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//===- Filesystem.cpp -----------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file contains a few utility functions to handle files.
//
//===----------------------------------------------------------------------===//
#include "lld/Common/Filesystem.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/Support/FileOutputBuffer.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/Parallel.h"
#include "llvm/Support/Path.h"
#if LLVM_ON_UNIX
#include <unistd.h>
#endif
#include <thread>
using namespace llvm;
using namespace lld;
// 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 remove the file.
// The calling thread returns almost immediately.
void lld::unlinkAsync(StringRef path) {
if (!sys::fs::exists(path) || !sys::fs::is_regular_file(path))
return;
// Removing a file is async on windows.
#if defined(_WIN32)
// On Windows co-operative programs can be expected to open LLD's
// output in FILE_SHARE_DELETE mode. This allows us to delete the
// file (by moving it to a temporary filename and then deleting
// it) so that we can link another output file that overwrites
// the existing file, even if the current file is in use.
//
// This is done on a best effort basis - we do not error if the
// operation fails. The consequence is merely that the user
// experiences an inconvenient work-flow.
//
// The code here allows LLD to work on all versions of Windows.
// However, at Windows 10 1903 it seems that the behavior of
// Windows has changed, so that we could simply delete the output
// file. This code should be simplified once support for older
// versions of Windows is dropped.
//
// Warning: It seems that the WINVER and _WIN32_WINNT preprocessor
// defines affect the behavior of the Windows versions of the calls
// we are using here. If this code stops working this is worth
// bearing in mind.
SmallString<128> tmpName;
if (!sys::fs::createUniqueFile(path + "%%%%%%%%.tmp", tmpName)) {
if (!sys::fs::rename(path, tmpName))
path = tmpName;
else
sys::fs::remove(tmpName);
}
sys::fs::remove(path);
#else
if (parallel::strategy.ThreadsRequested == 1)
return;
// We cannot just remove path from a different thread because we are now going
// to create path as a new file.
// Instead we open the file and unlink it on this thread. The unlink is fast
// since the open fd guarantees that it is not removing the last reference.
int fd;
std::error_code ec = sys::fs::openFileForRead(path, fd);
sys::fs::remove(path);
if (ec)
return;
// close and therefore remove TempPath in background.
std::mutex m;
std::condition_variable cv;
bool started = false;
std::thread([&, fd] {
{
std::lock_guard<std::mutex> l(m);
started = true;
cv.notify_all();
}
::close(fd);
}).detach();
// GLIBC 2.26 and earlier have race condition that crashes an entire process
// if the main thread calls exit(2) while other thread is starting up.
std::unique_lock<std::mutex> l(m);
cv.wait(l, [&] { return started; });
#endif
}
// 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 destination file until commit()
// is called. We use that class without calling commit() to predict
// if the given file is writable.
std::error_code lld::tryCreateFile(StringRef path) {
if (path.empty())
return std::error_code();
if (path == "-")
return std::error_code();
return errorToErrorCode(FileOutputBuffer::create(path, 1).takeError());
}
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