nixos: add 'localRecipients' config option for Postfix

The new option services.postfix.localRecipients allows
configuring the postfix option 'local_recipient_maps'. When
set to a list of user names (or patterns), that map
effectively replaces the lookup in the system's user
database that's used by default to determine which local
users are valid.

This option is useful to explicitly set local users that are
allowed to receive e-mail from the outside world. For local
injection i.e. via the 'sendmail' command this option has no
effect.

nixos/modules/services/mail/postfix.nix

@@ -13,6 +13,7 @@ let
                       || cfg.extraAliases != "";
   haveTransport = cfg.transport != "";
   haveVirtual = cfg.virtual != "";
+  haveLocalRecipients = cfg.localRecipients != null;
 
   clientAccess =
     optional (cfg.dnsBlacklistOverrides != "")
@@ -244,6 +245,7 @@ let
 
   aliasesFile = pkgs.writeText "postfix-aliases" aliases;
   virtualFile = pkgs.writeText "postfix-virtual" cfg.virtual;
+  localRecipientMapFile = pkgs.writeText "postfix-local-recipient-map" (concatMapStrings (x: x + " ACCEPT\n") cfg.localRecipients);
   checkClientAccessFile = pkgs.writeText "postfix-check-client-access" cfg.dnsBlacklistOverrides;
   mainCfFile = pkgs.writeText "postfix-main.cf" mainCf;
   masterCfFile = pkgs.writeText "postfix-master.cf" masterCfContent;
@@ -506,6 +508,19 @@ in
         '';
       };
 
+      localRecipients = mkOption {
+        type = with types; nullOr (listOf string);
+        default = null;
+        description = ''
+          List of accepted local users. Specify a bare username, an
+          <literal>"@domain.tld"</literal> wild-card, or a complete
+          <literal>"user@domain.tld"</literal> address. If set, these names end
+          up in the local recipient map -- see the local(8) man-page -- and
+          effectively replace the system user database lookup that's otherwise
+          used by default.
+        '';
+      };
+
       transport = mkOption {
         default = "";
         description = "
@@ -742,6 +757,7 @@ in
       // optionalAttrs haveAliases { alias_maps = [ "${cfg.aliasMapType}:/etc/postfix/aliases" ]; }
       // optionalAttrs haveTransport { transport_maps = [ "hash:/etc/postfix/transport" ]; }
       // optionalAttrs haveVirtual { virtual_alias_maps = [ "${cfg.virtualMapType}:/etc/postfix/virtual" ]; }
+      // optionalAttrs haveLocalRecipients { local_recipient_maps = [ "hash:/etc/postfix/local_recipients" ] ++ optional haveAliases "$alias_maps"; }
       // optionalAttrs (cfg.dnsBlacklists != []) { smtpd_client_restrictions = clientRestrictions; }
       // optionalAttrs cfg.useSrs {
         sender_canonical_maps = [ "tcp:127.0.0.1:10001" ];
@@ -869,6 +885,9 @@ in
     (mkIf haveVirtual {
       services.postfix.mapFiles."virtual" = virtualFile;
     })
+    (mkIf haveLocalRecipients {
+      services.postfix.mapFiles."local_recipients" = localRecipientMapFile;
+    })
     (mkIf cfg.enableHeaderChecks {
       services.postfix.mapFiles."header_checks" = headerChecksFile;
     })