scp: check for write errors

exits as soon as there is a write error instead of get the same error when
the transfer is closed

sftpd/internal_test.go

@@ -1534,7 +1534,18 @@ func TestSCPUploadFiledata(t *testing.T) {
 	addTransfer(&transfer)
 	err = scpCommand.getUploadFileData(0, &transfer)
 	if err != errTransferClosed {
-		t.Errorf("upload must fail, the transfer is already closed")
+		t.Errorf("upload must fail, the transfer is already closed, err: %v", err)
+	}
+	mockSSHChannel = MockChannel{
+		Buffer:       bytes.NewBuffer(buf),
+		StdErrBuffer: bytes.NewBuffer(stdErrBuf),
+		ReadError:    nil,
+		WriteError:   nil,
+	}
+	addTransfer(&transfer)
+	err = scpCommand.getUploadFileData(2, &transfer)
+	if !errors.Is(err, os.ErrClosed) {
+		t.Errorf("the file is closed and writes must fail, err: %v", err)
 	}
 	err = os.Remove(testfile)
 	if err != nil {

sftpd/scp.go

@@ -158,7 +158,12 @@ func (c *scpCommand) getUploadFileData(sizeToRead int64, transfer *Transfer) err
 				transfer.Close()
 				return err
 			}
-			transfer.WriteAt(buf[:n], sizeToRead-remaining)
+			_, err = transfer.WriteAt(buf[:n], sizeToRead-remaining)
+			if err != nil {
+				c.sendErrorMessage(err.Error())
+				transfer.Close()
+				return err
+			}
 			remaining -= int64(n)
 			if remaining <= 0 {
 				break