modules: add declarative git repository management

modules/git.nix

@@ -0,0 +1,68 @@
+{ config, lib, pkgs, ... }:
+with lib;
+let
+  cfg = config.niveum.git;
+
+  repositoryService = name: repository: nameValuePair "git-repository-${name}" {
+    enable = repository.enable;
+    startAt = "*/3:0";
+    wants = [ (if (repository.autoFetch != {}) then "network-online.target" else "multi-user.target") ];
+    after = [ (if (repository.autoFetch != {}) then "network-online.target" else "multi-user.target") ];
+    serviceConfig = {
+      Type = "oneshot";
+      RandomizedDelaySec = "300";
+    };
+    script =
+    let
+      branchSetup = flip map repository.branches (branch: ''
+        ${pkgs.git}/bin/git branch ${branch}
+      '');
+      remoteSetup = concatStringsSep "\n" (flip mapAttrsToList repository.remotes (name: url: ''
+        ${pkgs.git}/bin/git remote add ${name} ${url}
+      ''));
+      fetch = concatStringsSep "\n" (flip map repository.autoFetch ({ branch, remote }: ''
+        ${pkgs.git}/bin/git fetch ${remote} ${branch}
+      ''));
+      repositorySetup = ''
+        if [ ! -d ${repository.location}/.git ]; then
+          ${pkgs.git}/bin/git clone ${repository.remotes.origin} ${repository.location}
+        fi
+      '';
+    in ''
+      ${repositorySetup}
+      cd ${repository.location}
+
+      ${branchSetup}
+      ${remoteSetup}
+      ${fetch}
+    '';
+  };
+in {
+  options.niveum.git = {
+    enable = mkEnableOption "declarative repository management";
+    repositories = mkOption {
+      type = types.attrsOf (types.submodule {
+        options = {
+          enable = mkEnableOption "git repository";
+          location = mkOption { type = types.path; };
+          remotes = mkOption { type = types.attrsOf types.path; default = {}; };
+          branches = mkOption { type = types.listOf types.str; default = []; };
+          autoFetch = mkOption {
+            type = types.listOf (types.submodule {
+              options = {
+                branch = mkOption { type = types.str; };
+                remote = mkOption { type = types.str; };
+              };
+            });
+            default = {};
+          };
+        };
+      });
+      default = {};
+    };
+  };
+
+  config = mkIf cfg.enable {
+    systemd.services = attrsets.mapAttrs' repositoryService cfg.repositories;
+  };
+}