Function to do runtime relative directory mapping

Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > Tom Lane wrote:
> >> I guess what you are saying is we should have a configure-time option to
> >> address configured directories via relative paths from the executable's
> >> directory, rather than absolute paths? Seems reasonable ...
>
> > Yep. In fact, why would we not use that by default?
>
> Because it'll be slower. Instead of
> /usr/local/pgsql/lib
> we'd be using something like
> /usr/local/pgsql/bin/../lib
> which is not too bad here but would get worse if the directories are not
> so close.
>
> But perhaps we can arrange for the path to be simplified down to an
> absolute form when it's constructed at backend startup? You'd need a
> routine anyway to combine the bindir path (determined by FindExec) with
> the relative path provided by configure, so maybe this routine could be
> smart about leading ../ in the configure path.

I wrote relative_path() which does the mapping from compiled src/dst to
a new path. For example:

$ tst1 /usr/local/pgsql/bin /usr/local/pgsql/lib /users/fred/pgsql/bin
/users/fred/pgsql/lib

$ tst1 /a/b/c /a/d /f/g/h
/f/d

This can be used in the backend to map from your bindir to a relative
libdir at runtime, and share too. It returns [mp]alloc'ed values, or
NULL if it can't do the tranformation.

Patch attached.

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