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//===--- LockFileManager.cpp - File-level Locking Utility------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#include "llvm/Support/LockFileManager.h"
#include "llvm/ADT/None.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/Support/Errc.h"
#include "llvm/Support/ErrorOr.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/MemoryBuffer.h"
#include "llvm/Support/Process.h"
#include "llvm/Support/Signals.h"
#include "llvm/Support/raw_ostream.h"
#include <cerrno>
#include <chrono>
#include <ctime>
#include <memory>
#include <random>
#include <sys/stat.h>
#include <sys/types.h>
#include <system_error>
#include <thread>
#include <tuple>
#ifdef _WIN32
#include <windows.h>
#include <unistd.h>
#if defined(__APPLE__) && defined(__MAC_OS_X_VERSION_MIN_REQUIRED) && (__MAC_OS_X_VERSION_MIN_REQUIRED > 1050)
#include <uuid/uuid.h>
using namespace llvm;
/// Attempt to read the lock file with the given name, if it exists.
/// \param LockFileName The name of the lock file to read.
/// \returns The process ID of the process that owns this lock file
Optional<std::pair<std::string, int> >
LockFileManager::readLockFile(StringRef LockFileName) {
// Read the owning host and PID out of the lock file. If it appears that the
// owning process is dead, the lock file is invalid.
ErrorOr<std::unique_ptr<MemoryBuffer>> MBOrErr =
if (!MBOrErr) {
return None;
MemoryBuffer &MB = *MBOrErr.get();
StringRef Hostname;
StringRef PIDStr;
std::tie(Hostname, PIDStr) = getToken(MB.getBuffer(), " ");
PIDStr = PIDStr.substr(PIDStr.find_first_not_of(" "));
int PID;
if (!PIDStr.getAsInteger(10, PID)) {
auto Owner = std::make_pair(std::string(Hostname), PID);
if (processStillExecuting(Owner.first, Owner.second))
return Owner;
// Delete the lock file. It's invalid anyway.
return None;
static std::error_code getHostID(SmallVectorImpl<char> &HostID) {
// On OS X, use the more stable hardware UUID instead of hostname.
struct timespec wait = {1, 0}; // 1 second.
uuid_t uuid;
if (gethostuuid(uuid, &wait) != 0)
return std::error_code(errno, std::system_category());
uuid_string_t UUIDStr;
uuid_unparse(uuid, UUIDStr);
StringRef UUIDRef(UUIDStr);
HostID.append(UUIDRef.begin(), UUIDRef.end());
char HostName[256];
HostName[255] = 0;
HostName[0] = 0;
gethostname(HostName, 255);
StringRef HostNameRef(HostName);
HostID.append(HostNameRef.begin(), HostNameRef.end());
StringRef Dummy("localhost");
HostID.append(Dummy.begin(), Dummy.end());
return std::error_code();
bool LockFileManager::processStillExecuting(StringRef HostID, int PID) {
#if LLVM_ON_UNIX && !defined(__ANDROID__)
SmallString<256> StoredHostID;
if (getHostID(StoredHostID))
return true; // Conservatively assume it's executing on error.
// Check whether the process is dead. If so, we're done.
if (StoredHostID == HostID && getsid(PID) == -1 && errno == ESRCH)
return false;
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;
RemoveUniqueLockFileOnSignal(StringRef Name)
: Filename(Name), RemoveImmediately(true) {
sys::RemoveFileOnSignal(Filename, nullptr);
~RemoveUniqueLockFileOnSignal() {
if (!RemoveImmediately) {
// Leave the signal handler enabled. It will be removed when the lock is
// released.
void lockAcquired() { RemoveImmediately = false; }
} // 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 ");
setError(EC, S);
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)))
// 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 ");
setError(EC, S);
// Write our process ID to our unique lock file.
SmallString<256> HostID;
if (auto EC = getHostID(HostID)) {
setError(EC, "failed to get host id");
raw_fd_ostream Out(UniqueLockFileID, /*shouldClose=*/true);
Out << HostID << ' ' << sys::Process::getProcessId();
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 ");
setError(Out.error(), S);
// 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);
if (!EC) {
if (EC != errc::file_exists) {
std::string S("failed to create link ");
raw_string_ostream OSS(S);
OSS << LockFileName.str() << " to " << UniqueLockFileName.str();
setError(EC, OSS.str());
// 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)
if (!sys::fs::exists(LockFileName)) {
// The previous owner released the lock file before we could read it.
// Try to get ownership again.
// 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 ");
setError(EC, S);
LockFileManager::LockFileState LockFileManager::getState() const {
if (Owner)
return LFS_Shared;
if (ErrorCode)
return LFS_Error;
return LFS_Owned;
std::string LockFileManager::getErrorMessage() const {
if (ErrorCode) {
std::string Str(ErrorDiagMsg);
std::string ErrCodeMsg = ErrorCode.message();
raw_string_ostream OSS(Str);
if (!ErrCodeMsg.empty())
OSS << ": " << ErrCodeMsg;
return OSS.str();
return "";
LockFileManager::~LockFileManager() {
if (getState() != LFS_Owned)
// Since we own the lock, remove the lock file and our own unique lock file.
// The unique file is now gone, so remove it from the signal handler. This
// matches a sys::RemoveFileOnSignal() in LockFileManager().
LockFileManager::waitForUnlock(const unsigned MaxSeconds) {
if (getState() != LFS_Shared)
return Res_Success;
// Since we don't yet have an event-based method to wait for the lock file,
// implement randomized exponential backoff, similar to Ethernet collision
// algorithm. This improves performance on machines with high core counts
// when the file lock is heavily contended by multiple clang processes
const unsigned long MinWaitDurationMS = 10;
const unsigned long MaxWaitMultiplier = 50; // 500ms max wait
unsigned long WaitMultiplier = 1;
unsigned long ElapsedTimeSeconds = 0;
std::random_device Device;
std::default_random_engine Engine(Device());
auto StartTime = std::chrono::steady_clock::now();
do {
// FIXME: implement event-based waiting
// Sleep for the designated interval, to allow the owning process time to
// finish up and remove the lock file.
std::uniform_int_distribution<unsigned long> Distribution(1,
unsigned long WaitDurationMS = MinWaitDurationMS * Distribution(Engine);
if (sys::fs::access(LockFileName.c_str(), sys::fs::AccessMode::Exist) ==
errc::no_such_file_or_directory) {
// If the original file wasn't created, somone thought the lock was dead.
if (!sys::fs::exists(FileName))
return Res_OwnerDied;
return Res_Success;
// If the process owning the lock died without cleaning up, just bail out.
if (!processStillExecuting((*Owner).first, (*Owner).second))
return Res_OwnerDied;
WaitMultiplier *= 2;
if (WaitMultiplier > MaxWaitMultiplier) {
WaitMultiplier = MaxWaitMultiplier;
ElapsedTimeSeconds = std::chrono::duration_cast<std::chrono::seconds>(
std::chrono::steady_clock::now() - StartTime)
} while (ElapsedTimeSeconds < MaxSeconds);
// Give up.
return Res_Timeout;
std::error_code LockFileManager::unsafeRemoveLockFile() {
return sys::fs::remove(LockFileName);