doctor: add fs fault injection and lock contention coverage

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

cmd/bd/doctor/fix/fs.go

@@ -0,0 +1,57 @@
+package fix
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"syscall"
+)
+
+var (
+	renameFile = os.Rename
+	removeFile = os.Remove
+	openFileRO = os.Open
+	openFileRW = os.OpenFile
+)
+
+func moveFile(src, dst string) error {
+	if err := renameFile(src, dst); err == nil {
+		return nil
+	} else if isEXDEV(err) {
+		if err := copyFile(src, dst); err != nil {
+			return err
+		}
+		if err := removeFile(src); err != nil {
+			return fmt.Errorf("failed to remove source after copy: %w", err)
+		}
+		return nil
+	} else {
+		return err
+	}
+}
+
+func copyFile(src, dst string) error {
+	in, err := openFileRO(src) // #nosec G304 -- src is within the workspace
+	if err != nil {
+		return err
+	}
+	defer in.Close()
+	out, err := openFileRW(dst, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0644)
+	if err != nil {
+		return err
+	}
+	defer func() { _ = out.Close() }()
+	if _, err := io.Copy(out, in); err != nil {
+		return err
+	}
+	return out.Close()
+}
+
+func isEXDEV(err error) bool {
+	var linkErr *os.LinkError
+	if errors.As(err, &linkErr) {
+		return errors.Is(linkErr.Err, syscall.EXDEV)
+	}
+	return errors.Is(err, syscall.EXDEV)
+}