Allow to use "zero-copy" write

It can provide substantial performance benefit like 300->600 MB/s write (LOL),
but it also leads to significant performance degradation caused by inefficient
operations with the []FileBuffer inode.buffers list that grows over time.

internal/file.go

@@ -327,7 +327,7 @@ func (inode *Inode) ResizeUnlocked(newSize uint64) {
 	inode.Attributes.Size = newSize
 }
 
-func (fh *FileHandle) WriteFile(offset int64, data []byte) (err error) {
+func (fh *FileHandle) WriteFile(offset int64, data []byte, copyData bool) (err error) {
 	fh.inode.logFuse("WriteFile", offset, len(data))
 
 	end := uint64(offset)+uint64(len(data))
@@ -351,7 +351,7 @@ func (fh *FileHandle) WriteFile(offset int64, data []byte) (err error) {
 		fh.inode.ResizeUnlocked(end)
 	}
 
-	allocated := fh.inode.addBuffer(uint64(offset), data, BUF_DIRTY, true)
+	allocated := fh.inode.addBuffer(uint64(offset), data, BUF_DIRTY, copyData)
 
 	fh.inode.lastWriteEnd = end
 	if fh.inode.CacheState == ST_CACHED {

internal/goofys.go

@@ -1313,8 +1313,12 @@ func (fs *Goofys) WriteFile(
 	}
 	fs.mu.RUnlock()
 
-	err = fh.WriteFile(op.Offset, op.Data)
+	// fuse binding leaves extra room for header, so we
+	// account for it when we decide whether to do "zero-copy" write
+	copyData := len(op.Data) < cap(op.Data)-4096
+	err = fh.WriteFile(op.Offset, op.Data, copyData)
 	err = mapAwsError(err)
+	op.SuppressReuse = !copyData
 
 	return
 }