From c00d17aae388bca8bf5657a2f312a1e436e7aef0 Mon Sep 17 00:00:00 2001
From: Alexey Lebedeff <alexey.lebedeff@booking.com>
Date: Thu, 25 Jan 2018 20:28:58 +0100
Subject: [PATCH] epmd: Introduce erlang port mapper daemon service

Having socket-activated epmd means that there always be only a single
instance managed centrally. Because Erlang also starts it
automatically if not available, and in worst case scenario 'epmd' can
be started by some Erlang application running under systemd. And then
restarting this application unit will cause complete loss of names in
'epmd' (if other Erlang system are also installed on this host).

E.g. see at which lengths RabbitMQ goes to recover from such
situations:
https://github.com/rabbitmq/rabbitmq-server/blame/7741b37b1efa97ac9b17685cc626bd35ee52ca16/src/rabbit_epmd_monitor.erl#L36

Having the only one socket-activated epmd completely solves this
problem.
---
 nixos/modules/module-list.nix              |  1 +
 nixos/modules/services/networking/epmd.nix | 56 ++++++++++++++++++++++
 2 files changed, 57 insertions(+)
 create mode 100644 nixos/modules/services/networking/epmd.nix

diff --git a/nixos/modules/module-list.nix b/nixos/modules/module-list.nix
index cd00bc8d6bc..aa79849d18a 100644
--- a/nixos/modules/module-list.nix
+++ b/nixos/modules/module-list.nix
@@ -484,6 +484,7 @@
   ./services/networking/dnsdist.nix
   ./services/networking/dnsmasq.nix
   ./services/networking/ejabberd.nix
+  ./services/networking/epmd.nix
   ./services/networking/fakeroute.nix
   ./services/networking/ferm.nix
   ./services/networking/firefox/sync-server.nix
diff --git a/nixos/modules/services/networking/epmd.nix b/nixos/modules/services/networking/epmd.nix
new file mode 100644
index 00000000000..692b75e4f08
--- /dev/null
+++ b/nixos/modules/services/networking/epmd.nix
@@ -0,0 +1,56 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.services.epmd;
+
+in
+
+{
+  ###### interface
+  options.services.epmd = {
+    enable = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to enable socket activation for Erlang Port Mapper Daemon (epmd),
+        which acts as a name server on all hosts involved in distributed
+        Erlang computations.
+      '';
+    };
+    package = mkOption {
+      type = types.package;
+      default = pkgs.erlang;
+      description = ''
+        The Erlang package to use to get epmd binary. That way you can re-use
+        an Erlang runtime that is already installed for other purposes.
+      '';
+    };
+  };
+
+  ###### implementation
+  config = mkIf cfg.enable {
+    systemd.sockets.epmd = rec {
+      description = "Erlang Port Mapper Daemon Activation Socket";
+      wantedBy = [ "sockets.target" ];
+      before = wantedBy;
+      socketConfig = {
+        ListenStream = "4369";
+        Accept = "false";
+      };
+    };
+
+    systemd.services.epmd = {
+      description = "Erlang Port Mapper Daemon";
+      after = [ "network.target" ];
+      requires = [ "epmd.socket" ];
+
+      serviceConfig = {
+        DynamicUser = true;
+        ExecStart = "${cfg.package}/bin/epmd -systemd";
+        Type = "notify";
+      };
+    };
+  };
+}