Perl's "open '-|' $program" feature, which is used to pipe the output of
a process to a filehandle variable, is implemented internally using
pipes along with a fork/exec construct. The pipes are created, your
process automatically forks, and the child exec(2)'s the command you
wished to run.

This is all documented, and I thought I had a handle on this, but Perl's
open() function's error checking is based only on whether fork(2)
succeeded, not on whether exec(2) succeeded.

What I think we get when we don't check for errors is some kind of
raised signal (e.g. SIGPIPE) which is then duly received by the existing
async-IPC code (which is already doing multi-proc), which then kills the
IPC procs due to the 'impossible SIGPIPE' happening and crashes
kdesrc-build.

This fix, to check for an executable before trying to run, should at
least make this kind of error more apparent (this isn't perfect yet
either since 'bzr' failing to exist during pretend mode should be listed
as an update failure, not update success). I would still need to find a
better way to handle this since this is still a race condition.

CCBUG:345425