[PATCH] www+httpd: lower priority of large mbox downloads

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

While each git blob request is treated fairly w.r.t other git
blob requests, responses triggering thousands of git blob
requests can still noticeably increase latency for
less-expensive responses.

Move large mbox results and the nasty all.mbox endpoint to
a low priority queue which only fires once per-event loop
iteration.  This reduces the response time of short HTTP
responses while many gigantic mboxes are being downloaded
simultaneously, but still maximizes use of available I/O
when there's no inexpensive HTTP responses happening.

This only affects PublicInbox::WWW users who use
public-inbox-httpd, not generic PSGI servers.
---
 lib/PublicInbox/GzipFilter.pm | 7 ++++++-
 lib/PublicInbox/Mbox.pm       | 7 +++++--
 lib/PublicInbox/WWW.pm        | 9 +++++++++
 3 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/lib/PublicInbox/GzipFilter.pm b/lib/PublicInbox/GzipFilter.pm
index 334d6581..c50c26c5 100644
--- a/lib/PublicInbox/GzipFilter.pm
+++ b/lib/PublicInbox/GzipFilter.pm
@@ -175,7 +175,12 @@ sub async_blob_cb { # git->cat_async callback
 	$smsg->{blob} eq $oid or bail($self, "BUG: $smsg->{blob} != $oid");
 	eval { $self->async_eml(PublicInbox::Eml->new($bref)) };
 	bail($self, "E: async_eml: $@") if $@;
-	$http->next_step($self->can('async_next'));
+	if ($self->{-low_prio}) {
+		push(@{$self->{www}->{-low_prio_q}}, $self) == 1 and
+				PublicInbox::DS::requeue($self->{www});
+	} else {
+		$http->next_step($self->can('async_next'));
+	}
 }
 
 sub smsg_blob {
diff --git a/lib/PublicInbox/Mbox.pm b/lib/PublicInbox/Mbox.pm
index f72af26b..cec76182 100644
--- a/lib/PublicInbox/Mbox.pm
+++ b/lib/PublicInbox/Mbox.pm
@@ -174,6 +174,7 @@ sub mbox_all_ids {
 		[404, [qw(Content-Type text/plain)], ["No results found\n"]];
 	$ctx->{ids} = $ids;
 	$ctx->{prev} = $prev;
+	$ctx->{-low_prio} = 1;
 	require PublicInbox::MboxGz;
 	PublicInbox::MboxGz::mbox_gz($ctx, \&all_ids_cb, 'all');
 }
@@ -192,12 +193,13 @@ sub results_cb {
 			my $smsg = $over->get_art($num) or next;
 			return $smsg;
 		}
-		# refill result set
+		# refill result set, deprioritize since there's many results
 		my $srch = $ctx->{ibx}->isrch or return gone($ctx, 'search');
 		my $mset = $srch->mset($ctx->{query}, $ctx->{qopts});
 		my $size = $mset->size or return;
 		$ctx->{qopts}->{offset} += $size;
 		$ctx->{ids} = $srch->mset_to_artnums($mset, $ctx->{qopts});
+		$ctx->{-low_prio} = 1;
 	}
 }
 
@@ -214,12 +216,13 @@ sub results_thread_cb {
 		# refills ctx->{xids}
 		next if $over->expand_thread($ctx);
 
-		# refill result set
+		# refill result set, deprioritize since there's many results
 		my $srch = $ctx->{ibx}->isrch or return gone($ctx, 'search');
 		my $mset = $srch->mset($ctx->{query}, $ctx->{qopts});
 		my $size = $mset->size or return;
 		$ctx->{qopts}->{offset} += $size;
 		$ctx->{ids} = $srch->mset_to_artnums($mset, $ctx->{qopts});
+		$ctx->{-low_prio} = 1;
 	}
 
 }
diff --git a/lib/PublicInbox/WWW.pm b/lib/PublicInbox/WWW.pm
index 570e690e..a7c961f4 100644
--- a/lib/PublicInbox/WWW.pm
+++ b/lib/PublicInbox/WWW.pm
@@ -659,4 +659,13 @@ sub get_description {
 	};
 }
 
+sub event_step { # called via requeue
+	my ($self) = @_;
+	# gzf = PublicInbox::GzipFilter == $ctx
+	my $gzf = shift(@{$self->{-low_prio_q}}) // return;
+	PublicInbox::DS::requeue($self) if scalar(@{$self->{-low_prio_q}});
+	my $http = $gzf->{env}->{'psgix.io'}; # PublicInbox::HTTP
+	$http->next_step($gzf->can('async_next'));
+}
+
 1;