FileOutputBuffer : Fix undefined behavior on windows

[shankare]

Summary

A memory buffer backedup by a fd, shows undefined behavior if the fd is not resized appropriately. This shows up on file systems mounted on windows, the files end up with all bytes 0'ed.

[ruiu]

What is the problem you are trying to fix? The current code is working fine for both non-Windows and Windows. This patch would make this function very slow on Windows.

[shankare]

I was trying to debug a problem with the usage of FileOutputBuffer where the memory contents are not being synced to disk. I traced every call in FileOutputBuffer and none of the calls return an error, but the file is not synced. This happens on windows systems with FAST IO enabled. Resizing the underlying file makes the problem disappear.

[ruiu]

Please share minimal code to reproduce the problem to let me assess it.

[shankare]

I totally forgot about your comment, I will respond to you with a testcase shortly.

lib/Support/FileOutputBuffer.cpp

FileOutputBuffer::create(StringRef FilePath, size_t Size, unsigned Flags) {
   int FD;
   EC = sys::fs::createUniqueFile(Twine(FilePath) + ".tmp%%%%%%%", FD,
                                  TempFilePath, Mode);
   if (EC)
     return EC;
 
   sys::RemoveFileOnSignal(TempFilePath);
 
-#ifndef LLVM_ON_WIN32
-  // On Windows, CreateFileMapping (the mmap function on Windows)
-  // automatically extends the underlying file. We don't need to
-  // extend the file beforehand. _chsize (ftruncate on Windows) is
-  // pretty slow just like it writes specified amount of bytes,
-  // so we should avoid calling that.
+  // The behavior is defined by the file system. On filesystems
+  // backed by NFS, this is essential.
   EC = sys::fs::resize_file(FD, Size);
   if (EC)
     return EC;
-#endif
 
   auto MappedFile = llvm::make_unique<mapped_file_region>(
       FD, mapped_file_region::readwrite, Size, 0, EC);
   int Ret = close(FD);
   if (EC)
     return EC;
   if (Ret)
     return std::error_code(errno, std::generic_category());