restore: allow overwrite to replace empty directories and symlinks

With an already existing file tree an old directory or symlink may exist
in a place where restore wants to create a new file. Thus, check for
unexpected file types and clean up if necessary.

internal/restorer/fileswriter_test.go

@@ -1,9 +1,13 @@
 package restorer
 
 import (
+	"fmt"
 	"os"
+	"path/filepath"
+	"syscall"
 	"testing"
 
+	"github.com/restic/restic/internal/errors"
 	rtest "github.com/restic/restic/internal/test"
 )
 
@@ -34,3 +38,76 @@ func TestFilesWriterBasic(t *testing.T) {
 	rtest.OK(t, err)
 	rtest.Equals(t, []byte{2, 2}, buf)
 }
+
+func TestCreateFile(t *testing.T) {
+	basepath := filepath.Join(t.TempDir(), "test")
+
+	scenarios := []struct {
+		name   string
+		create func(t testing.TB, path string)
+		err    error
+	}{
+		{
+			"file",
+			func(t testing.TB, path string) {
+				rtest.OK(t, os.WriteFile(path, []byte("test-test-test-data"), 0o400))
+			},
+			nil,
+		},
+		{
+			"empty dir",
+			func(t testing.TB, path string) {
+				rtest.OK(t, os.Mkdir(path, 0o400))
+			},
+			nil,
+		},
+		{
+			"symlink",
+			func(t testing.TB, path string) {
+				rtest.OK(t, os.Symlink("./something", path))
+			},
+			nil,
+		},
+		{
+			"filled dir",
+			func(t testing.TB, path string) {
+				rtest.OK(t, os.Mkdir(path, 0o700))
+				rtest.OK(t, os.WriteFile(filepath.Join(path, "file"), []byte("data"), 0o400))
+			},
+			syscall.ENOTEMPTY,
+		},
+	}
+
+	tests := []struct {
+		size     int64
+		isSparse bool
+	}{
+		{5, false},
+		{21, false},
+		{100, false},
+		{5, true},
+		{21, true},
+		{100, true},
+	}
+
+	for i, sc := range scenarios {
+		t.Run(sc.name, func(t *testing.T) {
+			for _, test := range tests {
+				path := basepath + fmt.Sprintf("%v", i)
+				sc.create(t, path)
+				f, err := createFile(path, test.size, test.isSparse)
+				if sc.err == nil {
+					rtest.OK(t, err)
+					fi, err := f.Stat()
+					rtest.OK(t, err)
+					rtest.Assert(t, fi.Mode().IsRegular(), "wrong filetype %v", fi.Mode())
+					rtest.Assert(t, fi.Size() <= test.size, "unexpected file size expected %v, got %v", test.size, fi.Size())
+					rtest.OK(t, f.Close())
+				} else {
+					rtest.Assert(t, errors.Is(err, sc.err), "unexpected error got %v expected %v", err, sc.err)
+				}
+				rtest.OK(t, os.RemoveAll(path))
+			}
+		})
+	}
+}