[PATCH 3/2] qspawn: workaround Perl 5.16.3 leak, re-enable Deflater

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

The httpd-supplied write callback is the leak culprit under Perl
5.16.3.  undef-ing it immediately after use keeps a repeated
"git fetch" loop from monotonically increasing memory and FD use
on the Perl shipped with RHEL/CentOS 7.x.

Other endpoints tested showed no increase in memory use under
constant load with "ab -HAccept-Encoding:gzip -k", including the
async psgi_qx code path used by $INBOX_URL/$OBJECT_ID/s/ via
SolverGit module.
---
 Note: I initially tried this change, but thought it only slowed
 down the leaking because I had not yet discovered the
 workaround in commit cd71a869c7e9c811
 ("ds: workaround a memory leak in Perl 5.16.x").

 Now that both leaks are worked around, memory usage is completely
 flat when repeating a single request of any type with gzip-enabled.

 lib/PublicInbox/Qspawn.pm | 4 ++++
 script/public-inbox-httpd | 8 +-------
 2 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/lib/PublicInbox/Qspawn.pm b/lib/PublicInbox/Qspawn.pm
index 5a30064..cb3dc51 100644
--- a/lib/PublicInbox/Qspawn.pm
+++ b/lib/PublicInbox/Qspawn.pm
@@ -281,6 +281,10 @@ sub psgi_return {
 								$buf, $filter);
 			$wcb->($r);
 		}
+
+		# Workaround a leak under Perl 5.16.3 when combined with
+		# Plack::Middleware::Deflater:
+		$wcb = undef;
 	};
 	$limiter ||= $def_limiter ||= PublicInbox::Qspawn::Limiter->new(32);
 	my $start_cb = sub { # may run later, much later...
diff --git a/script/public-inbox-httpd b/script/public-inbox-httpd
index 9b869f9..b2464f4 100755
--- a/script/public-inbox-httpd
+++ b/script/public-inbox-httpd
@@ -24,13 +24,7 @@ my $refresh = sub {
 		my $www = PublicInbox::WWW->new;
 		$www->preload;
 		$app = builder {
-			# 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 {
+			eval {
 				enable 'Deflater',
 					content_type => [ qw(
 						text/html
-- 
EW