nixos/systemd: allow using writeShellApplication for systemd unit scripts

nixos/doc/manual/release-notes/rl-2411.section.md

@@ -62,6 +62,15 @@
   - The OCaml-based Xen Store can now be configured using  [`virtualisation.xen.store.settings`](#opt-virtualisation.xen.store.settings).
   - The `virtualisation.xen.bridge` options have been deprecated in this release cycle. Users who need network bridges are encouraged to set up their own networking configurations.
 
+- A new option [`systemd.enableStrictShellChecks`](#opt-systemd.enableStrictShellChecks) has been added. When enabled, all systemd scripts generated by NixOS will
+  be checked with [shellcheck](https://www.shellcheck.net) and any errors or warnings will cause the build to fail.
+  This affects all scripts that have been created through the `script`, `reload`, `preStart`, `postStart`, `preStop` and `postStop` options for systemd services.
+  This does not affect commandlines passed directly to `ExecStart`, `ExecReload`, `ExecStartPre`, `ExecStartPost`, `ExecStop` or `ExecStopPost`.
+  It therefore also does not affect systemd units that are coming from packages and that are not defined through the NixOS config.
+  This option is disabled by default, and although some services have already been fixed, it is still likely that you will encounter build failures when enabling this.
+  We encourage people to enable this option when they are willing and able to submit fixes for potential build failures to nixpkgs.
+  The option can also be enabled or disabled for individual services using the `enableStrictShellChecks` option on the service itself, which will take precedence over the global setting.
+
 ## New Modules {#sec-release-24.11-new-modules}
 
 - [TaskChampion Sync-Server](https://github.com/GothenburgBitFactory/taskchampion-sync-server), a [Taskwarrior 3](https://taskwarrior.org/docs/upgrade-3/) sync server, replacing Taskwarrior 2's sync server named [`taskserver`](https://github.com/GothenburgBitFactory/taskserver).

nixos/lib/systemd-lib.nix

@@ -386,18 +386,27 @@ in rec {
       ''}
     ''; # */
 
-  makeJobScript = name: text:
+  makeJobScript = { name, text, enableStrictShellChecks }:
     let
       scriptName = replaceStrings [ "\\" "@" ] [ "-" "_" ] (shellEscape name);
-      out = (pkgs.writeShellScriptBin scriptName ''
-        set -e
-        ${text}
-      '').overrideAttrs (_: {
+      out = (
+        if ! enableStrictShellChecks then
+          pkgs.writeShellScriptBin scriptName ''
+            set -e
+
+            ${text}
+          ''
+        else
+          pkgs.writeShellApplication {
+            name = scriptName;
+            inherit text;
+          }
+      ).overrideAttrs (_: {
         # The derivation name is different from the script file name
         # to keep the script file name short to avoid cluttering logs.
         name = "unit-script-${scriptName}";
       });
-    in "${out}/bin/${scriptName}";
+    in lib.getExe out;
 
   unitConfig = { config, name, options, ... }: {
     config = {
@@ -448,10 +457,16 @@ in rec {
     };
   };
 
-  serviceConfig = { name, config, ... }: {
+  serviceConfig =
+  let
+    nixosConfig = config;
+  in
+  { name, lib, config, ... }: {
     config = {
       name = "${name}.service";
       environment.PATH = mkIf (config.path != []) "${makeBinPath config.path}:${makeSearchPathOutput "bin" "sbin" config.path}";
+
+      enableStrictShellChecks = lib.mkOptionDefault nixosConfig.systemd.enableStrictShellChecks;
     };
   };
 

nixos/lib/systemd-unit-options.nix

@@ -17,6 +17,7 @@ let
     concatMap
     filterOverrides
     isList
+    literalExpression
     mergeEqualOption
     mkIf
     mkMerge
@@ -357,6 +358,14 @@ in rec {
         '';
       };
 
+      enableStrictShellChecks = mkOption {
+        type = types.bool;
+        description = "Enable running shellcheck on the generated scripts for this unit.";
+        # The default gets set in systemd-lib.nix because we don't have access to
+        # the full NixOS config here.
+        defaultText = literalExpression "config.systemd.enableStrictShellChecks";
+      };
+
       script = mkOption {
         type = types.lines;
         default = "";
@@ -428,27 +437,51 @@ in rec {
 
     config = mkMerge [
       (mkIf (config.preStart != "") rec {
-        jobScripts = makeJobScript "${name}-pre-start" config.preStart;
+        jobScripts = makeJobScript {
+          name = "${name}-pre-start";
+          text = config.preStart;
+          inherit (config) enableStrictShellChecks;
+        };
         serviceConfig.ExecStartPre = [ jobScripts ];
       })
       (mkIf (config.script != "") rec {
-        jobScripts = makeJobScript "${name}-start" config.script;
+        jobScripts = makeJobScript {
+          name = "${name}-start";
+          text = config.script;
+          inherit (config) enableStrictShellChecks;
+        };
         serviceConfig.ExecStart = jobScripts + " " + config.scriptArgs;
       })
       (mkIf (config.postStart != "") rec {
-        jobScripts = (makeJobScript "${name}-post-start" config.postStart);
+        jobScripts = makeJobScript {
+          name = "${name}-post-start";
+          text = config.postStart;
+          inherit (config) enableStrictShellChecks;
+        };
         serviceConfig.ExecStartPost = [ jobScripts ];
       })
       (mkIf (config.reload != "") rec {
-        jobScripts = makeJobScript "${name}-reload" config.reload;
+        jobScripts = makeJobScript {
+          name = "${name}-reload";
+          text = config.reload;
+          inherit (config) enableStrictShellChecks;
+        };
         serviceConfig.ExecReload = jobScripts;
       })
       (mkIf (config.preStop != "") rec {
-        jobScripts = makeJobScript "${name}-pre-stop" config.preStop;
+        jobScripts = makeJobScript {
+          name = "${name}-pre-stop";
+          text = config.preStop;
+          inherit (config) enableStrictShellChecks;
+        };
         serviceConfig.ExecStop = jobScripts;
       })
       (mkIf (config.postStop != "") rec {
-        jobScripts = makeJobScript "${name}-post-stop" config.postStop;
+        jobScripts = makeJobScript {
+          name = "${name}-post-stop";
+          text = config.postStop;
+          inherit (config) enableStrictShellChecks;
+        };
         serviceConfig.ExecStopPost = jobScripts;
       })
     ];

nixos/modules/system/boot/systemd.nix

@@ -197,6 +197,8 @@ in
 
     package = mkPackageOption pkgs "systemd" {};
 
+    enableStrictShellChecks = mkEnableOption "running shellcheck on the generated scripts for systemd units.";
+
     units = mkOption {
       description = "Definition of systemd units; see {manpage}`systemd.unit(5)`.";
       default = {};