[Support] Add mapped_file_region::sync(), equivalent to msync

llvm/include/llvm/Support/FileSystem.h

@@ -1342,6 +1342,11 @@ class mapped_file_region {
   LLVM_ABI size_t size() const;
   LLVM_ABI char *data() const;
 
+  /// Write changes to disk and synchronize. Equivalent to POSIX msync. This
+  /// will wait for flushing memory-mapped region back to disk and can be very
+  /// slow.
+  LLVM_ABI std::error_code sync() const;
+
   /// Get a const view of the data. Modifying this memory has undefined
   /// behavior.
   LLVM_ABI const char *const_data() const;

llvm/lib/Support/Unix/Path.inc

@@ -876,6 +876,12 @@ void mapped_file_region::unmapImpl() {
     ::munmap(Mapping, Size);
 }
 
+std::error_code mapped_file_region::sync() const {
+  if (int Res = ::msync(Mapping, Size, MS_SYNC))
+    return std::error_code(Res, std::generic_category());
+  return std::error_code();
+}
+
 void mapped_file_region::dontNeedImpl() {
   assert(Mode == mapped_file_region::readonly);
   if (!Mapping)

llvm/lib/Support/Windows/Path.inc

@@ -1006,6 +1006,14 @@ void mapped_file_region::unmapImpl() {
 
 void mapped_file_region::dontNeedImpl() {}
 
+std::error_code mapped_file_region::sync() const {
+  if (!::FlushViewOfFile(Mapping, Size))
+    return mapWindowsError(GetLastError());
+  if (!::FlushFileBuffers(FileHandle))
+    return mapWindowsError(GetLastError());
+  return std::error_code();
+}
+
 int mapped_file_region::alignment() {
   SYSTEM_INFO SysInfo;
   ::GetSystemInfo(&SysInfo);

llvm/unittests/Support/Path.cpp

@@ -1471,6 +1471,43 @@ TEST_F(FileSystemTest, FileMapping) {
   ASSERT_NO_ERROR(fs::remove(TempPath));
 }
 
+TEST_F(FileSystemTest, FileMappingSync) {
+  // Create a temp file.
+  SmallString<0> TempPath(TestDirectory);
+  sys::path::append(TempPath, "test-%%%%");
+  auto TempFileOrError = fs::TempFile::create(TempPath);
+  ASSERT_TRUE((bool)TempFileOrError);
+  fs::TempFile File = std::move(*TempFileOrError);
+  StringRef Content("hello there");
+  std::string FileName = File.TmpName;
+  ASSERT_NO_ERROR(
+      fs::resize_file_before_mapping_readwrite(File.FD, Content.size()));
+  {
+    // Map in the file and write some content.
+    std::error_code EC;
+    fs::mapped_file_region MFR(fs::convertFDToNativeFile(File.FD),
+                               fs::mapped_file_region::readwrite,
+                               Content.size(), 0, EC);
+
+    // Keep the file so it can be read.
+    ASSERT_FALSE((bool)File.keep());
+
+    // Write content through mapped memory.
+    ASSERT_NO_ERROR(EC);
+    std::copy(Content.begin(), Content.end(), MFR.data());
+
+    // Synchronize to file system.
+    ASSERT_FALSE((bool)MFR.sync());
+
+    // Check the file content using file IO APIs.
+    auto Buffer = MemoryBuffer::getFile(FileName);
+    ASSERT_TRUE((bool)Buffer);
+    ASSERT_EQ(Content, Buffer->get()->getBuffer());
+  }
+  // Manually remove the test file.
+  ASSERT_FALSE((bool)fs::remove(FileName));
+}
+
 TEST(Support, NormalizePath) {
   //                           Input,        Expected Win, Expected Posix
   using TestTuple = std::tuple<const char *, const char *, const char *>;