nixos/munin: fix broken plugin wrappers

The munin-node service used wrapProgram to inject environment variables.
This doesn't work because munin plugins depend on argv[0], which is
overwritten when the executable is a script with a shebang line (example
below).

This commit removes the wrappers and instead passes the required
environment variables to munin-node.

Eliminating the wrappers resulted in some broken plugins, e.g., meminfo
and hddtemp_smartctl. That was fixed with the per-plugin configuration.

Example:

The plugin if_eth0 is a symlink to /.../plugins/if_, which uses $0
to determine that it should monitor traffic on the eth0 interface.

if_ is a wrapped program, and runs `exec -a "$0" .if_-wrapped`

.if_-wrapped has a "#!/nix/.../bash" line, which results in bash
changing $0, and as a result the plugin thinks my interface
is called "-wrapped".

nixos/modules/services/monitoring/munin.nix

···
   nodeCfg = config.services.munin-node;
   cronCfg = config.services.munin-cron;
 
-  muninPlugins = pkgs.stdenv.mkDerivation {
-    name = "munin-available-plugins";
-    buildCommand = ''
-      mkdir -p $out
-
-      cp --preserve=mode ${pkgs.munin}/lib/plugins/* $out/
-
-      for file in $out/*; do
-        case "$file" in
-            */plugin.sh|*/plugins.history)
-              chmod +x "$file"
-              continue;;
-        esac
-
-        # read magic makers from the file
-        family=$(sed -nr 's/.*#%#\s+family\s*=\s*(\S+)\s*/\1/p' $file)
-        cap=$(sed -nr 's/.*#%#\s+capabilities\s*=\s*(.+)/\1/p' $file)
-
-        wrapProgram $file \
-          --set PATH "/run/wrappers/bin:/run/current-system/sw/bin" \
-          --set MUNIN_LIBDIR "${pkgs.munin}/lib" \
-          --set MUNIN_PLUGSTATE "/var/run/munin"
-
-        # munin uses markers to tell munin-node-configure what a plugin can do
-        echo "#%# family=$family" >> $file
-        echo "#%# capabilities=$cap" >> $file
-      done
-
-      # NOTE: we disable disktstats because plugin seems to fail and it hangs html generation (100% CPU + memory leak)
-      rm -f $out/diskstats
-    '';
-    buildInputs = [ pkgs.makeWrapper ];
-  };
-
   muninConf = pkgs.writeText "munin.conf"
     ''
       dbdir     /var/lib/munin
···
 
       ${nodeCfg.extraConfig}
     '';
+
+  pluginConf = pkgs.writeText "munin-plugin-conf"
+    ''
+      [hddtemp_smartctl]
+      user root
+      group root
+
+      [meminfo]
+      user root
+      group root
+
+      [ipmi*]
+      user root
+      group root
+    '';
+
+  pluginConfDir = pkgs.stdenv.mkDerivation {
+    name = "munin-plugin-conf.d";
+    buildCommand = ''
+      mkdir $out
+      ln -s ${pluginConf} $out/nixos-config
+    '';
+  };
 in
 
 {
···
       description = "Munin Node";
       after = [ "network.target" ];
       wantedBy = [ "multi-user.target" ];
-      path = [ pkgs.munin ];
+      path = with pkgs; [ munin smartmontools "/run/current-system/sw" "/run/wrappers" ];
+      environment.MUNIN_LIBDIR = "${pkgs.munin}/lib";
       environment.MUNIN_PLUGSTATE = "/var/run/munin";
+      environment.MUNIN_LOGDIR = "/var/log/munin";
       preStart = ''
         echo "updating munin plugins..."
 
         mkdir -p /etc/munin/plugins
         rm -rf /etc/munin/plugins/*
-        PATH="/run/wrappers/bin:/run/current-system/sw/bin" ${pkgs.munin}/sbin/munin-node-configure --shell --families contrib,auto,manual --config ${nodeConf} --libdir=${muninPlugins} --servicedir=/etc/munin/plugins 2>/dev/null | ${pkgs.bash}/bin/bash
+        ${pkgs.munin}/bin/munin-node-configure --suggest --shell --families contrib,auto,manual --config ${nodeConf} --libdir=${pkgs.munin}/lib/plugins --servicedir=/etc/munin/plugins --sconfdir=${pluginConfDir} 2>/dev/null | ${pkgs.bash}/bin/bash
+
+        # NOTE: we disable disktstats because plugin seems to fail and it hangs html generation (100% CPU + memory leak)
+        rm /etc/munin/plugins/diskstats || true
       '';
       serviceConfig = {
-        ExecStart = "${pkgs.munin}/sbin/munin-node --config ${nodeConf} --servicedir /etc/munin/plugins/";
+        ExecStart = "${pkgs.munin}/sbin/munin-node --config ${nodeConf} --servicedir /etc/munin/plugins/ --sconfdir=${pluginConfDir}";
       };
     };