nixos/prometheus-exporters: add mail exporter module

nixos/modules/services/monitoring/prometheus/exporters.nix

@@ -28,6 +28,7 @@ let
     "dovecot"
     "fritzbox"
     "json"
+    "mail"
     "minio"
     "nginx"
     "node"
@@ -168,6 +169,12 @@ in
         Please ensure you have either `services.prometheus.exporters.snmp.configuration'
           or `services.prometheus.exporters.snmp.configurationPath' set!
       '';
+    } {
+      assertion = (cfg.mail.configFile == null) != (cfg.mail.configuration == {});
+      message = ''
+        Please specify either 'services.prometheus.exporters.mail.configuration'
+          or 'services.prometheus.exporters.mail.configFile'.
+      '';
     } ];
   }] ++ [(mkIf config.services.minio.enable {
     services.prometheus.exporters.minio.minioAddress  = mkDefault "http://localhost:9000";

nixos/modules/services/monitoring/prometheus/exporters/mail.nix

@@ -0,0 +1,156 @@
+{ config, lib, pkgs, options }:
+
+with lib;
+
+let
+  cfg = config.services.prometheus.exporters.mail;
+
+  configurationFile = pkgs.writeText "prometheus-mail-exporter.conf" (builtins.toJSON (
+    # removes the _module attribute, null values and converts attrNames to lowercase
+    mapAttrs' (name: value:
+      if name == "servers"
+      then nameValuePair (toLower name)
+        ((map (srv: (mapAttrs' (n: v: nameValuePair (toLower n) v)
+          (filterAttrs (n: v: !(n == "_module" || v == null)) srv)
+        ))) value)
+      else nameValuePair (toLower name) value
+    ) (filterAttrs (n: _: !(n == "_module")) cfg.configuration)
+  ));
+
+  serverOptions.options = {
+    name = mkOption {
+      type = types.str;
+      description = ''
+        Value for label 'configname' which will be added to all metrics.
+      '';
+    };
+    server = mkOption {
+      type = types.str;
+      description = ''
+        Hostname of the server that should be probed.
+      '';
+    };
+    port = mkOption {
+      type = types.int;
+      example = 587;
+      description = ''
+        Port to use for SMTP.
+      '';
+    };
+    from = mkOption {
+      type = types.str;
+      example = "exporteruser@domain.tld";
+      description = ''
+        Content of 'From' Header for probing mails.
+      '';
+    };
+    to = mkOption {
+      type = types.str;
+      example = "exporteruser@domain.tld";
+      description = ''
+        Content of 'To' Header for probing mails.
+      '';
+    };
+    detectionDir = mkOption {
+      type = types.path;
+      example = "/var/spool/mail/exporteruser/new";
+      description = ''
+        Directory in which new mails for the exporter user are placed.
+        Note that this needs to exist when the exporter starts.
+      '';
+    };
+    login = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      example = "exporteruser@domain.tld";
+      description = ''
+        Username to use for SMTP authentication.
+      '';
+    };
+    passphrase = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = ''
+        Password to use for SMTP authentication.
+      '';
+    };
+  };
+
+  exporterOptions.options = {
+    monitoringInterval = mkOption {
+      type = types.str;
+      example = "10s";
+      description = ''
+        Time interval between two probe attempts.
+      '';
+    };
+    mailCheckTimeout = mkOption {
+      type = types.str;
+      description = ''
+        Timeout until mails are considered "didn't make it".
+      '';
+    };
+    disableFileDelition = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Disables the exporter's function to delete probing mails.
+      '';
+    };
+    servers = mkOption {
+      type = types.listOf (types.submodule serverOptions);
+      default = [];
+      example = literalExample ''
+        [ {
+          name = "testserver";
+          server = "smtp.domain.tld";
+          port = 587;
+          from = "exporteruser@domain.tld";
+          to = "exporteruser@domain.tld";
+          detectionDir = "/path/to/Maildir/new";
+        } ]
+      '';
+      description = ''
+        List of servers that should be probed.
+      '';
+    };
+  };
+in
+{
+  port = 9225;
+  extraOpts = {
+    configFile = mkOption {
+      type = types.nullOr types.path;
+      default = null;
+      description = ''
+        Specify the mailexporter configuration file to use.
+      '';
+    };
+    configuration = mkOption {
+      type = types.submodule exporterOptions;
+      default = {};
+      description = ''
+        Specify the mailexporter configuration file to use.
+      '';
+    };
+    telemetryPath = mkOption {
+      type = types.str;
+      default = "/metrics";
+      description = ''
+        Path under which to expose metrics.
+      '';
+    };
+  };
+  serviceOpts = {
+    serviceConfig = {
+      ExecStart = ''
+        ${pkgs.prometheus-mail-exporter}/bin/mailexporter \
+          --web.listen-address ${cfg.listenAddress}:${toString cfg.port} \
+          --config.file ${
+            if cfg.configuration != {} then configurationFile else cfg.configFile
+          } \
+          ${concatStringsSep " \\\n  " cfg.extraFlags}
+      '';
+    };
+  };
+}