%");
+  if (!Temp) {
+    std::error_code EC = errorToErrorCode(Temp.takeError());
+    std::string S("failed to create unique file with prefix ");
+    S.append(LockFileName.str());
     setError(EC, S);
     return;
   }
+  UniqueLockFile = std::move(*Temp);
+
+  // Make sure we discard the temporary file on exit.
+  RAIICleanup RemoveTempFile([&]() {
+    if (Error E = UniqueLockFile->discard())
+      setError(errorToErrorCode(std::move(E)));
+  });
 
   // Write our process ID to our unique lock file.
   {
     SmallString<256> HostID;
     if (auto EC = getHostID(HostID)) {
       setError(EC, "failed to get host id");
       return;
     }
 
-    raw_fd_ostream Out(UniqueLockFileID, /*shouldClose=*/true);
+    raw_fd_ostream Out(UniqueLockFile->FD, /*shouldClose=*/false);
     Out << HostID << ' ';
 #if LLVM_ON_UNIX
     Out << getpid();
 #else
     Out << "1";
 #endif
-    Out.close();
+    Out.flush();
 
     if (Out.has_error()) {
       // We failed to write out PID, so report the error, remove the
       // unique lock file, and fail.
       std::string S("failed to write to ");
-      S.append(UniqueLockFileName.str());
+      S.append(UniqueLockFile->TmpName);
       setError(Out.error(), S);
-      sys::fs::remove(UniqueLockFileName);
       return;
     }
   }
 
-  // Clean up the unique file on signal, which also releases the lock if it is
-  // held since the .lock symlink will point to a nonexistent file.
-  RemoveUniqueLockFileOnSignal RemoveUniqueFile(UniqueLockFileName);
-
   while (true) {
     // Create a link from the lock file name. If this succeeds, we're done.
     std::error_code EC =
-        sys::fs::create_link(UniqueLockFileName, LockFileName);
+        sys::fs::create_link(UniqueLockFile->TmpName, LockFileName);
     if (!EC) {
-      RemoveUniqueFile.lockAcquired();
+      RemoveTempFile.cancel();
       return;
     }
 
     if (EC != errc::file_exists) {
       std::string S("failed to create link ");
       raw_string_ostream OSS(S);
-      OSS << LockFileName.str() << " to " << UniqueLockFileName.str();
+      OSS << LockFileName.str() << " to " << UniqueLockFile->TmpName;
       setError(EC, OSS.str());
       return;
     }
 
     // Someone else managed to create the lock file first. Read the process ID
     // from the lock file.
-    if ((Owner = readLockFile(LockFileName))) {
-      // Wipe out our unique lock file (it's useless now)
-      sys::fs::remove(UniqueLockFileName);
-      return;
-    }
+    if ((Owner = readLockFile(LockFileName)))
+      return; // RemoveTempFile will delete out our unique lock file.
 
     if (!sys::fs::exists(LockFileName)) {
       // The previous owner released the lock file before we could read it.
       // Try to get ownership again.
       continue;
     }
 
     // There is a lock file that nobody owns; try to clean it up and get
     // ownership.
     if ((EC = sys::fs::remove(LockFileName))) {
       std::string S("failed to remove lockfile ");
-      S.append(UniqueLockFileName.str());
+      S.append(LockFileName.str());
       setError(EC, S);
       return;
     }
   }
 }
 
 LockFileManager::LockFileState LockFileManager::getState() const {
   if (Owner)

 }
 
 LockFileManager::~LockFileManager() {
   if (getState() != LFS_Owned)
     return;
 
   // Since we own the lock, remove the lock file and our own unique lock file.
   sys::fs::remove(LockFileName);
-  sys::fs::remove(UniqueLockFileName);
-  // The unique file is now gone, so remove it from the signal handler. This
-  // matches a sys::RemoveFileOnSignal() in LockFileManager().
-  sys::DontRemoveFileOnSignal(UniqueLockFileName);
+  consumeError(UniqueLockFile->discard());
 }
 
 LockFileManager::WaitForUnlockResult LockFileManager::waitForUnlock() {
   if (getState() != LFS_Shared)
     return Res_Success;
 
 #if LLVM_ON_WIN32
   unsigned long Interval = 1;