smtpsrv: Implement a post-DATA hook

This patch implements a post-DATA hook, which is run after receiving the
data but before sending a reply.

It can be used to implement content filtering when receiving email, for
example for passing the email through an anti-spam or an anti-virus.

test/t-10-hooks/run.sh

@@ -0,0 +1,64 @@
+#!/bin/bash
+
+set -e
+. $(dirname ${0})/../util/lib.sh
+
+init
+
+generate_certs_for testserver
+add_user testserver user secretpassword
+add_user testserver someone secretpassword
+add_user testserver blockme secretpassword
+
+mkdir -p .logs
+chasquid -v=2 --log_dir=.logs --config_dir=config &
+wait_until_ready 1025
+
+cp config/hooks/post-data.good config/hooks/post-data
+
+run_msmtp someone@testserver < content
+
+wait_for_file .mail/someone@testserver
+
+mail_diff content .mail/someone@testserver
+
+if ! grep -q "X-Post-Data: success" .mail/someone@testserver; then
+	echo "missing X-Post-Data header"
+	exit 1
+fi
+
+function check() {
+	if ! grep -q "$1" .data/post-data.out; then
+		echo missing: $1
+		exit 1
+	fi
+}
+
+# Verify that the environment for the hook was reasonable.
+check "RCPT_TO=someone@testserver"
+check "MAIL_FROM=user@testserver"
+check "USER=$USER"
+check "PWD=$PWD/config"
+check "FROM_LOCAL_DOMAIN=1"
+check "ON_TLS=1"
+check "AUTH_AS=user@testserver"
+check "PATH="
+check "REMOTE_ADDR="
+
+
+# Check that a failure in the script results in failing delivery.
+if run_msmtp blockme@testserver < content 2>/dev/null; then
+	echo "ERROR: hook did not block email as expected"
+	exit 1
+fi
+
+# Check that the bad hooks don't prevent delivery.
+for i in config/hooks/post-data.bad*; do
+	cp $i config/hooks/post-data
+
+	run_msmtp someone@testserver < content
+	wait_for_file .mail/someone@testserver
+	mail_diff content .mail/someone@testserver
+done
+
+success