httpd: disable Deflater middleware by default on Perl <5.18
Testing with perl-5.16.3-294.el7_6 RPM package on RHEL/CentOS 7,
the Deflater middleware triggers a leak when used in conjunction
with our push-based responses from PublicInbox::Qspawn.
I could not find another solution to workaround the memory leak
in this case, and I could not find a specific leak fix in
the perl5180delta manpage[1] which looked like it would
solve our problem.
Attempting to workaround the issue proved futile. Using
internal Deflater-specific keys to prevent deflating in
GitHTTPBackend and Qspawn did not solve the problem:
So this appears to be a problem with Plack::Util::response_cb
somewhere.
This does NOT appear to be a problem with ref() leaking as in
DS::next_tick[2], since I couldn't find where
Plack::Middleware::Deflater or Plack::Util::response_cb would be
calling ref() on a blessed reference to trigger a leak.
Also, oddly enough, the ref() use for backwards compatibility at
the top of PublicInbox::GitHTTPBackend::serve does NOT seem to
trigger a leak on 5.16.3 due to [2]:
# XXX compatibility... ugh, can we stop supporting this?
$git = PublicInbox::Git->new($git) unless ref($git);