WIP: Add smarthost support

WORK IN PROGRESS -- WORK IN PROGRESS -- WORK IN PROGRESS

This patch adds support for delivering mail via a smarthost.

In this mode, all accepted mail gets delivered through an SMTP
connection to a specific host, statically configured.

test/t-20-smarthost/A/chasquid.conf

@@ -0,0 +1,17 @@
+smtp_address: ":1025"
+submission_address: ":1587"
+submission_over_tls_address: ":1465"
+monitoring_address: ":1099"
+
+# We don't expect this to be used, pick something that will error to ease
+# troubleshooting.
+mail_delivery_agent_bin: "no_mda_needed"
+
+data_dir: "../.data-A"
+mail_log_path: "../.logs/mail_log-A"
+
+# srv-b is our smarthost.
+# We use tls protocol because the smtp one is already well exercised in the
+# package tests.
+smarthost_url: "tls://userB@srv-b:userB@srv-b:2465"
+

test/t-20-smarthost/B/chasquid.conf

@@ -0,0 +1,10 @@
+smtp_address: ":2025"
+submission_address: ":2587"
+submission_over_tls_address: ":2465"
+monitoring_address: ":2099"
+
+mail_delivery_agent_bin: "test-mda"
+mail_delivery_agent_args: "%to%"
+
+data_dir: "../.data-B"
+mail_log_path: "../.logs/mail_log-B"

test/t-20-smarthost/C/chasquid.conf

@@ -0,0 +1,10 @@
+smtp_address: ":3025"
+submission_address: ":3587"
+submission_over_tls_address: ":3465"
+monitoring_address: ":3099"
+
+mail_delivery_agent_bin: "test-mda"
+mail_delivery_agent_args: "%to%"
+
+data_dir: "../.data-C"
+mail_log_path: "../.logs/mail_log-C"

test/t-20-smarthost/content

@@ -0,0 +1,9 @@
+From: userA@srv-A
+To: userC@srv-C
+Subject: Los espejos
+
+Yo que sentí el horror de los espejos
+no sólo ante el cristal impenetrable
+donde acaba y empieza, inhabitable,
+un imposible espacio de reflejos
+

test/t-20-smarthost/hosts

@@ -0,0 +1,3 @@
+srv-A localhost
+srv-B localhost
+srv-C localhost

test/t-20-smarthost/msmtprc

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+account default
+
+host srv-A
+port 1587
+
+tls on
+tls_trust_file A/certs/srv-A/fullchain.pem
+
+from userA@srv-A
+
+auth on
+user userA@srv-A
+password userA
+

test/t-20-smarthost/run.sh

@@ -0,0 +1,60 @@
+#!/bin/bash
+
+set -e
+. $(dirname ${0})/../util/lib.sh
+
+init
+
+rm -rf .data-A .data-B .data-C .mail .logs
+
+# Build with the DNS override, so we can fake DNS records.
+export GOTAGS="dnsoverride"
+
+# Launch minidns in the background using our configuration.
+minidns_bg --addr=":9053" -zones=zones >> .minidns.log 2>&1
+
+# 3 servers:
+# A - listens on :1025, hosts srv-A
+# B - listens on :2015, hosts srv-B
+# C - listens on :3015, hosts srv-C
+#
+# B and C are normal servers.
+# A will use B as a smarthost.
+#
+# We will send an email from A to C, and expect it to go through B.
+
+mkdir -p .certs
+for i in A B C; do
+		CONFDIR=${i} generate_certs_for srv-${i}
+		CONFDIR=${i} add_user user${i}@srv-${i} user${i}
+		mkdir -p .logs-${i}
+		cp ${i}/certs/srv-${i}/fullchain.pem .certs/cert-${i}.pem
+done
+
+# Make the servers trust each other.
+export SSL_CERT_DIR="$PWD/.certs/"
+
+chasquid -v=2 --logfile=.logs-A/chasquid.log --config_dir=A \
+	--testing__dns_addr=127.0.0.1:9053 &
+chasquid -v=2 --logfile=.logs-B/chasquid.log --config_dir=B \
+	--testing__dns_addr=127.0.0.1:9053 \
+	--testing__outgoing_smtp_port=3025 &
+chasquid -v=2 --logfile=.logs-C/chasquid.log --config_dir=C \
+	--testing__dns_addr=127.0.0.1:9053 \
+	--testing__outgoing_smtp_port=2025 &
+
+wait_until_ready 1025
+wait_until_ready 2025
+wait_until_ready 3025
+
+# Use A to send to C, and wait for delivery.
+run_msmtp userC@srv-c < content
+wait_for_file .mail/userc@srv-c
+mail_diff content .mail/userc@srv-c
+
+# Check that it went through B.
+if ! grep -q "from=userA@srv-a to=userC@srv-c sent" .logs/mail_log-B; then
+	fail "can't find record of delivery on the smarthost B"
+fi
+
+success

test/t-20-smarthost/zones

@@ -0,0 +1,6 @@
+srv-a  A     127.0.0.1
+srv-a  AAAA  ::1
+srv-b  A     127.0.0.1
+srv-b  AAAA  ::1
+srv-c  A     127.0.0.1
+srv-c  AAAA  ::1