Do not remove a target file in FileOutputBuffer::create().

[ruiu]

Summary

FileOutputBuffer::create() attempts to remove a target file if the file
is a regular one, which results in an unexpected result in a failure
scenario.

If something goes wrong and the user of FileOutputBuffer decides to not
call commit(), it leaves nothing. An existing file was removed, and no
new file was created.

What we should do is to atomically replace an existing file with a new
file using rename(), so that it wouldn't remove an existing file without
creating a new one.

[bd1976llvm]

Hi Ruiu,

I like this change however, it causes a change in behaviour in lld. Please take a look at: https://reviews.llvm.org/D38401.

llvm/trunk/lib/Support/FileOutputBuffer.cpp

switch (Stat.type()) {
     case sys::fs::file_type::directory_file:
       return errc::is_a_directory;
     default:
       if (EC)
         return EC;
       IsRegular = false;
   }
 
-  if (IsRegular) {
-    // Delete target file.
-    EC = sys::fs::remove(FilePath);
-    if (EC)
-      return EC;
-  }
-
   SmallString<128> TempFilePath;
   int FD;
   if (IsRegular) {
     unsigned Mode = sys::fs::all_read | sys::fs::all_write;
     // If requested, make the output file executable.
     if (Flags & F_executable)
       Mode |= sys::fs::all_exe;
     // Create new file in same directory but with random name.

 }
 
 std::error_code FileOutputBuffer::commit() {
   // Unmap buffer, letting OS flush dirty pages to file on disk.
   Region.reset();
 
   std::error_code EC;
   if (IsRegular) {
-    // Rename file to final name.
+    // Atomically replace the existing file with the new one.
     EC = sys::fs::rename(Twine(TempPath), Twine(FinalPath));
     sys::DontRemoveFileOnSignal(TempPath);
   } else {
     EC = sys::fs::copy_file(TempPath, FinalPath);
     std::error_code RMEC = sys::fs::remove(TempPath);
     sys::DontRemoveFileOnSignal(TempPath);
     if (RMEC)
       return RMEC;
   }
 
   return EC;
 }
 } // namespace