nixos/systemd-lib: fix restart/reloadTriggers when passing paths

When passing a path to restartTriggers or reloadTriggers, X-Restart/Reload-Triggers
will get populated by the absolute path of the file on the machine where the
config is evaluated. This patch corrects this behavior.

nixos/doc/manual/release-notes/rl-2405.section.md

@@ -276,6 +276,9 @@ The pre-existing [services.ankisyncd](#opt-services.ankisyncd.enable) has been m
 
 - Ada packages (libraries and tools) have been moved into the `gnatPackages` scope. `gnatPackages` uses the default GNAT compiler, `gnat12Packages` and `gnat13Packages` use the respective matching compiler version.
 
+- Paths provided as `restartTriggers` and `reloadTriggers` for systemd units will now be copied into the nix store to make the behavior consistent.
+  Previously, `restartTriggers = [ ./config.txt ]` would either always or never trigger a restart when switching to a new configuration, depending on if the source was in a flake or not.
+
 - `spark2014` has been renamed to `gnatprove`. A version of `gnatprove` matching different GNAT versions is available from the different `gnatPackages` sets.
 
 - `services.resolved.fallbackDns` can now be used to disable the upstream fallback servers entirely by setting it to an empty list. To get the previous behaviour of the upstream defaults set it to null, the new default, instead.

nixos/lib/systemd-lib.nix

@@ -14,10 +14,12 @@ let
     elem
     filter
     filterAttrs
+    flatten
     flip
     head
     isInt
     isList
+    isPath
     length
     makeBinPath
     makeSearchPathOutput
@@ -28,6 +30,7 @@ let
     optional
     optionalAttrs
     optionalString
+    pipe
     range
     replaceStrings
     reverseList
@@ -362,9 +365,17 @@ in rec {
         // optionalAttrs (config.requisite != [])
           { Requisite = toString config.requisite; }
         // optionalAttrs (config ? restartTriggers && config.restartTriggers != [])
-          { X-Restart-Triggers = "${pkgs.writeText "X-Restart-Triggers-${name}" (toString config.restartTriggers)}"; }
+          { X-Restart-Triggers = "${pkgs.writeText "X-Restart-Triggers-${name}" (pipe config.restartTriggers [
+              flatten
+              (map (x: if isPath x then "${x}" else x))
+              toString
+            ])}"; }
         // optionalAttrs (config ? reloadTriggers && config.reloadTriggers != [])
-          { X-Reload-Triggers = "${pkgs.writeText "X-Reload-Triggers-${name}" (toString config.reloadTriggers)}"; }
+          { X-Reload-Triggers = "${pkgs.writeText "X-Reload-Triggers-${name}" (pipe config.reloadTriggers [
+              flatten
+              (map (x: if isPath x then "${x}" else x))
+              toString
+            ])}"; }
         // optionalAttrs (config.description != "") {
           Description = config.description; }
         // optionalAttrs (config.documentation != []) {