termstatus: allow retrieving the underlying writer

This is intended for special cases where it must be guaranteed that the
output on stdout exactly matches what was written to the io.Writer.

internal/ui/mock.go

@@ -1,5 +1,9 @@
 package ui
 
+import "io"
+
+var _ Terminal = &MockTerminal{}
+
 type MockTerminal struct {
 	Output []string
 	Errors []string
@@ -20,3 +24,7 @@ func (m *MockTerminal) SetStatus(lines []string) {
 func (m *MockTerminal) CanUpdateStatus() bool {
 	return true
 }
+
+func (m *MockTerminal) OutputRaw() io.Writer {
+	return nil
+}

internal/ui/terminal.go

@@ -1,5 +1,7 @@
 package ui
 
+import "io"
+
 // Terminal is used to write messages and display status lines which can be
 // updated. See termstatus.Terminal for a concrete implementation.
 type Terminal interface {
@@ -7,4 +9,5 @@ type Terminal interface {
 	Error(line string)
 	SetStatus(lines []string)
 	CanUpdateStatus() bool
+	OutputRaw() io.Writer
 }

internal/ui/termstatus/status.go

@@ -12,8 +12,11 @@ import (
 	"golang.org/x/text/width"
 
 	"github.com/restic/restic/internal/terminal"
+	"github.com/restic/restic/internal/ui"
 )
 
+var _ ui.Terminal = &Terminal{}
+
 // Terminal is used to write messages and display status lines which can be
 // updated. When the output is redirected to a file, the status lines are not
 // printed.
@@ -83,6 +86,13 @@ func (t *Terminal) CanUpdateStatus() bool {
 	return t.canUpdateStatus
 }
 
+// OutputRaw returns the output writer. Should only be used if there is no
+// other option. Must not be used in combination with Print, Error, SetStatus
+// or any other method that writes to the terminal.
+func (t *Terminal) OutputRaw() io.Writer {
+	return t.wr
+}
+
 // Run updates the screen. It should be run in a separate goroutine. When
 // ctx is cancelled, the status lines are cleanly removed.
 func (t *Terminal) Run(ctx context.Context) {