It would be useful for us if an optional script could be specified with the
build periodically trigger such that if the script returns an exit code of 0 the
build will be started. If the exit code is non zero the build isn't started.

e.g. a build periodically spec of:

0 1 * * * /home/loadbuilder/buildscripts/genbuild

Would run the genbuild script on the target host and if genbuild had an exit
code of 0 the build would proceed. Otherwise the build would be ignored.

The problem we have is that our build process checks in files to the source
repository as part of the build process (a build branding tag). This will kick
off a build because repository files have changed. We'd like to use the script
to see if the repository changes are relevant to trigger a build and ignore
files checked in by the last build.

Right now, we run the script at the start of the build script, but this causes a
new Hudson build even though nothing has been built.