git-sync: add darwin support

- On darwin, creates a launch agent to run git-sync on an interval and
  when the `path` changes.

- The `uri` option is not used on Darwin. The auto-creation of the
  local git directory from the `uri` is a feature of the
  git-sync-on-inotify [1] wrapper (which won't work on Darwin afaik)
  and not `git-sync` itself.

[1] https://github.com/simonthum/git-sync/blob/master/contrib/git-sync-on-inotify

modules/services/git-sync.nix

@@ -24,9 +24,21 @@ let
     };
   };
 
+  mkAgent = name: repo: {
+    enable = true;
+    config = {
+      StartInterval = repo.interval;
+      ProcessType = "Background";
+      WorkingDirectory = "${repo.path}";
+      WatchPaths = [ "${repo.path}" ];
+      ProgramArguments = [ "${cfg.package}/bin/git-sync" ];
+    };
+  };
+
+  mkService = if pkgs.stdenv.isLinux then mkUnit else mkAgent;
   services = mapAttrs' (name: repo: {
     name = "git-sync-${name}";
-    value = mkUnit name repo;
+    value = mkService name repo;
   }) cfg.repositories;
 
   repositoryType = types.submodule ({ name, ... }: {
@@ -51,6 +63,8 @@ let
           event that the directory does not already exist. See
           <https://git-scm.com/docs/git-clone#_git_urls>
           for the supported URIs.
+
+          This option is not supported on Darwin.
         '';
       };
 
@@ -66,7 +80,8 @@ let
   });
 
 in {
-  meta.maintainers = [ maintainers.imalison maintainers.cab404 ];
+  meta.maintainers =
+    [ maintainers.imalison maintainers.cab404 maintainers.ryane ];
 
   options = {
     services.git-sync = {
@@ -90,12 +105,9 @@ in {
     };
   };
 
-  config = mkIf cfg.enable {
-    assertions = [
-      (lib.hm.assertions.assertPlatform "services.git-sync" pkgs
-        lib.platforms.linux)
-    ];
+  config = mkIf cfg.enable (mkMerge [
+    (mkIf pkgs.stdenv.isLinux { systemd.user.services = services; })
+    (mkIf pkgs.stdenv.isDarwin { launchd.agents = services; })
+  ]);
 
-    systemd.user.services = services;
-  };
 }

tests/default.nix

@@ -147,6 +147,7 @@ import nmt {
     ./modules/xresources
   ] ++ lib.optionals isDarwin [
     ./modules/launchd
+    ./modules/services/git-sync-darwin
     ./modules/services/imapnotify-darwin
     ./modules/targets-darwin
   ] ++ lib.optionals isLinux [

tests/modules/services/git-sync-darwin/basic.nix

@@ -0,0 +1,18 @@
+{ config, ... }:
+
+{
+  services.git-sync = {
+    enable = true;
+    package = config.lib.test.mkStubPackage { outPath = "@git-sync@"; };
+    repositories.test = {
+      path = "/a/path";
+      uri = "git+ssh://user@example.com:/~user/path/to/repo.git";
+    };
+  };
+
+  nmt.script = ''
+    serviceFile=LaunchAgents/org.nix-community.home.git-sync-test.plist
+    assertFileExists "$serviceFile"
+    assertFileContent "$serviceFile" ${./expected-agent.plist}
+  '';
+}

tests/modules/services/git-sync-darwin/default.nix

@@ -0,0 +1 @@
+{ git-sync = ./basic.nix; }

tests/modules/services/git-sync-darwin/expected-agent.plist

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>Label</key>
+	<string>org.nix-community.home.git-sync-test</string>
+	<key>ProcessType</key>
+	<string>Background</string>
+	<key>ProgramArguments</key>
+	<array>
+		<string>@git-sync@/bin/git-sync</string>
+	</array>
+	<key>StartInterval</key>
+	<integer>500</integer>
+	<key>WatchPaths</key>
+	<array>
+		<string>/a/path</string>
+	</array>
+	<key>WorkingDirectory</key>
+	<string>/a/path</string>
+</dict>
+</plist>