[Support] Add file lock/unlock functions

This is recommit of f51bc4f, reverted in 8577595, because
the function `flock` is not available on Solaris. In this variant
`flock` was replaced with `fcntl`, which is a POSIX function.

New functions `lockFile`, `tryLockFile` and `unlockFile` implement
simple file locking. They lock or unlock entire file. This must be
enough to support simulataneous writes to log files in parallel builds.

Differential Revision: https://reviews.llvm.org/D78896

llvm/include/llvm/Support/FileSystem.h

@@ -1131,6 +1131,43 @@ Expected<file_t>
 openNativeFileForRead(const Twine &Name, OpenFlags Flags = OF_None,
                       SmallVectorImpl<char> *RealPath = nullptr);
 
+/// Try to locks the file during the specified time.
+///
+/// This function implements advisory locking on entire file. If it returns
+/// <em>errc::success</em>, the file is locked by the calling process. Until the
+/// process unlocks the file by calling \a unlockFile, all attempts to lock the
+/// same file will fail/block. The process that locked the file may assume that
+/// none of other processes read or write this file, provided that all processes
+/// lock the file prior to accessing its content.
+///
+/// @param File    The descriptor representing the file to lock.
+/// @param Timeout Time in milliseconds that the process should wait before
+///                reporting lock failure. Zero value means try to get lock only
+///                once.
+/// @returns errc::success if lock is successfully obtained,
+/// errc::no_lock_available if the file cannot be locked, or platform-specific
+/// error_code otherwise.
+///
+/// @note Care should be taken when using this function in a multithreaded
+/// context, as it may not prevent other threads in the same process from
+/// obtaining a lock on the same file, even if they are using a different file
+/// descriptor.
+std::error_code
+tryLockFile(int FD,
+            std::chrono::milliseconds Timeout = std::chrono::milliseconds(0));
+
+/// Lock the file.
+///
+/// This function acts as @ref tryLockFile but it waits infinitely.
+std::error_code lockFile(int FD);
+
+/// Unlock the file.
+///
+/// @param File The descriptor representing the file to unlock.
+/// @returns errc::success if lock is successfully released or platform-specific
+/// error_code otherwise.
+std::error_code unlockFile(int FD);
+
 /// @brief Close the file object.  This should be used instead of ::close for
 /// portability. On error, the caller should assume the file is closed, as is
 /// the case for Process::SafelyCloseFileDescriptor

llvm/lib/Support/Unix/Path.inc

@@ -33,6 +33,7 @@
 
 #include <dirent.h>
 #include <pwd.h>
+#include <sys/file.h>
 
 #ifdef __APPLE__
 #include <mach-o/dyld.h>
@@ -1078,6 +1079,50 @@ Expected<size_t> readNativeFileSlice(file_t FD, MutableArrayRef<char> Buf,
   return NumRead;
 }
 
+std::error_code tryLockFile(int FD, std::chrono::milliseconds Timeout) {
+  auto Start = std::chrono::steady_clock::now();
+  auto End = Start + Timeout;
+  do {
+    struct flock Lock;
+    memset(&Lock, 0, sizeof(Lock));
+    Lock.l_type = F_WRLCK;
+    Lock.l_whence = SEEK_SET;
+    Lock.l_start = 0;
+    Lock.l_len = 0;
+    if (::fcntl(FD, F_SETLK, &Lock) != -1)
+      return std::error_code();
+    int Error = errno;
+    if (Error != EACCES && Error != EAGAIN)
+      return std::error_code(Error, std::generic_category());
+    usleep(1000);
+  } while (std::chrono::steady_clock::now() < End);
+  return make_error_code(errc::no_lock_available);
+}
+
+std::error_code lockFile(int FD) {
+  struct flock Lock;
+  memset(&Lock, 0, sizeof(Lock));
+  Lock.l_type = F_WRLCK;
+  Lock.l_whence = SEEK_SET;
+  Lock.l_start = 0;
+  Lock.l_len = 0;
+  if (::fcntl(FD, F_SETLKW, &Lock) != -1)
+    return std::error_code();
+  int Error = errno;
+  return std::error_code(Error, std::generic_category());
+}
+
+std::error_code unlockFile(int FD) {
+  struct flock Lock;
+  Lock.l_type = F_UNLCK;
+  Lock.l_whence = SEEK_SET;
+  Lock.l_start = 0;
+  Lock.l_len = 0;
+  if (::fcntl(FD, F_SETLK, &Lock) != -1)
+    return std::error_code();
+  return std::error_code(errno, std::generic_category());
+}
+
 std::error_code closeFile(file_t &F) {
   file_t TmpF = F;
   F = kInvalidFile;

llvm/lib/Support/Windows/Path.inc

@@ -1260,6 +1260,43 @@ Expected<size_t> readNativeFileSlice(file_t FileHandle,
   return readNativeFileImpl(FileHandle, Buf, &Overlapped);
 }
 
+std::error_code tryLockFile(int FD, std::chrono::milliseconds Timeout) {
+  DWORD Flags = LOCKFILE_EXCLUSIVE_LOCK | LOCKFILE_FAIL_IMMEDIATELY;
+  OVERLAPPED OV = {0};
+  file_t File = convertFDToNativeFile(FD);
+  auto Start = std::chrono::steady_clock::now();
+  auto End = Start + Timeout;
+  do {
+    if (::LockFileEx(File, Flags, 0, MAXDWORD, MAXDWORD, &OV))
+      return std::error_code();
+    DWORD Error = ::GetLastError();
+    if (Error == ERROR_LOCK_VIOLATION) {
+      ::Sleep(1);
+      continue;
+    }
+    return mapWindowsError(Error);
+  } while (std::chrono::steady_clock::now() < End);
+  return mapWindowsError(ERROR_LOCK_VIOLATION);
+}
+
+std::error_code lockFile(int FD) {
+  DWORD Flags = LOCKFILE_EXCLUSIVE_LOCK;
+  OVERLAPPED OV = {0};
+  file_t File = convertFDToNativeFile(FD);
+  if (::LockFileEx(File, Flags, 0, MAXDWORD, MAXDWORD, &OV))
+    return std::error_code();
+  DWORD Error = ::GetLastError();
+  return mapWindowsError(Error);
+}
+
+std::error_code unlockFile(int FD) {
+  OVERLAPPED OV = {0};
+  file_t File = convertFDToNativeFile(FD);
+  if (::UnlockFileEx(File, 0, MAXDWORD, MAXDWORD, &OV))
+    return std::error_code();
+  return mapWindowsError(::GetLastError());
+}
+
 std::error_code closeFile(file_t &F) {
   file_t TmpF = F;
   F = kInvalidFile;

llvm/unittests/Support/ProgramTest.cpp

@@ -14,6 +14,7 @@
 #include "llvm/Support/Path.h"
 #include "gtest/gtest.h"
 #include <stdlib.h>
+#include <thread>
 #if defined(__APPLE__)
 # include <crt_externs.h>
 #elif !defined(_MSC_VER)
@@ -361,4 +362,57 @@ TEST_F(ProgramEnvTest, TestExecuteAndWaitStatistics) {
   ASSERT_GE(ProcStat->TotalTime, ProcStat->UserTime);
 }
 
+TEST_F(ProgramEnvTest, TestLockFile) {
+  using namespace llvm::sys;
+
+  if (const char *LockedFile = getenv("LLVM_PROGRAM_TEST_LOCKED_FILE")) {
+    // Child process.
+    int FD2;
+    ASSERT_NO_ERROR(fs::openFileForReadWrite(LockedFile, FD2,
+                                             fs::CD_OpenExisting, fs::OF_None));
+
+    std::error_code ErrC = fs::tryLockFile(FD2, std::chrono::seconds(5));
+    ASSERT_NO_ERROR(ErrC);
+    ASSERT_NO_ERROR(fs::unlockFile(FD2));
+    close(FD2);
+    exit(0);
+  }
+
+  // Create file that will be locked.
+  SmallString<64> LockedFile;
+  int FD1;
+  ASSERT_NO_ERROR(
+      fs::createTemporaryFile("TestLockFile", "temp", FD1, LockedFile));
+
+  std::string Executable =
+      sys::fs::getMainExecutable(TestMainArgv0, &ProgramTestStringArg1);
+  StringRef argv[] = {Executable, "--gtest_filter=ProgramEnvTest.TestLockFile"};
+
+  // Add LLVM_PROGRAM_TEST_LOCKED_FILE to the environment of the child.
+  std::string EnvVar = "LLVM_PROGRAM_TEST_LOCKED_FILE=";
+  EnvVar += LockedFile.str();
+  addEnvVar(EnvVar);
+
+  // Lock the file.
+  ASSERT_NO_ERROR(fs::tryLockFile(FD1));
+
+  std::string Error;
+  bool ExecutionFailed;
+  ProcessInfo PI2 = ExecuteNoWait(Executable, argv, getEnviron(), {}, 0, &Error,
+                                  &ExecutionFailed);
+  ASSERT_FALSE(ExecutionFailed) << Error;
+  ASSERT_TRUE(Error.empty());
+  ASSERT_NE(PI2.Pid, ProcessInfo::InvalidPid) << "Invalid process id";
+
+  // Wait some time to give the child process a chance to start.
+  std::this_thread::sleep_for(std::chrono::milliseconds(100));
+
+  ASSERT_NO_ERROR(fs::unlockFile(FD1));
+  ProcessInfo WaitResult = llvm::sys::Wait(PI2, 5 /* seconds */, true, &Error);
+  ASSERT_TRUE(Error.empty());
+  ASSERT_EQ(0, WaitResult.ReturnCode);
+  ASSERT_EQ(WaitResult.Pid, PI2.Pid);
+  sys::fs::remove(LockedFile);
+}
+
 } // end anonymous namespace