fix: file consistency issue on SFTP upload (#18422)

* creating a byte buffer for SFTP file segments
* Adding an error condition for when there are 
  remaining segments in the queue
* Simplification of the queue using a map

cmd/sftp-server-driver.go

@@ -201,19 +201,57 @@ func (f *sftpDriver) Fileread(r *sftp.Request) (ra io.ReaderAt, err error) {
 	return obj, nil
 }
 
-type writerAt struct {
-	w  *io.PipeWriter
-	wg *sync.WaitGroup
-}
-
-func (w *writerAt) Close() error {
-	err := w.w.Close()
+func (w *writerAt) Close() (err error) {
+	if len(w.buffer) > 0 {
+		err = w.w.CloseWithError(errors.New("some file segments were not flushed from the queue"))
+		for i := range w.buffer {
+			delete(w.buffer, i)
+		}
+	} else {
+		err = w.w.Close()
+	}
 	w.wg.Wait()
 	return err
 }
 
+type writerAt struct {
+	w      *io.PipeWriter
+	wg     *sync.WaitGroup
+	buffer map[int64][]byte
+
+	nextOffset int64
+	m          sync.Mutex
+}
+
 func (w *writerAt) WriteAt(b []byte, offset int64) (n int, err error) {
-	return w.w.Write(b)
+	w.m.Lock()
+	defer w.m.Unlock()
+
+	if w.nextOffset == offset {
+		n, err = w.w.Write(b)
+		w.nextOffset += int64(n)
+	} else {
+		w.buffer[offset] = make([]byte, len(b))
+		copy(w.buffer[offset], b)
+		n = len(b)
+	}
+
+again:
+	nextOut, ok := w.buffer[w.nextOffset]
+	if ok {
+		n, err = w.w.Write(nextOut)
+		delete(w.buffer, w.nextOffset)
+		w.nextOffset += int64(n)
+		if n != len(nextOut) {
+			return 0, fmt.Errorf("expected write size %d but wrote %d bytes", len(nextOut), n)
+		}
+		if err != nil {
+			return 0, err
+		}
+		goto again
+	}
+
+	return len(b), nil
 }
 
 func (f *sftpDriver) Filewrite(r *sftp.Request) (w io.WriterAt, err error) {
@@ -238,7 +276,11 @@ func (f *sftpDriver) Filewrite(r *sftp.Request) (w io.WriterAt, err error) {
 
 	pr, pw := io.Pipe()
 
-	wa := &writerAt{w: pw, wg: &sync.WaitGroup{}}
+	wa := &writerAt{
+		buffer: make(map[int64][]byte),
+		w:      pw,
+		wg:     &sync.WaitGroup{},
+	}
 	wa.wg.Add(1)
 	go func() {
 		_, err := clnt.PutObject(r.Context(), bucket, object, pr, -1, minio.PutObjectOptions{SendContentMd5: true})