ELF/Filesystem.cpp

//===- Filesystem.cpp -----------------------------------------------------===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains a few utility functions to handle files.
//
//===----------------------------------------------------------------------===//

#include "Filesystem.h"
#include "Config.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/FileOutputBuffer.h"
#include <thread>

using namespace llvm;

using namespace lld;
using namespace lld::elf;

// Removes a given file asynchronously. This is a performance hack,
// so remove this when operating systems are improved.
//
// On Linux (and probably on other Unix-like systems), unlink(2) is a
// noticeably slow system call. As of 2016, unlink takes 250
// milliseconds to remove a 1 GB file on ext4 filesystem on my machine.
//
// To create a new result file, we first remove existing file. So, if
// you repeatedly link a 1 GB program in a regular compile-link-debug
// cycle, every cycle wastes 250 milliseconds only to remove a file.
// Since LLD can link a 1 GB binary in about 5 seconds, that waste
// actually counts.
//
// This function spawns a background thread to call unlink.
// The calling thread returns almost immediately.
void elf::unlinkAsync(StringRef Path) {
  if (!Config->Threads || !sys::fs::exists(Config->OutputFile) ||
      !sys::fs::is_regular_file(Config->OutputFile))
    return;

  // First, rename Path to avoid race condition. We cannot remove
  // Path from a different thread because we are now going to create
  // Path as a new file. If we do that in a different thread, the new
  // thread can remove the new file.
  SmallString<128> TempPath;
  if (sys::fs::createUniqueFile(Path + "tmp%%%%%%%%", TempPath))
    return;
  if (sys::fs::rename(Path, TempPath)) {
    sys::fs::remove(TempPath);
    return;
  }

  // Remove TempPath in background.
  std::thread([=] { ::remove(TempPath.str().str().c_str()); }).detach();
}

// Simulate file creation to see if Path is writable.
//
// Determining whether a file is writable or not is amazingly hard,
// and after all the only reliable way of doing that is to actually
// create a file. But we don't want to do that in this function
// because LLD shouldn't update any file if it will end in a failure.
// We also don't want to reimplement heuristics to determine if a
// file is writable. So we'll let FileOutputBuffer do the work.
//
// FileOutputBuffer doesn't touch a desitnation file until commit()
// is called. We use that class without calling commit() to predict
// if the given file is writable.
std::error_code elf::tryCreateFile(StringRef Path) {
  if (Path.empty())
    return std::error_code();
  return FileOutputBuffer::create(Path, 1).getError();
}