Tests: add AF_UNIX upstream write test for PROXY protocol v2.

Test nginx as a stream proxy with a Unix domain socket downstream
connection.  Verifies that ngx_proxy_protocol_v2_write() emits
family_transport 0x31 (AF_UNIX, STREAM), addr block length 216,
empty src_addr for an unbound client, the correct dst_addr path,
and that the payload follows the 232-byte header intact.

t/stream_proxy_protocol_v2_unix.t

@@ -0,0 +1,149 @@
+#!/usr/bin/perl
+
+# (C) Nginx, Inc.
+
+# Tests for PROXY protocol v2 write with AF_UNIX downstream in stream proxy.
+
+###############################################################################
+
+use warnings;
+use strict;
+
+use Test::More;
+
+use IO::Select;
+use IO::Socket::INET;
+use IO::Socket::UNIX;
+
+BEGIN { use FindBin; chdir($FindBin::Bin); }
+
+use lib 'lib';
+use Test::Nginx;
+
+###############################################################################
+
+select STDERR; $| = 1;
+select STDOUT; $| = 1;
+
+# PPv2 magic signature (12 bytes)
+my $SIG = "\x0D\x0A\x0D\x0A\x00\x0D\x0A\x51\x55\x49\x54\x0A";
+
+# Fixed header: 16 bytes.  Unix address block: 216 bytes.  Total: 232 bytes.
+my $PPV2_HDR = 232;
+
+my $t = Test::Nginx->new()->has(qw/stream/)->plan(7)
+	->write_file_expand('nginx.conf', <<'EOF');
+
+%%TEST_GLOBALS%%
+
+daemon off;
+
+events {
+}
+
+stream {
+    %%TEST_GLOBALS_STREAM%%
+
+    server {
+        listen          unix:%%TESTDIR%%/nginx.sock;
+        proxy_pass      127.0.0.1:8080;
+        proxy_protocol  on;
+        proxy_protocol_version 2;
+    }
+}
+
+EOF
+
+$t->run_daemon(\&stream_daemon, port(8080));
+$t->run();
+$t->waitforsocket('127.0.0.1:' . port(8080));
+
+###############################################################################
+
+my $ngx_sock = $t->testdir() . '/nginx.sock';
+
+my $s = IO::Socket::UNIX->new(Peer => $ngx_sock)
+	or die "Can't connect to $ngx_sock: $!\n";
+
+$s->syswrite('hello') or die "send: $!\n";
+
+my $data = '';
+my $sel  = IO::Select->new($s);
+while ($sel->can_read(1)) {
+	my $n = $s->sysread(my $chunk, 65536);
+	last unless $n;
+	$data .= $chunk;
+}
+
+my $src = substr($data, 16, 108);
+$src =~ s/\x00.*//s;
+
+my $dst = substr($data, 124, 108);
+$dst =~ s/\x00.*//s;
+
+is(substr($data, 0, 12), $SIG,                     'unix v2 signature');
+is(unpack('C', substr($data, 12, 1)), 0x21,         'unix v2 version and command');
+is(unpack('C', substr($data, 13, 1)), 0x31,         'unix v2 AF_UNIX STREAM');
+is(unpack('n', substr($data, 14, 2)), 216,           'unix v2 addr block length');
+is($src, '',                                         'unix v2 src addr (unbound)');
+is($dst, $ngx_sock,                                  'unix v2 dst addr');
+is(substr($data, $PPV2_HDR), 'hello',                'unix v2 payload after header');
+
+###############################################################################
+
+sub stream_daemon {
+	my ($port) = @_;
+
+	my $server = IO::Socket::INET->new(
+		Proto     => 'tcp',
+		LocalAddr => '127.0.0.1:' . $port,
+		Listen    => 5,
+		Reuse     => 1,
+	) or die "Can't create listening socket: $!\n";
+
+	my $sel = IO::Select->new($server);
+
+	local $SIG{PIPE} = 'IGNORE';
+
+	while (my @ready = $sel->can_read) {
+		foreach my $fh (@ready) {
+			if ($server == $fh) {
+				my $new = $fh->accept;
+				$new->autoflush(1);
+				$sel->add($new);
+
+			} elsif (stream_handle_client($fh)) {
+				$sel->remove($fh);
+				$fh->close;
+			}
+		}
+	}
+}
+
+sub stream_handle_client {
+	my ($client) = @_;
+
+	log2c("(new connection $client)");
+
+	my $buffer = '';
+	my $csel   = IO::Select->new($client);
+	while ($csel->can_read(0.5)) {
+		my $n = $client->sysread(my $chunk, 65536);
+		last unless $n;
+		$buffer .= $chunk;
+	}
+
+	log2i("$client recv " . length($buffer) . " bytes");
+
+	$client->syswrite($buffer);
+
+	log2o("$client sent " . length($buffer) . " bytes");
+
+	return 1;
+}
+
+sub log2i { Test::Nginx::log_core('|| <<', @_); }
+sub log2o { Test::Nginx::log_core('|| >>', @_); }
+sub log2c { Test::Nginx::log_core('||',    @_); }
+
+###############################################################################