[PATCH] qspawn: clarify and improve error handling

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

EINTR should not happen when using non-blocking sockets like we
do in our daemons, but maybe some OSes allow it to happen and
edge-triggered notifications won't notify us again.

So always retry immediately on EINTR without relying on kqueue
or epoll to notify us, and log any other unrecoverable errors
which may happen while we're at it.
---
 lib/PublicInbox/Qspawn.pm | 38 +++++++++++++++++++++++++++-----------
 1 file changed, 27 insertions(+), 11 deletions(-)

diff --git a/lib/PublicInbox/Qspawn.pm b/lib/PublicInbox/Qspawn.pm
index 73fd3225..4b9bca5a 100644
--- a/lib/PublicInbox/Qspawn.pm
+++ b/lib/PublicInbox/Qspawn.pm
@@ -73,6 +73,11 @@ sub child_err ($) {
 	$msg;
 }
 
+sub log_err ($$) {
+	my ($env, $msg) = @_;
+	$env->{'psgi.errors'}->print($msg, "\n");
+}
+
 # callback for dwaitpid
 sub waitpid_err ($$) {
 	my ($self, $pid) = @_;
@@ -99,8 +104,7 @@ sub waitpid_err ($$) {
 	$self->{err} = $err;
 	my $env = $self->{env} or return;
 	if (!$env->{'qspawn.quiet'}) {
-		$err = join(' ', @{$self->{args}->[0]}).": $err\n";
-		$env->{'psgi.errors'}->print($err);
+		log_err($env, join(' ', @{$self->{args}->[0]}) . ": $err");
 	}
 }
 
@@ -155,6 +159,8 @@ sub psgi_qx {
 	my $scalar = '';
 	open(my $qx, '+>', \$scalar) or die; # PerlIO::scalar
 	my $end = sub {
+		my $err = $_[0]; # $!
+		log_err($env, "psgi_qx: $err") if defined($err);
 		finish($self, $env);
 		eval { $qx_cb->(\$scalar) };
 		$qx = $scalar = undef;
@@ -162,14 +168,17 @@ sub psgi_qx {
 	my $rpipe; # comes from popen_rd
 	my $async = $env->{'pi-httpd.async'};
 	my $cb = sub {
-		my $r = sysread($rpipe, my $buf, 65536);
+		my ($r, $buf);
+reread:
+		$r = sysread($rpipe, $buf, 65536);
 		if ($async) {
 			$async->async_pass($env->{'psgix.io'}, $qx, \$buf);
 		} elsif (defined $r) {
 			$r ? $qx->write($buf) : $end->();
 		} else {
-			return if $! == EAGAIN || $! == EINTR; # loop again
-			$end->();
+			return if $! == EAGAIN; # try again when notified
+			goto reread if $! == EINTR;
+			$end->($!);
 		}
 	};
 	$limiter ||= $def_limiter ||= PublicInbox::Qspawn::Limiter->new(32);
@@ -223,6 +232,8 @@ sub psgi_return {
 	my ($self, $env, $limiter, $parse_hdr) = @_;
 	my ($fh, $rpipe);
 	my $end = sub {
+		my $err = $_[0]; # $!
+		log_err($env, "psgi_return: $err") if defined($err);
 		finish($self, $env);
 		$fh->close if $fh; # async-only
 	};
@@ -233,14 +244,19 @@ sub psgi_return {
 		# we must loop until EAGAIN for EPOLLET in HTTPD/Async.pm
 		# We also need to check EINTR for generic PSGI servers.
 		my $ret;
-		my $n = 0;
+		my $total_rd = 0;
 		do {
 			my $r = sysread($rpipe, $buf, 4096, length($buf));
-			return if !defined($r) && $! == EAGAIN || $! == EINTR;
-
-			# $r may be undef, here:
-			$n += $r if $r;
-			$ret = $parse_hdr->($r ? $n : $r, \$buf);
+			if (defined($r)) {
+				$total_rd += $r;
+				$ret = $parse_hdr->($r ? $total_rd : 0, \$buf);
+			} else {
+				# caller should notify us when it's ready:
+				return if $! == EAGAIN;
+				next if $! == EINTR; # immediate retry
+				log_err($env, "error reading header: $!");
+				$ret = [ 500, [], [ "Internal error\n" ] ];
+			}
 		} until (defined $ret);
 		$ret;
 	};
-- 
EW