[PATCH 09/18] qspawn: introduce new psgi_yield API

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

This is intended to replace psgi_return and HTTPD/Async
entirely, hopefully making our code less convoluted while
maintaining the ability to handle slow clients on
memory-constrained systems

This was made possible by the philosophy shift in commit 21a539a2df0c
(httpd/async: switch to buffering-as-fast-as-possible, 2019-06-28).

We'll still support generic PSGI via the `pull' model with a
GetlineResponse class which is similar to the old GetlineBody.
---
 MANIFEST                           |   1 +
 lib/PublicInbox/GetlineResponse.pm |  40 ++++++++++
 lib/PublicInbox/GitHTTPBackend.pm  |   4 +-
 lib/PublicInbox/GzipFilter.pm      |   3 +-
 lib/PublicInbox/HTTP.pm            |   8 +-
 lib/PublicInbox/InputPipe.pm       |  12 +--
 lib/PublicInbox/LEI.pm             |   2 +-
 lib/PublicInbox/Qspawn.pm          | 119 ++++++++++++++++++++++++++++-
 8 files changed, 176 insertions(+), 13 deletions(-)
 create mode 100644 lib/PublicInbox/GetlineResponse.pm

diff --git a/MANIFEST b/MANIFEST
index f087621c..420b40a1 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -204,6 +204,7 @@ lib/PublicInbox/Filter/Vger.pm
 lib/PublicInbox/Gcf2.pm
 lib/PublicInbox/Gcf2Client.pm
 lib/PublicInbox/GetlineBody.pm
+lib/PublicInbox/GetlineResponse.pm
 lib/PublicInbox/Git.pm
 lib/PublicInbox/GitAsyncCat.pm
 lib/PublicInbox/GitCredential.pm
diff --git a/lib/PublicInbox/GetlineResponse.pm b/lib/PublicInbox/GetlineResponse.pm
new file mode 100644
index 00000000..290cce74
--- /dev/null
+++ b/lib/PublicInbox/GetlineResponse.pm
@@ -0,0 +1,40 @@
+# Copyright (C) all contributors <meta@public-inbox.org>
+# License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
+
+# For generic PSGI servers (not public-inbox-httpd/netd) which assumes their
+# getline response bodies can be backpressure-aware for slow clients
+# This depends on rpipe being _blocking_ on getline.
+package PublicInbox::GetlineResponse;
+use v5.12;
+
+sub response {
+	my ($qsp) = @_;
+	my ($res, $rbuf);
+	do { # read header synchronously
+		sysread($qsp->{rpipe}, $rbuf, 65536);
+		$res = $qsp->parse_hdr_done($rbuf); # fills $bref
+	} until defined($res);
+	my ($wcb, $filter) = $qsp->yield_pass(undef, $res) or return;
+	my $self = $res->[2] = bless {
+		qsp => $qsp,
+		filter => $filter,
+	}, __PACKAGE__;
+	my ($bref) = @{delete $qsp->{yield_parse_hdr}};
+	$self->{rbuf} = $$bref if $$bref ne '';
+	$wcb->($res);
+}
+
+sub getline {
+	my ($self) = @_;
+	my $rpipe = $self->{qsp}->{rpipe} // do {
+		delete($self->{qsp})->finish;
+		return; # EOF was set on previous call
+	};
+	my $buf = delete($self->{rbuf}) // $rpipe->getline;
+	$buf // delete($self->{qsp}->{rpipe}); # set EOF for next call
+	$self->{filter} ? $self->{filter}->translate($buf) : $buf;
+}
+
+sub close {}
+
+1;
diff --git a/lib/PublicInbox/GitHTTPBackend.pm b/lib/PublicInbox/GitHTTPBackend.pm
index edbc0157..d7e0bced 100644
--- a/lib/PublicInbox/GitHTTPBackend.pm
+++ b/lib/PublicInbox/GitHTTPBackend.pm
@@ -79,7 +79,7 @@ sub serve_dumb {
 	PublicInbox::WwwStatic::response($env, $h, $path, $type);
 }
 
-sub git_parse_hdr { # {parse_hdr} for Qspawn
+sub ghb_parse_hdr { # header parser for Qspawn
 	my ($r, $bref, @dumb_args) = @_;
 	my $res = parse_cgi_headers($r, $bref) or return; # incomplete
 	$res->[0] == 403 ? serve_dumb(@dumb_args) : $res;
@@ -106,7 +106,7 @@ sub serve_smart {
 	$env{PATH_TRANSLATED} = "$git->{git_dir}/$path";
 	my $rdr = input_prepare($env) or return r(500);
 	my $qsp = PublicInbox::Qspawn->new([qw(git http-backend)], \%env, $rdr);
-	$qsp->psgi_return($env, $limiter, \&git_parse_hdr, $env, $git, $path);
+	$qsp->psgi_yield($env, $limiter, \&ghb_parse_hdr, $env, $git, $path);
 }
 
 sub input_prepare {
diff --git a/lib/PublicInbox/GzipFilter.pm b/lib/PublicInbox/GzipFilter.pm
index db8e8397..d6ecd5ba 100644
--- a/lib/PublicInbox/GzipFilter.pm
+++ b/lib/PublicInbox/GzipFilter.pm
@@ -123,9 +123,10 @@ sub http_out ($) {
 	};
 }
 
+# returns undef if HTTP client disconnected, may return 0
+# because ->translate can return ''
 sub write {
 	my $self = shift;
-	# my $ret = bytes::length($_[1]); # XXX does anybody care?
 	http_out($self)->write($self->translate(@_));
 }
 
diff --git a/lib/PublicInbox/HTTP.pm b/lib/PublicInbox/HTTP.pm
index ca162939..edc88fe8 100644
--- a/lib/PublicInbox/HTTP.pm
+++ b/lib/PublicInbox/HTTP.pm
@@ -455,11 +455,12 @@ sub next_step {
 # They may be exposed to the PSGI application when the PSGI app
 # returns a CODE ref for "push"-based responses
 package PublicInbox::HTTP::Chunked;
-use strict;
+use v5.12;
 
 sub write {
 	# ([$http], $buf) = @_;
-	PublicInbox::HTTP::chunked_write($_[0]->[0], $_[1])
+	PublicInbox::HTTP::chunked_write($_[0]->[0], $_[1]);
+	$_[0]->[0]->{sock} ? length($_[1]) : undef;
 }
 
 sub close {
@@ -468,12 +469,13 @@ sub close {
 }
 
 package PublicInbox::HTTP::Identity;
-use strict;
+use v5.12;
 our @ISA = qw(PublicInbox::HTTP::Chunked);
 
 sub write {
 	# ([$http], $buf) = @_;
 	PublicInbox::HTTP::identity_write($_[0]->[0], $_[1]);
+	$_[0]->[0]->{sock} ? length($_[1]) : undef;
 }
 
 1;
diff --git a/lib/PublicInbox/InputPipe.pm b/lib/PublicInbox/InputPipe.pm
index b38d8270..f4d57e7d 100644
--- a/lib/PublicInbox/InputPipe.pm
+++ b/lib/PublicInbox/InputPipe.pm
@@ -39,14 +39,16 @@ sub consume {
 	if ($@) { # regular file (but not w/ select|IO::Poll backends)
 		$self->{-need_rq} = 1;
 		$self->requeue;
-	} elsif (-p $in || -S _) { # O_NONBLOCK for sockets and pipes
+	} elsif (do { no warnings 'unopened'; !stat($in) }) { # ProcessIONBF
+	} elsif (-p _ || -S _) { # O_NONBLOCK for sockets and pipes
 		$in->blocking(0);
 	} elsif (-t $in) { # isatty(3) can't use `_' stat cache
 		unblock_tty($self);
 	}
+	$self;
 }
 
-sub close {
+sub close { # idempotent
 	my ($self) = @_;
 	if (my $t = delete($self->{restore_termios})) {
 		my $fd = fileno($self->{sock} // return);
@@ -60,16 +62,16 @@ sub event_step {
 	my $r = sysread($self->{sock} // return, my $rbuf, 65536);
 	eval {
 		if ($r) {
-			$self->{cb}->(@{$self->{args}}, $rbuf);
+			$self->{cb}->($self, @{$self->{args}}, $rbuf);
 			$self->requeue if $self->{-need_rq};
 		} elsif (defined($r)) { # EOF
-			$self->{cb}->(@{$self->{args}}, '');
+			$self->{cb}->($self, @{$self->{args}}, '');
 			$self->close
 		} elsif ($!{EAGAIN}) { # rely on EPOLLIN
 		} elsif ($!{EINTR}) { # rely on EPOLLIN for sockets/pipes/tty
 			$self->requeue if $self->{-need_rq};
 		} else { # another error
-			$self->{cb}->(@{$self->{args}}, undef);
+			$self->{cb}->($self, @{$self->{args}}, undef);
 			$self->close;
 		}
 	};
diff --git a/lib/PublicInbox/LEI.pm b/lib/PublicInbox/LEI.pm
index 56e4c001..7bc7b2dc 100644
--- a/lib/PublicInbox/LEI.pm
+++ b/lib/PublicInbox/LEI.pm
@@ -1567,7 +1567,7 @@ sub request_umask {
 }
 
 sub _stdin_cb { # PublicInbox::InputPipe::consume callback for --stdin
-	my ($lei, $cb) = @_; # $_[-1] = $rbuf
+	my (undef, $lei, $cb) = @_; # $_[-1] = $rbuf
 	$_[1] // return $lei->fail("error reading stdin: $!");
 	$lei->{stdin_buf} .= $_[-1];
 	do_env($lei, $cb) if $_[-1] eq '';
diff --git a/lib/PublicInbox/Qspawn.pm b/lib/PublicInbox/Qspawn.pm
index 9a7e8734..203d8f41 100644
--- a/lib/PublicInbox/Qspawn.pm
+++ b/lib/PublicInbox/Qspawn.pm
@@ -31,6 +31,9 @@ use PublicInbox::GzipFilter;
 use Scalar::Util qw(blessed);
 use PublicInbox::Limiter;
 use PublicInbox::Aspawn qw(run_await);
+use PublicInbox::Syscall qw(EPOLLIN);
+use PublicInbox::InputPipe;
+use Carp qw(carp confess);
 
 # n.b.: we get EAGAIN with public-inbox-httpd, and EINTR on other PSGI servers
 use Errno qw(EAGAIN EINTR);
@@ -61,7 +64,7 @@ sub _do_spawn {
 	if ($start_cb) {
 		eval { # popen_rd may die on EMFILE, ENFILE
 			$self->{rpipe} = popen_rd($cmd, $cmd_env, \%o,
-						\&waitpid_err, $self);
+						\&waitpid_err, $self, \%o);
 			$start_cb->($self); # EPOLL_CTL_ADD may ENOSPC/ENOMEM
 		};
 	} else {
@@ -126,6 +129,20 @@ sub wait_await { # run_await cb
 	waitpid_err($pid, $self, $opt);
 }
 
+sub yield_chunk { # $_[-1] is sysread buffer (or undef)
+	my ($self, $ipipe) = @_;
+	if (!defined($_[-1])) {
+		warn "error reading body: $!";
+	} elsif ($_[-1] eq '') { # normal EOF
+		$self->finish;
+		$self->{qfh}->close;
+	} elsif (defined($self->{qfh}->write($_[-1]))) {
+		return; # continue while HTTP client is reading our writes
+	} # else { # HTTP client disconnected
+	delete $self->{rpipe};
+	$ipipe->close;
+}
+
 sub finish ($;$) {
 	my ($self, $err) = @_;
 	$self->{_err} //= $err; # only for $@
@@ -201,6 +218,39 @@ EOM
 	$ret;
 }
 
+sub yield_pass {
+	my ($self, $ipipe, $res) = @_; # $ipipe = InputPipe
+	my $env = $self->{psgi_env};
+	my $wcb = delete $env->{'qspawn.wcb'} // confess('BUG: no qspawn.wcb');
+	if (ref($res) eq 'CODE') { # chain another command
+		delete $self->{rpipe};
+		$ipipe->close if $ipipe;
+		$res->($wcb);
+		$self->{passed} = 1;
+		return; # all done
+	}
+	confess("BUG: $res unhandled") if ref($res) ne 'ARRAY';
+
+	my $filter = blessed($res->[2]) && $res->[2]->can('attach') ?
+			pop(@$res) : delete($env->{'qspawn.filter'});
+	$filter //= PublicInbox::GzipFilter::qsp_maybe($res->[1], $env);
+
+	if (scalar(@$res) == 3) { # done early (likely error or static file)
+		delete $self->{rpipe};
+		$ipipe->close if $ipipe;
+		$wcb->($res); # all done
+		return;
+	}
+	scalar(@$res) == 2 or confess("BUG: scalar(res) != 2: @$res");
+	return ($wcb, $filter) if !$ipipe; # generic PSGI
+	# streaming response
+	my $qfh = $wcb->($res); # get PublicInbox::HTTP::(Chunked|Identity)
+	$qfh = $filter->attach($qfh) if $filter;
+	my ($bref) = @{delete $self->{yield_parse_hdr}};
+	$qfh->write($$bref) if $$bref ne '';
+	$self->{qfh} = $qfh; # keep $ipipe open
+}
+
 sub psgi_return_init_cb { # this may be PublicInbox::HTTPD::Async {cb}
 	my ($self) = @_;
 	my $r = rd_hdr($self) or return; # incomplete
@@ -257,6 +307,55 @@ sub psgi_return_start { # may run later, much later...
 	}
 }
 
+sub r500 () { [ 500, [], [ "Internal error\n" ] ] }
+
+sub parse_hdr_done ($$) {
+	my ($self) = @_;
+	my $ret;
+	if (defined $_[-1]) {
+		my ($bref, $ph_cb, @ph_arg) = @{$self->{yield_parse_hdr}};
+		$$bref .= $_[-1];
+		$ret = eval { $ph_cb->(length($_[-1]), $bref, @ph_arg) };
+		if ($@) {
+			carp "parse_hdr (@{$self->{cmd}}): $@\n";
+			$ret = r500();
+		} elsif (!$ret && $_[-1] eq '') {
+			carp <<EOM;
+EOF parsing headers from @{$self->{cmd}} ($self->{psgi_env}->{REQUEST_URI})
+EOM
+			$ret = r500();
+		}
+	} else {
+		carp <<EOM;
+E: parsing headers: $! from @{$self->{cmd}} ($self->{psgi_env}->{REQUEST_URI})
+EOM
+		$ret = r500();
+	}
+	$ret; # undef if headers incomplete
+}
+
+sub ipipe_cb { # InputPipe callback
+	my ($ipipe, $self) = @_; # $_[-1] rbuf
+	if ($self->{qfh}) { # already streaming
+		yield_chunk($self, $ipipe, $_[-1]);
+	} elsif (my $res = parse_hdr_done($self, $_[-1])) {
+		yield_pass($self, $ipipe, $res);
+	} # else: headers incomplete, keep reading
+}
+
+sub _yield_start { # may run later, much later...
+	my ($self) = @_;
+	if ($self->{psgi_env}->{'pi-httpd.async'}) {
+		require PublicInbox::ProcessIONBF;
+		my $rpipe = $self->{rpipe};
+		PublicInbox::ProcessIONBF->replace($rpipe);
+		PublicInbox::InputPipe::consume($rpipe, \&ipipe_cb, $self);
+	} else {
+		require PublicInbox::GetlineResponse;
+		PublicInbox::GetlineResponse::response($self);
+	}
+}
+
 # Used for streaming the stdout of one process as a PSGI response.
 #
 # $env is the PSGI env.
@@ -302,4 +401,22 @@ sub psgi_return {
 	}
 }
 
+sub psgi_yield {
+	my ($self, $env, $limiter, @parse_hdr_arg)= @_;
+	$self->{psgi_env} = $env;
+	$self->{yield_parse_hdr} = [ \(my $buf = ''), @parse_hdr_arg ];
+	$limiter ||= $def_limiter ||= PublicInbox::Limiter->new(32);
+
+	# the caller already captured the PSGI write callback from
+	# the PSGI server, so we can call ->start, here:
+	$env->{'qspawn.wcb'} ? start($self, $limiter, \&_yield_start) : sub {
+		# the caller will return this sub to the PSGI server, so
+		# it can set the response callback (that is, for
+		# PublicInbox::HTTP, the chunked_wcb or identity_wcb callback),
+		# but other HTTP servers are supported:
+		$env->{'qspawn.wcb'} = $_[0];
+		start($self, $limiter, \&_yield_start);
+	}
+}
+
 1;