mem.ReadAt/mem.WriteAt violates spec and subject to race conditions

[k3an3]

Note in the Go 1.23.3 docs for the io.WriterAt interface:

If WriteAt is writing to a destination with a seek offset, WriteAt should not affect nor be affected by the underlying seek offset.

Clients of WriteAt can execute parallel WriteAt calls on the same destination if the ranges do not overlap.

However, the current implementations of MemFS ReadAt/WriteAt look like this, respectively:

afero/mem/file.go

Lines 226 to 232 in 5c4385a

	func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
	prev := atomic.LoadInt64(&f.at)
	atomic.StoreInt64(&f.at, off)
	n, err = f.Read(b)
	atomic.StoreInt64(&f.at, prev)
	return
	}

afero/mem/file.go

Lines 300 to 303 in 5c4385a

	func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
	atomic.StoreInt64(&f.at, off)
	return f.Write(b)
	}

If multiple calls are made in parallel on the same file, a race will occur between modifying the stored file offset and reading/writing the file, causing these functions to return or write incorrect data. Additionally, it is not expected that the underlying seek offset is modified at all. A corrected implementation will likely have to extract and re-implement parts of Read/Write in order to not affect the saved offset.