1//===--- LockFileManager.cpp - File-level Locking Utility------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#include "llvm/Support/LockFileManager.h"
10#include "llvm/ADT/None.h"
11#include "llvm/ADT/SmallVector.h"
12#include "llvm/ADT/StringExtras.h"
13#include "llvm/Support/Errc.h"
14#include "llvm/Support/ErrorOr.h"
15#include "llvm/Support/FileSystem.h"
16#include "llvm/Support/MemoryBuffer.h"
17#include "llvm/Support/Process.h"
18#include "llvm/Support/Signals.h"
19#include "llvm/Support/raw_ostream.h"
20#include <cerrno>
21#include <chrono>
22#include <ctime>
23#include <memory>
24#include <random>
25#include <sys/stat.h>
26#include <sys/types.h>
27#include <system_error>
28#include <thread>
29#include <tuple>
30
31#ifdef _WIN32
32#include <windows.h>
33#endif
34#if LLVM_ON_UNIX
35#include <unistd.h>
36#endif
37
38#if defined(__APPLE__) && defined(__MAC_OS_X_VERSION_MIN_REQUIRED) && (__MAC_OS_X_VERSION_MIN_REQUIRED > 1050)
39#define USE_OSX_GETHOSTUUID 1
40#else
41#define USE_OSX_GETHOSTUUID 0
42#endif
43
44#if USE_OSX_GETHOSTUUID
45#include <uuid/uuid.h>
46#endif
47
48using namespace llvm;
49
50/// Attempt to read the lock file with the given name, if it exists.
51///
52/// \param LockFileName The name of the lock file to read.
53///
54/// \returns The process ID of the process that owns this lock file
55Optional<std::pair<std::string, int> >
56LockFileManager::readLockFile(StringRef LockFileName) {
57  // Read the owning host and PID out of the lock file. If it appears that the
58  // owning process is dead, the lock file is invalid.
59  ErrorOr<std::unique_ptr<MemoryBuffer>> MBOrErr =
60      MemoryBuffer::getFile(LockFileName);
61  if (!MBOrErr) {
62    sys::fs::remove(LockFileName);
63    return None;
64  }
65  MemoryBuffer &MB = *MBOrErr.get();
66
67  StringRef Hostname;
68  StringRef PIDStr;
69  std::tie(Hostname, PIDStr) = getToken(MB.getBuffer(), " ");
70  PIDStr = PIDStr.substr(PIDStr.find_first_not_of(" "));
71  int PID;
72  if (!PIDStr.getAsInteger(10, PID)) {
73    auto Owner = std::make_pair(std::string(Hostname), PID);
74    if (processStillExecuting(Owner.first, Owner.second))
75      return Owner;
76  }
77
78  // Delete the lock file. It's invalid anyway.
79  sys::fs::remove(LockFileName);
80  return None;
81}
82
83static std::error_code getHostID(SmallVectorImpl<char> &HostID) {
84  HostID.clear();
85
86#if USE_OSX_GETHOSTUUID
87  // On OS X, use the more stable hardware UUID instead of hostname.
88  struct timespec wait = {1, 0}; // 1 second.
89  uuid_t uuid;
90  if (gethostuuid(uuid, &wait) != 0)
91    return std::error_code(errno, std::system_category());
92
93  uuid_string_t UUIDStr;
94  uuid_unparse(uuid, UUIDStr);
95  StringRef UUIDRef(UUIDStr);
96  HostID.append(UUIDRef.begin(), UUIDRef.end());
97
98#elif LLVM_ON_UNIX
99  char HostName[256];
100  HostName[255] = 0;
101  HostName[0] = 0;
102  gethostname(HostName, 255);
103  StringRef HostNameRef(HostName);
104  HostID.append(HostNameRef.begin(), HostNameRef.end());
105
106#else
107  StringRef Dummy("localhost");
108  HostID.append(Dummy.begin(), Dummy.end());
109#endif
110
111  return std::error_code();
112}
113
114bool LockFileManager::processStillExecuting(StringRef HostID, int PID) {
115#if LLVM_ON_UNIX && !defined(__ANDROID__)
116  SmallString<256> StoredHostID;
117  if (getHostID(StoredHostID))
118    return true; // Conservatively assume it's executing on error.
119
120  // Check whether the process is dead. If so, we're done.
121  if (StoredHostID == HostID && getsid(PID) == -1 && errno == ESRCH)
122    return false;
123#endif
124
125  return true;
126}
127
128namespace {
129
130/// An RAII helper object ensure that the unique lock file is removed.
131///
132/// Ensures that if there is an error or a signal before we finish acquiring the
133/// lock, the unique file will be removed. And if we successfully take the lock,
134/// the signal handler is left in place so that signals while the lock is held
135/// will remove the unique lock file. The caller should ensure there is a
136/// matching call to sys::DontRemoveFileOnSignal when the lock is released.
137class RemoveUniqueLockFileOnSignal {
138  StringRef Filename;
139  bool RemoveImmediately;
140public:
141  RemoveUniqueLockFileOnSignal(StringRef Name)
142  : Filename(Name), RemoveImmediately(true) {
143    sys::RemoveFileOnSignal(Filename, nullptr);
144  }
145
146  ~RemoveUniqueLockFileOnSignal() {
147    if (!RemoveImmediately) {
148      // Leave the signal handler enabled. It will be removed when the lock is
149      // released.
150      return;
151    }
152    sys::fs::remove(Filename);
153    sys::DontRemoveFileOnSignal(Filename);
154  }
155
156  void lockAcquired() { RemoveImmediately = false; }
157};
158
159} // end anonymous namespace
160
161LockFileManager::LockFileManager(StringRef FileName)
162{
163  this->FileName = FileName;
164  if (std::error_code EC = sys::fs::make_absolute(this->FileName)) {
165    std::string S("failed to obtain absolute path for ");
166    S.append(std::string(this->FileName.str()));
167    setError(EC, S);
168    return;
169  }
170  LockFileName = this->FileName;
171  LockFileName += ".lock";
172
173  // If the lock file already exists, don't bother to try to create our own
174  // lock file; it won't work anyway. Just figure out who owns this lock file.
175  if ((Owner = readLockFile(LockFileName)))
176    return;
177
178  // Create a lock file that is unique to this instance.
179  UniqueLockFileName = LockFileName;
180  UniqueLockFileName += "-%%%%%%%%";
181  int UniqueLockFileID;
182  if (std::error_code EC = sys::fs::createUniqueFile(
183          UniqueLockFileName, UniqueLockFileID, UniqueLockFileName)) {
184    std::string S("failed to create unique file ");
185    S.append(std::string(UniqueLockFileName.str()));
186    setError(EC, S);
187    return;
188  }
189
190  // Write our process ID to our unique lock file.
191  {
192    SmallString<256> HostID;
193    if (auto EC = getHostID(HostID)) {
194      setError(EC, "failed to get host id");
195      return;
196    }
197
198    raw_fd_ostream Out(UniqueLockFileID, /*shouldClose=*/true);
199    Out << HostID << ' ' << sys::Process::getProcessId();
200    Out.close();
201
202    if (Out.has_error()) {
203      // We failed to write out PID, so report the error, remove the
204      // unique lock file, and fail.
205      std::string S("failed to write to ");
206      S.append(std::string(UniqueLockFileName.str()));
207      setError(Out.error(), S);
208      sys::fs::remove(UniqueLockFileName);
209      return;
210    }
211  }
212
213  // Clean up the unique file on signal, which also releases the lock if it is
214  // held since the .lock symlink will point to a nonexistent file.
215  RemoveUniqueLockFileOnSignal RemoveUniqueFile(UniqueLockFileName);
216
217  while (true) {
218    // Create a link from the lock file name. If this succeeds, we're done.
219    std::error_code EC =
220        sys::fs::create_link(UniqueLockFileName, LockFileName);
221    if (!EC) {
222      RemoveUniqueFile.lockAcquired();
223      return;
224    }
225
226    if (EC != errc::file_exists) {
227      std::string S("failed to create link ");
228      raw_string_ostream OSS(S);
229      OSS << LockFileName.str() << " to " << UniqueLockFileName.str();
230      setError(EC, OSS.str());
231      return;
232    }
233
234    // Someone else managed to create the lock file first. Read the process ID
235    // from the lock file.
236    if ((Owner = readLockFile(LockFileName))) {
237      // Wipe out our unique lock file (it's useless now)
238      sys::fs::remove(UniqueLockFileName);
239      return;
240    }
241
242    if (!sys::fs::exists(LockFileName)) {
243      // The previous owner released the lock file before we could read it.
244      // Try to get ownership again.
245      continue;
246    }
247
248    // There is a lock file that nobody owns; try to clean it up and get
249    // ownership.
250    if ((EC = sys::fs::remove(LockFileName))) {
251      std::string S("failed to remove lockfile ");
252      S.append(std::string(UniqueLockFileName.str()));
253      setError(EC, S);
254      return;
255    }
256  }
257}
258
259LockFileManager::LockFileState LockFileManager::getState() const {
260  if (Owner)
261    return LFS_Shared;
262
263  if (ErrorCode)
264    return LFS_Error;
265
266  return LFS_Owned;
267}
268
269std::string LockFileManager::getErrorMessage() const {
270  if (ErrorCode) {
271    std::string Str(ErrorDiagMsg);
272    std::string ErrCodeMsg = ErrorCode.message();
273    raw_string_ostream OSS(Str);
274    if (!ErrCodeMsg.empty())
275      OSS << ": " << ErrCodeMsg;
276    return OSS.str();
277  }
278  return "";
279}
280
281LockFileManager::~LockFileManager() {
282  if (getState() != LFS_Owned)
283    return;
284
285  // Since we own the lock, remove the lock file and our own unique lock file.
286  sys::fs::remove(LockFileName);
287  sys::fs::remove(UniqueLockFileName);
288  // The unique file is now gone, so remove it from the signal handler. This
289  // matches a sys::RemoveFileOnSignal() in LockFileManager().
290  sys::DontRemoveFileOnSignal(UniqueLockFileName);
291}
292
293LockFileManager::WaitForUnlockResult
294LockFileManager::waitForUnlock(const unsigned MaxSeconds) {
295  if (getState() != LFS_Shared)
296    return Res_Success;
297
298  // Since we don't yet have an event-based method to wait for the lock file,
299  // implement randomized exponential backoff, similar to Ethernet collision
300  // algorithm. This improves performance on machines with high core counts
301  // when the file lock is heavily contended by multiple clang processes
302  const unsigned long MinWaitDurationMS = 10;
303  const unsigned long MaxWaitMultiplier = 50; // 500ms max wait
304  unsigned long WaitMultiplier = 1;
305  unsigned long ElapsedTimeSeconds = 0;
306
307  std::random_device Device;
308  std::default_random_engine Engine(Device());
309
310  auto StartTime = std::chrono::steady_clock::now();
311
312  do {
313    // FIXME: implement event-based waiting
314
315    // Sleep for the designated interval, to allow the owning process time to
316    // finish up and remove the lock file.
317    std::uniform_int_distribution<unsigned long> Distribution(1,
318                                                              WaitMultiplier);
319    unsigned long WaitDurationMS = MinWaitDurationMS * Distribution(Engine);
320    std::this_thread::sleep_for(std::chrono::milliseconds(WaitDurationMS));
321
322    if (sys::fs::access(LockFileName.c_str(), sys::fs::AccessMode::Exist) ==
323        errc::no_such_file_or_directory) {
324      // If the original file wasn't created, somone thought the lock was dead.
325      if (!sys::fs::exists(FileName))
326        return Res_OwnerDied;
327      return Res_Success;
328    }
329
330    // If the process owning the lock died without cleaning up, just bail out.
331    if (!processStillExecuting((*Owner).first, (*Owner).second))
332      return Res_OwnerDied;
333
334    WaitMultiplier *= 2;
335    if (WaitMultiplier > MaxWaitMultiplier) {
336      WaitMultiplier = MaxWaitMultiplier;
337    }
338
339    ElapsedTimeSeconds = std::chrono::duration_cast<std::chrono::seconds>(
340                             std::chrono::steady_clock::now() - StartTime)
341                             .count();
342
343  } while (ElapsedTimeSeconds < MaxSeconds);
344
345  // Give up.
346  return Res_Timeout;
347}
348
349std::error_code LockFileManager::unsafeRemoveLockFile() {
350  return sys::fs::remove(LockFileName);
351}
352