As updatedb is currently implemented with find it is possible to use patterns which are then passed as prune arguments. So this is a workaround I use: /path/to/directory?with?space and it works. Of course it matches a bit too much but I have not had problems with that. Of course you could use also some other pattern-based workaround.

Sounds great.  Moving away from the shell to avoid (or at least reduce) quoting problems and supporting multiple --prunepath options are definitely good moves.  And maintaining backward compatibility by providing a new option with the correct semantics makes sense -- just so long as the correct semantics are available!

I did a quick scan of the C implementation of updatedb, and it doesn't look like it implements the desired semantics yet, so I haven't tried it out.  But I'm looking forward to the new version!

FindUtils 4.2.20 on Mac OS X, although a brief look at the source for 4.2.33 suggests that it shares this behavior.

Because the --prunepaths switch takes a space-separated list of directories as its argument, there's no way to include a directory whose name contains whitespace as part of this argument.  The script interprets it as two separate directories.  Trying to escape the spaces with backslashes or quotes doesn't work; the escape characters get interpreted as part of the directory name as well.