sftpd: return the error Operation Unsupported for unexpected reads

a cloud based file cannot be opened for read and write at the same
time. Return a proper error if a client try to do this.

It can happen only for SFTP

sftpd/internal_test.go

@@ -1680,6 +1680,13 @@ func TestTransferFailingReader(t *testing.T) {
 	err = tr.Close()
 	assert.NoError(t, err)
 
+	tr = newTransfer(baseTransfer, nil, nil, errRead)
+	_, err = tr.ReadAt(buf, 0)
+	assert.EqualError(t, err, errRead.Error())
+
+	err = tr.Close()
+	assert.NoError(t, err)
+
 	err = os.Remove(fsPath)
 	assert.NoError(t, err)
 	assert.Len(t, connection.GetTransfers(), 0)

sftpd/transfer.go

@@ -32,6 +32,9 @@ func (r *failingReader) ReadAt(p []byte, off int64) (n int, err error) {
 }
 
 func (r *failingReader) Close() error {
+	if r.innerReader == nil {
+		return nil
+	}
 	return r.innerReader.Close()
 }
 
@@ -70,6 +73,12 @@ func newTransfer(baseTransfer *common.BaseTransfer, pipeWriter *vfs.PipeWriter,
 			}
 		}
 	}
+	if baseTransfer.File == nil && errForRead != nil && pipeReader == nil {
+		reader = &failingReader{
+			innerReader: nil,
+			errRead:     errForRead,
+		}
+	}
 	return &transfer{
 		BaseTransfer: baseTransfer,
 		writerAt:     writer,