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.go

@@ -1,11 +1,15 @@
 package restorer
 
 import (
+	"fmt"
+	stdfs "io/fs"
 	"os"
 	"sync"
+	"syscall"
 
 	"github.com/cespare/xxhash/v2"
 	"github.com/restic/restic/internal/debug"
+	"github.com/restic/restic/internal/errors"
 	"github.com/restic/restic/internal/fs"
 )
 
@@ -39,13 +43,26 @@ func newFilesWriter(count int) *filesWriter {
 	}
 }
 
-func createFile(path string, createSize int64, sparse bool) (*os.File, error) {
-	f, err := os.OpenFile(path, os.O_CREATE|os.O_WRONLY, 0600)
+func openFile(path string) (*os.File, error) {
+	f, err := os.OpenFile(path, os.O_WRONLY|fs.O_NOFOLLOW, 0600)
 	if err != nil {
-		if !fs.IsAccessDenied(err) {
-			return nil, err
-		}
+		return nil, err
+	}
+	fi, err := f.Stat()
+	if err != nil {
+		_ = f.Close()
+		return nil, err
+	}
+	if !fi.Mode().IsRegular() {
+		_ = f.Close()
+		return nil, fmt.Errorf("unexpected file type %v at %q", fi.Mode().Type(), path)
+	}
+	return f, nil
+}
 
+func createFile(path string, createSize int64, sparse bool) (*os.File, error) {
+	f, err := os.OpenFile(path, os.O_CREATE|os.O_WRONLY|fs.O_NOFOLLOW, 0600)
+	if err != nil && fs.IsAccessDenied(err) {
 		// If file is readonly, clear the readonly flag by resetting the
 		// permissions of the file and try again
 		// as the metadata will be set again in the second pass and the
@@ -53,40 +70,75 @@ func createFile(path string, createSize int64, sparse bool) (*os.File, error) {
 		if err = fs.ResetPermissions(path); err != nil {
 			return nil, err
 		}
-		if f, err = os.OpenFile(path, os.O_WRONLY, 0600); err != nil {
+		if f, err = os.OpenFile(path, os.O_WRONLY|fs.O_NOFOLLOW, 0600); err != nil {
+			return nil, err
+		}
+	} else if err != nil && (errors.Is(err, syscall.ELOOP) || errors.Is(err, syscall.EISDIR)) {
+		// symlink or directory, try to remove it later on
+		f = nil
+	} else if err != nil {
+		return nil, err
+	}
+
+	var fi stdfs.FileInfo
+	if f != nil {
+		// stat to check that we've opened a regular file
+		fi, err = f.Stat()
+		if err != nil {
+			_ = f.Close()
+			return nil, err
+		}
+	}
+	if f == nil || !fi.Mode().IsRegular() {
+		// close handle if we still have it
+		if f != nil {
+			if err := f.Close(); err != nil {
+				return nil, err
+			}
+		}
+
+		// not what we expected, try to get rid of it
+		if err := os.Remove(path); err != nil {
+			return nil, err
+		}
+		// create a new file, pass O_EXCL to make sure there are no surprises
+		f, err = os.OpenFile(path, os.O_CREATE|os.O_WRONLY|os.O_EXCL|fs.O_NOFOLLOW, 0600)
+		if err != nil {
+			return nil, err
+		}
+		fi, err = f.Stat()
+		if err != nil {
+			_ = f.Close()
 			return nil, err
 		}
 	}
 
+	return ensureSize(f, fi, createSize, sparse)
+}
+
+func ensureSize(f *os.File, fi stdfs.FileInfo, createSize int64, sparse bool) (*os.File, error) {
 	if sparse {
-		err = truncateSparse(f, createSize)
+		err := truncateSparse(f, createSize)
 		if err != nil {
 			_ = f.Close()
 			return nil, err
 		}
-	} else {
-		info, err := f.Stat()
+	} else if fi.Size() > createSize {
+		// file is too long must shorten it
+		err := f.Truncate(createSize)
 		if err != nil {
 			_ = f.Close()
 			return nil, err
 		}
-		if info.Size() > createSize {
-			// file is too long must shorten it
-			err = f.Truncate(createSize)
-			if err != nil {
-				_ = f.Close()
-				return nil, err
-			}
-		} else if createSize > 0 {
-			err := fs.PreallocateFile(f, createSize)
-			if err != nil {
-				// Just log the preallocate error but don't let it cause the restore process to fail.
-				// Preallocate might return an error if the filesystem (implementation) does not
-				// support preallocation or our parameters combination to the preallocate call
-				// This should yield a syscall.ENOTSUP error, but some other errors might also
-				// show up.
-				debug.Log("Failed to preallocate %v with size %v: %v", path, createSize, err)
-			}
+	} else if createSize > 0 {
+		err := fs.PreallocateFile(f, createSize)
+		if err != nil {
+			// Just log the preallocate error but don't let it cause the restore process to fail.
+			// Preallocate might return an error if the filesystem (implementation) does not
+			// support preallocation or our parameters combination to the preallocate call
+			// This should yield a syscall.ENOTSUP error, but some other errors might also
+			// show up.
+			debug.Log("Failed to preallocate %v with size %v: %v", f.Name(), createSize, err)
 		}
 	}
 	return f, nil
@@ -110,7 +162,7 @@ func (w *filesWriter) writeToFile(path string, blob []byte, offset int64, create
 			if err != nil {
 				return nil, err
 			}
-		} else if f, err = os.OpenFile(path, os.O_WRONLY, 0600); err != nil {
+		} else if f, err = openFile(path); err != nil {
 			return nil, err
 		}