aliases: Implement aliases hooks

This patch implements two new hooks: alias-resolve and alias-exists.

They are called during the aliases resolution process, to allow for more
complex integration with other systems, such as storing the aliases in a
database.

See the included documentation for more details.

test/t-04-aliases/alias-exists-hook

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+case "$1" in
+"vicuña@testserver")
+	exit 0
+	;;
+"ñandú@testserver")
+	exit 0
+	;;
+"roto@testserver")
+	exit 0
+	;;
+esac
+
+exit 1

test/t-04-aliases/alias-resolve-hook

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+case "$1" in
+"vicuña@testserver")
+	# Test one naked, one full. These exist in the static aliases file.
+	echo pepe, joan@testserver
+	;;
+"ñandú@testserver")
+	echo "| writemailto ../.data/pipe_alias_worked"
+	;;
+"roto@testserver")
+	exit 1
+	;;
+esac

test/t-04-aliases/run.sh

@@ -22,6 +22,10 @@ function send_and_check() {
 	done
 }
 
+# Remove the hooks that could be left over from previous failed tests.
+rm -f config/hooks/alias-resolve
+rm -f config/hooks/alias-exists
+
 # Test email aliases.
 send_and_check pepe jose
 send_and_check joan juan
@@ -39,5 +43,32 @@ run_msmtp tubo@testserver < content
 wait_for_file .data/pipe_alias_worked
 mail_diff content .data/pipe_alias_worked
 
+# Set up the hooks.
+mkdir -p config/hooks/
+cp alias-exists-hook config/hooks/alias-exists
+cp alias-resolve-hook config/hooks/alias-resolve
+
+# Test email aliases.
+send_and_check vicuña juan jose
+
+# Test the pipe alias separately.
+rm -f .data/pipe_alias_worked
+run_msmtp ñandú@testserver < content
+wait_for_file .data/pipe_alias_worked
+mail_diff content .data/pipe_alias_worked
+
+# Test when alias-resolve exits with an error
+if run_msmtp roto@testserver < content 2> .logs/msmtp.out; then
+	echo "expected delivery to roto@ to fail, but succeeded"
+fi
+
+# Test a non-existent alias.
+if run_msmtp nono@testserver < content 2> .logs/msmtp.out; then
+	echo "expected delivery to nono@ to fail, but succeeded"
+fi
+
+# Remove the hooks, leave a clean state.
+rm -f config/hooks/alias-resolve
+rm -f config/hooks/alias-exists
 
 success