lib/model: Don't panic on failed chmod-back on directory (fixes #5836) (#5896)

* lib/model: Don't panic on failed chmod-back on directory (fixes #5836)

This makes the "in writable dir"-wrapper log chmod-back errors instead
of panicking. To do that we need a logger so the function moved into the
model package which is also the only place it's used. The tests came
along.

(The test also exercised osutil.RenameOrCopy like some sort of
piggybacking. I removed that.)

lib/osutil/osutil.go

@@ -8,7 +8,6 @@
 package osutil
 
 import (
-	"errors"
 	"io"
 	"path/filepath"
 	"runtime"
@@ -81,38 +80,6 @@ func Copy(src, dst fs.Filesystem, from, to string) (err error) {
 	})
 }
 
-// InWritableDir calls fn(path), while making sure that the directory
-// containing `path` is writable for the duration of the call.
-func InWritableDir(fn func(string) error, fs fs.Filesystem, path string) error {
-	dir := filepath.Dir(path)
-	info, err := fs.Stat(dir)
-	if err != nil {
-		return err
-	}
-	if !info.IsDir() {
-		return errors.New("Not a directory: " + path)
-	}
-	if info.Mode()&0200 == 0 {
-		// A non-writeable directory (for this user; we assume that's the
-		// relevant part). Temporarily change the mode so we can delete the
-		// file or directory inside it.
-		err = fs.Chmod(dir, 0755)
-		if err == nil {
-			defer func() {
-				err = fs.Chmod(dir, info.Mode())
-				if err != nil {
-					// We managed to change the permission bits like a
-					// millisecond ago, so it'd be bizarre if we couldn't
-					// change it back.
-					panic(err)
-				}
-			}()
-		}
-	}
-
-	return fn(path)
-}
-
 // Tries hard to succeed on various systems by temporarily tweaking directory
 // permissions and removing the destination file when necessary.
 func withPreparedTarget(filesystem fs.Filesystem, from, to string, f func() error) error {