[PATCH 2/2] httpd: disable Deflater middleware by default on Perl <5.18

[Eric Wong <e@80x24.org>]

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:

        $env->{"plack.skip-deflater"} = 1;
        $env->{"psgix.no-compress"} = 1;

Nor did forcing an invalid encoding via "git fetch":

	git -c http.extraheader=Accept-Encoding:gzap fetch

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);

[1] https://perldoc.perl.org/perl5180delta.html
[2] https://rt.perl.org/Public/Bug/Display.html?id=114340
---
 script/public-inbox-httpd | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/script/public-inbox-httpd b/script/public-inbox-httpd
index b2464f4..9b869f9 100755
--- a/script/public-inbox-httpd
+++ b/script/public-inbox-httpd
@@ -24,7 +24,13 @@ my $refresh = sub {
 		my $www = PublicInbox::WWW->new;
 		$www->preload;
 		$app = builder {
-			eval {
+			# Perl 5.16.3 leaks in our "push" response code path
+			# (e.g. Qspawn) due to something in
+			# Plack::Util::response_cb, regardless of whether the
+			# client is sending Accept-Encoding:gzip requests.
+			# perl5180delta documents many leak fixes, so assume
+			# 5.18+ is safe for now and bump the check as-need:
+			$] >= 5.018000 and eval {
 				enable 'Deflater',
 					content_type => [ qw(
 						text/html
-- 
EW