Re: [PATCH v2] Add a setting to require a filter to be successful

A few test in t0021 use 'false' as the filter, which can exit without
reading any byte from us, before we start writing and causes us to die
with SIGPIPE, leading to intermittent test failure.  I think treating this
as a failure of running the filter (the end user's filter should read what
is fed in full, produce its output and write the result back to us) is the
right thing to do, and this patch needs more work to handle such a
situation better, probably by using sigchain_push(SIGPIPE) or something.