diff --git a/llvm/include/llvm/Support/FileSystem.h b/llvm/include/llvm/Support/FileSystem.h
--- a/llvm/include/llvm/Support/FileSystem.h
+++ b/llvm/include/llvm/Support/FileSystem.h
@@ -1131,6 +1131,28 @@
 openNativeFileForRead(const Twine &Name, OpenFlags Flags = OF_None,
                       SmallVectorImpl<char> *RealPath = nullptr);
 
+/// Locks the file.
+///
+/// The function locks the entire file for exclusive access by the calling
+/// process.
+///
+/// @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.
+std::error_code tryLockFile(int FD,
+    std::chrono::milliseconds Timeout = std::chrono::milliseconds(0));
+
+/// 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
diff --git a/llvm/lib/Support/Unix/Path.inc b/llvm/lib/Support/Unix/Path.inc
--- a/llvm/lib/Support/Unix/Path.inc
+++ b/llvm/lib/Support/Unix/Path.inc
@@ -1052,6 +1052,41 @@
   return NumRead;
 }
 
+std::error_code lockFile(int FD, std::chrono::milliseconds Timeout) {
+  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;
+  unsigned T = Timeout.count();
+  if (T == 0)
+    T = 1;
+  for (unsigned Counter = 0; Counter < T; ++Counter) {
+    if (::fcntl(FD, F_SETLK, &Lock) == -1) {
+      int Error = errno;
+      if (Error == EACCES || Error == EAGAIN) {
+        usleep(1000);
+        continue;
+      }
+      return std::error_code(Error, std::generic_category());
+    }
+    return std::error_code();
+  }
+  return std::error_code(EACCES, 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(errno, std::generic_category());
+  return std::error_code();
+}
+
 std::error_code closeFile(file_t &F) {
   file_t TmpF = F;
   F = kInvalidFile;
diff --git a/llvm/lib/Support/Windows/Path.inc b/llvm/lib/Support/Windows/Path.inc
--- a/llvm/lib/Support/Windows/Path.inc
+++ b/llvm/lib/Support/Windows/Path.inc
@@ -1260,6 +1260,38 @@
   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);
+  unsigned T = Timeout.count();
+  if (T == 0)
+    T = 1;
+  for (unsigned Counter = 0; Counter < T; ++Counter) {
+    if (::LockFileEx(File, Flags, 0, MAXDWORD, MAXDWORD, &OV)) {
+      return std::error_code();
+    } else {
+      DWORD Error = ::GetLastError();
+      if (Error == ERROR_LOCK_VIOLATION) {
+        ::Sleep(1);
+        continue;
+      } else {
+        return mapWindowsError(Error);
+      }
+    }
+  }
+  return mapWindowsError(ERROR_LOCK_VIOLATION);
+}
+
+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();
+  else
+    return mapWindowsError(::GetLastError());
+}
+
 std::error_code closeFile(file_t &F) {
   file_t TmpF = F;
   F = kInvalidFile;
diff --git a/llvm/unittests/Support/Path.cpp b/llvm/unittests/Support/Path.cpp
--- a/llvm/unittests/Support/Path.cpp
+++ b/llvm/unittests/Support/Path.cpp
@@ -1934,4 +1934,30 @@
 }
 #endif
 
+#ifdef _WIN32
+// Windows refuses lock request if file region is already locked by the same
+// process. POSIX system in this case updates the existing lock.
+TEST_F(FileSystemTest, lockFile) {
+  int FD1, FD2;
+  std::error_code EC;
+  SmallString<64> TempPath;
+  EC = fs::createTemporaryFile("test", "temp", FD1, TempPath);
+  ASSERT_NO_ERROR(EC);
+  FileRemover Cleanup(TempPath);
+  EC = fs::openFileForReadWrite(TempPath, FD2,
+                                fs::CD_OpenExisting, fs::OF_Append);
+  ASSERT_NO_ERROR(EC);
+  EC = fs::tryLockFile(FD1);
+  ASSERT_NO_ERROR(EC);
+  EC = fs::tryLockFile(FD2, std::chrono::milliseconds(5));
+  ASSERT_EQ(errc::no_lock_available, EC);
+  EC = fs::unlockFile(FD1);
+  ASSERT_NO_ERROR(EC);
+  EC = fs::tryLockFile(FD2);
+  ASSERT_NO_ERROR(EC);
+  EC = fs::unlockFile(FD2);
+  ASSERT_NO_ERROR(EC);
+}
+#endif
+
 } // anonymous namespace