scm: support --listen flag for uds

Support the --listen flag to mu scm, to start listening on a Unix domain socket.

mu/mu-options.cc

@@ -632,8 +632,11 @@ sub_server(CLI::App& sub, Options& opts)
 static void
 sub_scm(CLI::App& sub, Options& opts)
 {
+	sub.add_flag("--listen", opts.scm.listen,
+		       "Start listening on a domain socket");
 	sub.add_option("script-path", opts.scm.script_path, "Path to script")
-		->type_name("<path>");
+		->type_name("<path>")
+		->excludes("--listen");
 	sub.add_option("script-args", opts.scm.params, "Parameters for script")
 		->type_name("<parameters>");
 }

mu/mu-options.hh

@@ -259,6 +259,7 @@ struct Options {
 	struct Scm {
 		OptString script_path; /**< Path to script (optional) */
 		StringVec params;      /**< Parameters for script (after "--") */
+		bool listen; /**< Whether to start listening on a socket */
 	} scm;
 
 

scm/meson.build

@@ -17,8 +17,7 @@ mu_scm_dir=join_paths(datadir, 'mu', 'scm')
 mu_scm_dir_arg='-DMU_SCM_DIR="' + mu_scm_dir + '"'
 
 lib_mu_scm=static_library(
-  'mu-scm',
-  [
+  'mu-scm',  [
     'mu-scm.cc',
     'mu-scm-message.cc',
     'mu-scm-mime.cc',
@@ -33,7 +32,9 @@ lib_mu_scm=static_library(
   install: false,
   cpp_args: [mu_scm_dir_arg])
 
-install_data(['mu-scm.scm', 'mu-scm-shell.scm'], install_dir : mu_scm_dir)
+install_data(['mu-scm.scm',
+              'mu-scm-repl.scm'],
+             install_dir : mu_scm_dir)
 
 # note: top-level meson.build defines a dummy replacement for this.
 mu_scm_dep = declare_dependency(

scm/mu-scm-repl.scm

@@ -13,6 +13,16 @@
 ;; You should have received a copy of the GNU General Public License
 ;; along with this program; if not, write to the Free Software Foundation,
 ;; Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+(use-modules (system repl server)
+	     (ice-9 threads))
+(use-modules (mu))
+
+;; when a socket path is defined, listen on it (blocking)
+;; after printing UNIX-CONNECT:<socket-file>\n on stdout
+(let ((socket-path (getenv "MU_SCM_SOCKET_PATH")))
+  (when socket-path
+    (format #t "UNIX-CONNECT:~a\n" socket-path)
+    (run-server
+     (make-unix-domain-server-socket #:path socket-path))))
 
 (display "Welcome to the mu shell!\n\n")
-(use-modules (mu))

scm/mu-scm-store.cc

@@ -216,7 +216,6 @@ init_subrs()
 #pragma GCC diagnostic pop
 }
 
-
 void
 Mu::Scm::init_store(const Store& store)
 {

scm/mu-scm.cc

@@ -85,7 +85,9 @@ make_mu_scm_path(const std::string& fname) {
 
 namespace {
 static std::string mu_scm_path;
-static std::string mu_scm_shell_path;
+static std::string mu_scm_repl_path;
+static std::string mu_scm_socket_path;
+constexpr auto SOCKET_PATH_ENV = "MU_SCM_SOCKET_PATH";
 }
 
 static Result<void>
@@ -104,12 +106,11 @@ prepare_run(const Mu::Scm::Config& conf)
 	else
 		return Err(path.error());
 
-	if (const auto path = make_mu_scm_path("mu-scm-shell.scm"); path)
-		mu_scm_shell_path = *path;
+	if (const auto path = make_mu_scm_path("mu-scm-repl.scm"); path)
+		mu_scm_repl_path = *path;
 	else
 		return Err(path.error());
 
-
 	if (config->options.scm.script_path) {
 		const auto path{config->options.scm.script_path->c_str()};
 		if (const auto res = ::access(path, R_OK); res != 0) {
@@ -121,9 +122,28 @@ prepare_run(const Mu::Scm::Config& conf)
 	return Ok();
 }
 
-Result<void>
-Mu::Scm::run(const Mu::Scm::Config& conf) {
+// make a unique unix-socket path
+static std::string
+maybe_set_uds_path(bool set)
+{
+	if (set) {
+		GRand* grand{g_rand_new()};
+		auto path = join_paths(g_get_user_runtime_dir(),
+				       mu_format("mu-scm-socket-{:08x}",
+						 g_rand_int(grand)));
+		g_rand_free(grand);
+		g_setenv(SOCKET_PATH_ENV, path.c_str(), 1);
+		return path;
+	} else  {
+		g_unsetenv(SOCKET_PATH_ENV);
+		return {};
+	}
+}
 
+
+Result<void>
+Mu::Scm::run(const Mu::Scm::Config& conf)
+{
 	if (const auto res = prepare_run(conf); !res)
 		return Err(res.error());
 
@@ -151,10 +171,13 @@ Mu::Scm::run(const Mu::Scm::Config& conf) {
 					"-c", cmd.c_str()})
 				args.emplace_back(arg);
 		} else {
-			// otherwise, drop us into an interactive shell/repl (and
-			// shell spec)
+			// otherwise, drop us into an interactive shell/repl
+			// or start listening on a domain socket.
+			mu_scm_socket_path =
+				maybe_set_uds_path(config->options.scm.listen);
+			args.emplace_back("--no-auto-compile");
 			args.emplace_back("-l");
-			args.emplace_back(mu_scm_shell_path.c_str());
+			args.emplace_back(mu_scm_repl_path.c_str());
 		}
 		/* ahem...*/
 		scm_shell(std::size(args), const_cast<char**>(args.data()));