dovecot: respect the mailDirectory base for LDAP home directories

This change is safe, if you have not altered the default value of the
 `mailserver.mailDirectory` setting.

mail-server/assertions.nix

@@ -1,6 +1,21 @@
 { config, lib, ... }:
 {
-  assertions = lib.optionals config.mailserver.enable [
+  assertions = [
+    {
+      assertion = config.mailserver.stateVersion < 2
+        && config.mailserver.ldap.enable
+        && config.mailserver.mailDirectory != "/var/vmail";
+      message = ''
+        Issue: The dovecot homedir for LDAP users was previously not respecting `mailserver.mailDirectory`.
+        Remediation:
+          - Stop the `dovecot2.service`
+          - Move `/var/vmail/ldap` below your `mailserver.mailDirectory`
+          - Increase the `stateVersion` to 2.
+
+        Check https://nixos-mailserver.readthedocs.io/en/latest/migrations.html#ldap-home-directory-migration for more information.
+      ''; 
+    }
+  ] ++ lib.optionals config.mailserver.enable [
     {
       assertion = config.mailserver.stateVersion != null;
       message = "The `mailserver.stateVersion` option is not set. Check https://nixos-mailserver.readthedocs.io/en/latest/migrations.html to determine the proper value to initialize it at.";

mail-server/dovecot.nix

@@ -356,7 +356,7 @@ in
         userdb {
           driver = ldap
           args = ${ldapConfFile}
-          default_fields = home=/var/vmail/ldap/%{user} uid=${toString cfg.vmailUID} gid=${toString cfg.vmailUID}
+          default_fields = home=${cfg.mailDirectory}/ldap/%{user} uid=${toString cfg.vmailUID} gid=${toString cfg.vmailUID}
         }
         ''}