mailsystem: dovecot: Autolearn ham/spam when moving mails

mailsystem/dovecot.nix

@@ -92,6 +92,14 @@ with (import ./common.nix {inherit config;}); let
     chgrp "${cfg.vmailGroupName}" ${cfg.mailDirectory}
     chmod 02770 ${cfg.mailDirectory}
   '';
+
+  junkMailboxes = builtins.attrNames (lib.filterAttrs (n: v: v ? "specialUse" && v.specialUse == "Junk") dovecot2Cfg.mailboxes);
+  junkMailboxNumber = builtins.length junkMailboxes;
+  # The assertion guarantees that there is exactly one Junk mailbox.
+  junkMailboxName =
+    if junkMailboxNumber == 1
+    then builtins.elemAt junkMailboxes 0
+    else "";
 in {
   options.mailsystem.dovecot.dhparamSize = lib.mkOption {
     type = lib.types.int;
@@ -101,12 +109,18 @@ in {
 
   config = lib.mkIf cfg.enable {
     assertions =
-      lib.mapAttrsToList (user: value: [
+      [
         {
+          assertion = junkMailboxNumber == 1;
+          message = "mailnix requires exactly one dovecot mailbox with the 'special use' flag to 'Junk' (${builtins.toString junkMailboxNumber} have been found)";
+        }
+      ]
+      ++ lib.mapAttrsToList (
+        user: value: {
           assertion = value.hashedPasswordFile != null;
           message = "A file containing the hashed password for user ${user} needs to be set.";
         }
-      ])
+      )
       cfg.accounts;
 
     services.dovecot2 = {
@@ -136,6 +150,44 @@ in {
         sieve = "file:~/sieve;active=~/.dovecot.sieve";
       };
 
+      sieve = {
+        extensions = [
+          "fileinto"
+          "mailbox"
+        ];
+
+        scripts.after = builtins.toFile "spam.sieve" ''
+          require "fileinto";
+          require "mailbox";
+
+          if header :is "X-Spam" "Yes" {
+            fileinto :create "${junkMailboxName}";
+            stop;
+          }
+        '';
+
+        pipeBins = map lib.getExe [
+          (pkgs.writeShellScriptBin "learn-ham.sh"
+            "exec ${pkgs.rspamd}/bin/rspamc -h ${rspamdControllerSocket} learn_ham")
+          (pkgs.writeShellScriptBin "learn-spam.sh"
+            "exec ${pkgs.rspamd}/bin/rspamc -h ${rspamdControllerSocket} learn_spam")
+        ];
+      };
+
+      imapsieve.mailbox = [
+        {
+          name = junkMailboxName;
+          causes = ["COPY" "APPEND"];
+          before = ./dovecot/report-spam.sieve;
+        }
+        {
+          name = "*";
+          from = junkMailboxName;
+          causes = ["COPY"];
+          before = ./dovecot/report-ham.sieve;
+        }
+      ];
+
       # TODO: move configuration to default.nix?
       mailboxes = {
         Drafts = {