Merge pull request #287743 from nagisa/adds-systemd-upholds

systemd: add support for upholds and upheldBy

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

···
   [fileSystems.overlay](#opt-fileSystems._name_.overlay.lowerdir). See also the
   [NixOS docs](#sec-overlayfs).
 
+- systemd units can now specify the `Upholds=` and `UpheldBy=` unit dependencies via the aptly
+  named `upholds` and `upheldBy` options. These options get systemd to enforce that the
+  dependencies remain continuosly running for as long as the dependent unit is in a running state.
+
 - `stdenv`: The `--replace` flag in `substitute`, `substituteInPlace`, `substituteAll`, `substituteAllStream`, and `substituteStream` is now deprecated if favor of the new `--replace-fail`, `--replace-warn` and `--replace-quiet`. The deprecated `--replace` equates to `--replace-warn`.
 
 - A new hardening flag, `zerocallusedregs` was made available, corresponding to the gcc/clang option `-fzero-call-used-regs=used-gpr`.

nixos/lib/systemd-lib.nix

···
             ln -sfn '${name}' $out/'${name2}'
           '') (unit.aliases or [])) units)}
 
-      # Create .wants and .requires symlinks from the wantedBy and
+      # Create .wants, .upholds and .requires symlinks from the wantedBy, upheldBy and
       # requiredBy options.
       ${concatStrings (mapAttrsToList (name: unit:
           concatMapStrings (name2: ''
             mkdir -p $out/'${name2}.wants'
             ln -sfn '../${name}' $out/'${name2}.wants'/
           '') (unit.wantedBy or [])) units)}
+
+      ${concatStrings (mapAttrsToList (name: unit:
+          concatMapStrings (name2: ''
+            mkdir -p $out/'${name2}.upholds'
+            ln -sfn '../${name}' $out/'${name2}.upholds'/
+          '') (unit.upheldBy or [])) units)}
 
       ${concatStrings (mapAttrsToList (name: unit:
           concatMapStrings (name2: ''
···
           { Requires = toString config.requires; }
         // optionalAttrs (config.wants != [])
           { Wants = toString config.wants; }
+        // optionalAttrs (config.upholds != [])
+          { Upholds = toString config.upholds; }
         // optionalAttrs (config.after != [])
           { After = toString config.after; }
         // optionalAttrs (config.before != [])

nixos/lib/systemd-unit-options.nix

···
       '';
     };
 
+    upheldBy = mkOption {
+      default = [];
+      type = types.listOf unitNameType;
+      description = lib.mdDoc ''
+        Keep this unit running as long as the listed units are running. This is a continuously
+        enforced version of wantedBy.
+      '';
+    };
+
     wantedBy = mkOption {
       default = [];
       type = types.listOf unitNameType;
···
         type = types.listOf unitNameType;
         description = lib.mdDoc ''
           Start the specified units when this unit is started.
+        '';
+      };
+
+      upholds = mkOption {
+        default = [];
+        type = types.listOf unitNameType;
+        description = lib.mdDoc ''
+          Keeps the specified running while this unit is running. A continuous version of `wants`.
         '';
       };