nixos/gitolite: customize .gitolite.rc declaratively

Add the `extraGitoliteRc` option to customize the `.gitolite.rc`
configuration file declaratively.

Resolves #29249.

nixos/modules/services/misc/gitolite.nix

@@ -49,6 +49,35 @@ in
         '';
       };
 
+      extraGitoliteRc = mkOption {
+        type = types.lines;
+        default = "";
+        example = literalExample ''
+          $RC{UMASK} = 0027;
+          $RC{SITE_INFO} = 'This is our private repository host';
+          push( @{$RC{ENABLE}}, 'Kindergarten' ); # enable the command/feature
+          @{$RC{ENABLE}} = grep { $_ ne 'desc' } @{$RC{ENABLE}}; # disable the command/feature
+        '';
+        description = ''
+          Extra configuration to append to the default <literal>~/.gitolite.rc</literal>.
+
+          This should be Perl code that modifies the <literal>%RC</literal>
+          configuration variable. The default <literal>~/.gitolite.rc</literal>
+          content is generated by invoking <literal>gitolite print-default-rc</literal>,
+          and extra configuration from this option is appended to it. The result
+          is placed to Nix store, and the <literal>~/.gitolite.rc</literal> file
+          becomes a symlink to it.
+
+          If you already have a customized (or otherwise changed)
+          <literal>~/.gitolite.rc</literal> file, NixOS will refuse to replace
+          it with a symlink, and the `gitolite-init` initialization service
+          will fail. In this situation, in order to use this option, you
+          will need to take any customizations you may have in
+          <literal>~/.gitolite.rc</literal>, convert them to appropriate Perl
+          statements, add them to this option, and remove the file.
+        '';
+      };
+
       user = mkOption {
         type = types.str;
         default = "gitolite";
@@ -59,7 +88,34 @@ in
     };
   };
 
-  config = mkIf cfg.enable {
+  config = mkIf cfg.enable (
+  let
+    manageGitoliteRc = cfg.extraGitoliteRc != "";
+    rcDir = pkgs.runCommand "gitolite-rc" { } rcDirScript;
+    rcDirScript =
+      ''
+        mkdir "$out"
+        export HOME=temp-home
+        mkdir -p "$HOME/.gitolite/logs" # gitolite can't run without it
+        '${pkgs.gitolite}'/bin/gitolite print-default-rc >>"$out/gitolite.rc.default"
+        cat <<END >>"$out/gitolite.rc"
+        # This file is managed by NixOS.
+        # Use services.gitolite options to control it.
+
+        END
+        cat "$out/gitolite.rc.default" >>"$out/gitolite.rc"
+      '' +
+      optionalString (cfg.extraGitoliteRc != "") ''
+        echo -n ${escapeShellArg ''
+
+          # Added by NixOS:
+          ${removeSuffix "\n" cfg.extraGitoliteRc}
+
+          # per perl rules, this should be the last line in such a file:
+          1;
+        ''} >>"$out/gitolite.rc"
+      '';
+  in {
     users.extraUsers.${cfg.user} = {
       description     = "Gitolite user";
       home            = cfg.dataDir;
@@ -77,10 +133,50 @@ in
       serviceConfig.Type = "oneshot";
       serviceConfig.RemainAfterExit = true;
 
-      path = [ pkgs.gitolite pkgs.git pkgs.perl pkgs.bash config.programs.ssh.package ];
+      path = [ pkgs.gitolite pkgs.git pkgs.perl pkgs.bash pkgs.diffutils config.programs.ssh.package ];
-      script = ''
+      script =
+      let
+        rcSetupScriptIfCustomFile =
+          if manageGitoliteRc then ''
+            cat <<END
+            <3>ERROR: NixOS can't apply declarative configuration
+            <3>to your .gitolite.rc file, because it seems to be
+            <3>already customized manually.
+            <3>See the services.gitolite.extraGitoliteRc option
+            <3>in "man configuration.nix" for more information.
+            END
+            # Not sure if the line below addresses the issue directly or just
+            # adds a delay, but without it our error message often doesn't
+            # show up in `systemctl status gitolite-init`.
+            journalctl --flush
+            exit 1
+          '' else ''
+            :
+          '';
+        rcSetupScriptIfDefaultFileOrStoreSymlink =
+          if manageGitoliteRc then ''
+            ln -sf "${rcDir}/gitolite.rc" "$GITOLITE_RC"
+          '' else ''
+            [[ -L "$GITOLITE_RC" ]] && rm -f "$GITOLITE_RC"
+          '';
+      in
+        ''
           cd ${cfg.dataDir}
           mkdir -p .gitolite/logs
+
+          GITOLITE_RC=.gitolite.rc
+          GITOLITE_RC_DEFAULT=${rcDir}/gitolite.rc.default
+          if ( [[ ! -e "$GITOLITE_RC" ]] && [[ ! -L "$GITOLITE_RC" ]] ) ||
+             ( [[ -f "$GITOLITE_RC" ]] && diff -q "$GITOLITE_RC" "$GITOLITE_RC_DEFAULT" >/dev/null ) ||
+             ( [[ -L "$GITOLITE_RC" ]] && [[ "$(readlink "$GITOLITE_RC")" =~ ^/nix/store/ ]] )
+          then
+        '' + rcSetupScriptIfDefaultFileOrStoreSymlink +
+        ''
+          else
+        '' + rcSetupScriptIfCustomFile +
+        ''
+          fi
+
           if [ ! -d repositories ]; then
             gitolite setup -pk ${pubkeyFile}
           fi
@@ -93,5 +189,5 @@ in
     };
 
     environment.systemPackages = [ pkgs.gitolite pkgs.git ];
-  };
+  });
 }