scanner: Use context only for cancellation

When the scanner is slower than the actual backup, the tomb cancels the
context passed to Scan(), which then returns ctx.Err(). In the end, the
main function prints an error message that is not helpful ("Context
cancelled") and exits with an error code although no error occurred.

The code now ignores the error in the context and just uses it for
cancellation. The scanner is not supposed to return an error anyway.

Closes #1978

internal/archiver/scanner.go

@@ -52,20 +52,17 @@ func (s *Scanner) Scan(ctx context.Context, targets []string) error {
 		}
 
 		if ctx.Err() != nil {
-			return ctx.Err()
+			return nil
 		}
 	}
 
-	if ctx.Err() != nil {
-		return ctx.Err()
-	}
 	s.Result("", stats)
 	return nil
 }
 
 func (s *Scanner) scan(ctx context.Context, stats ScanStats, target string) (ScanStats, error) {
 	if ctx.Err() != nil {
-		return stats, ctx.Err()
+		return stats, nil
 	}
 
 	// exclude files by path before running stat to reduce number of lstat calls
@@ -89,10 +86,6 @@ func (s *Scanner) scan(ctx context.Context, stats ScanStats, target string) (Sca
 		stats.Files++
 		stats.Bytes += uint64(fi.Size())
 	case fi.Mode().IsDir():
-		if ctx.Err() != nil {
-			return stats, ctx.Err()
-		}
-
 		names, err := readdirnames(s.FS, target)
 		if err != nil {
 			return stats, s.Error(target, fi, err)
@@ -109,9 +102,6 @@ func (s *Scanner) scan(ctx context.Context, stats ScanStats, target string) (Sca
 		stats.Others++
 	}
 
-	if ctx.Err() != nil {
-		return stats, ctx.Err()
-	}
 	s.Result(target, stats)
 	return stats, nil
 }

internal/archiver/scanner_test.go

@@ -289,7 +289,7 @@ func TestScannerCancel(t *testing.T) {
 		"other": TestFile{Content: "other"},
 	}
 
-	result := ScanStats{Files: 2, Bytes: 6}
+	result := ScanStats{Files: 2, Dirs: 1, Bytes: 6}
 
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
@@ -319,12 +319,8 @@ func TestScannerCancel(t *testing.T) {
 	}
 
 	err = sc.Scan(ctx, []string{"."})
-	if err == nil {
-		t.Errorf("did not find expected error")
-	}
-
-	if err != context.Canceled {
-		t.Errorf("unexpected error found, want %v, got %v", context.Canceled, err)
+	if err != nil {
+		t.Errorf("unexpected error %v found", err)
 	}
 
 	if lastStats != result {