Add custom nix-daemon exporter

Allows us to see which derivations are being built in realtime. It is a
bit of a hack, but it seems to work. We simply look at the environment
of the child processes of nix-daemon (usually bash) and then look for
the $name variable which should hold the current derivation being
built. Needs root to be able to read the environ file of the different
nix-daemon processes as they are owned by the nixbld* users.

See: https://discourse.nixos.org/t/query-ongoing-builds/23486
Reviewed-by: Aleix Boné <abonerib@bsc.es>

m/hut/monitoring.nix

@@ -4,6 +4,7 @@
   imports = [
     ../module/slurm-exporter.nix
     ./gpfs-probe.nix
+    ./nix-daemon-exporter.nix
   ];
 
   age.secrets.grafanaJungleRobotPassword = {
@@ -108,6 +109,7 @@
             "127.0.0.1:${toString config.services.prometheus.exporters.smartctl.port}"
             "127.0.0.1:9341" # Slurm exporter
             "127.0.0.1:9966" # GPFS custom exporter
+            "127.0.0.1:9999" # Nix-daemon custom exporter
             "127.0.0.1:${toString config.services.prometheus.exporters.blackbox.port}"
           ];
         }];

m/hut/nix-daemon-builds.sh

@@ -0,0 +1,26 @@
+#!/bin/sh
+
+# Locate nix daemon pid
+nd=$(pgrep -o nix-daemon)
+
+# Locate children of nix-daemon
+pids1=$(tr ' ' '\n' < "/proc/$nd/task/$nd/children")
+
+# For each children, locate 2nd level children
+pids2=$(echo "$pids1" | xargs -I @ /bin/sh -c 'cat /proc/@/task/*/children' | tr ' ' '\n')
+
+cat <<EOF
+HTTP/1.1 200 OK
+Content-Type: text/plain; version=0.0.4; charset=utf-8; escaping=values
+
+# HELP nix_daemon_build Nix daemon derivation build state.
+# TYPE nix_daemon_build gauge
+EOF
+
+for pid in $pids2; do
+  name=$(cat /proc/$pid/environ 2>/dev/null | tr '\0' '\n' | rg "^name=(.+)" - --replace '$1' | tr -dc ' [:alnum:]_\-\.')
+  user=$(ps -o uname= -p "$pid")
+  if [ -n "$name" -a -n "$user" ]; then
+    printf 'nix_daemon_build{user="%s",name="%s"} 1\n' "$user" "$name"
+  fi
+done

m/hut/nix-daemon-exporter.nix

@@ -0,0 +1,23 @@
+{ pkgs, config, lib, ... }:
+let
+  script = pkgs.runCommand "nix-daemon-exporter.sh" { }
+    ''
+      cp ${./nix-daemon-builds.sh} $out;
+      chmod +x $out
+    ''
+  ;
+in
+{
+  systemd.services.nix-daemon-exporter = {
+    description = "Daemon to export nix-daemon metrics";
+    path = [ pkgs.procps pkgs.ripgrep ];
+    wantedBy = [ "default.target" ];
+    serviceConfig = {
+      Type = "simple";
+      ExecStart = "${pkgs.socat}/bin/socat TCP4-LISTEN:9999,fork EXEC:${script}";
+      # Needed root to read the environment, potentially unsafe
+      User = "root";
+      Group = "root";
+    };
+  };
+}