diff --git a/llvm/include/llvm/Support/raw_ostream.h b/llvm/include/llvm/Support/raw_ostream.h
--- a/llvm/include/llvm/Support/raw_ostream.h
+++ b/llvm/include/llvm/Support/raw_ostream.h
@@ -410,6 +410,12 @@
 
   uint64_t pos = 0;
 
+  /// If set, the stream tries to lock underlying file before writing to it.
+  bool LockForUpdate = false;
+
+  /// Time in milliseconds during which the stream tries to lock file.
+  unsigned LockTimeout = 1000;
+
   /// See raw_ostream::write_impl.
   void write_impl(const char *Ptr, size_t Size) override;
 
@@ -476,6 +482,15 @@
 
   void enable_colors(bool enable) override { ColorEnabled = enable; }
 
+  /// Requires locking file before write to it.
+  void lockForUpdate(bool X) { LockForUpdate = X; }
+  bool lockForUpdate() const { return LockForUpdate; }
+
+  /// Sets time in milliseconds during which the stream tries to lock
+  /// underlying file.
+  void lockTimeout(unsigned X) { LockTimeout = X; }
+  unsigned lockTimeout() const { return LockTimeout; }
+
   std::error_code error() const { return EC; }
 
   /// Return the value of the flag in this raw_fd_ostream indicating whether an
diff --git a/llvm/lib/Support/raw_ostream.cpp b/llvm/lib/Support/raw_ostream.cpp
--- a/llvm/lib/Support/raw_ostream.cpp
+++ b/llvm/lib/Support/raw_ostream.cpp
@@ -704,6 +704,12 @@
   MaxWriteSize = 1024 * 1024 * 1024;
 #endif
 
+  if (LockForUpdate)
+    if (std::error_code EC = llvm::sys::fs::lockFile(FD, LockTimeout)) {
+      error_detected(EC);
+      return;
+    }
+
   do {
     size_t ChunkSize = std::min(Size, MaxWriteSize);
     ssize_t ret = ::write(FD, Ptr, ChunkSize);
@@ -735,6 +741,10 @@
     Ptr += ret;
     Size -= ret;
   } while (Size > 0);
+
+  if (LockForUpdate)
+    if (std::error_code EC = llvm::sys::fs::unlockFile(FD))
+      error_detected(EC);
 }
 
 void raw_fd_ostream::close() {
diff --git a/llvm/unittests/Support/raw_ostream_test.cpp b/llvm/unittests/Support/raw_ostream_test.cpp
--- a/llvm/unittests/Support/raw_ostream_test.cpp
+++ b/llvm/unittests/Support/raw_ostream_test.cpp
@@ -7,7 +7,9 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/SmallString.h"
+#include "llvm/Support/Errc.h"
 #include "llvm/Support/FileSystem.h"
+#include "llvm/Support/FileUtilities.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/raw_ostream.h"
 #include "gtest/gtest.h"
@@ -356,4 +358,41 @@
   { raw_fd_ostream("-", EC, sys::fs::OpenFlags::OF_None); }
   { raw_fd_ostream("-", EC, sys::fs::OpenFlags::OF_None); }
 }
+
+#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(raw_fd_ostreamTest, LockForUpdate) {
+  int FD;
+  std::error_code EC;
+  SmallString<64> TempPath;
+  EC = sys::fs::createTemporaryFile("test", "temp", FD, TempPath);
+  ASSERT_FALSE(EC);
+  FileRemover Cleanup(TempPath);
+
+  raw_fd_ostream Stream(TempPath, EC);
+  ASSERT_FALSE(EC);
+  Stream.lockForUpdate(true);
+  Stream.write("ABCDE", 5);
+  Stream.flush();
+  ASSERT_FALSE(Stream.error());
+  Stream.write("FGHIJ", 5);
+  Stream.flush();
+  ASSERT_FALSE(Stream.error());
+
+  EC = sys::fs::lockFile(FD);
+  ASSERT_FALSE(EC);
+  Stream.write("12345", 5);
+  Stream.flush();
+  ASSERT_EQ(errc::no_lock_available, Stream.error());
+
+  Stream.clear_error();
+  EC = sys::fs::unlockFile(FD);
+  ASSERT_FALSE(EC);
+  Stream.write("abcde", 5);
+  Stream.flush();
+  ASSERT_FALSE(Stream.error());
+}
+#endif
+
 }