lib/systemd: new package for socket-based activation

The `lib/systemd` package implement function `Listeners` that return list
of file descriptor as [net.Listener], that enable program to run with
systemd.socket(5) based activation.

3 files changed, 37 insertions, 0 deletions

diff --git a/lib/systemd/testdata/cmd/daemonTCP/daemonTCP.service b/lib/systemd/testdata/cmd/daemonTCP/daemonTCP.service
new file mode 100644
index 00000000..cb37dd67
--- /dev/null
+++ b/lib/systemd/testdata/cmd/daemonTCP/daemonTCP.service
@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: BSD-3-Clause
+# SPDX-FileCopyrightText: 2026 M. Shulhan <ms@kilabit.info>
+
+[Service]
+ExecStart=/home/ms/go/bin/daemonTCP
+
+[Install]
+WantedBy=multi-user.target
diff --git a/lib/systemd/testdata/cmd/daemonTCP/daemonTCP.socket b/lib/systemd/testdata/cmd/daemonTCP/daemonTCP.socket
new file mode 100644
index 00000000..fe03e343
--- /dev/null
+++ b/lib/systemd/testdata/cmd/daemonTCP/daemonTCP.socket
@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: BSD-3-Clause
+# SPDX-FileCopyrightText: 2026 M. Shulhan <ms@kilabit.info>
+
+[Socket]
+ListenStream=127.0.0.1:20001
+
+[Install]
+WantedBy=sockets.target
diff --git a/lib/systemd/testdata/cmd/daemonTCP/main.go b/lib/systemd/testdata/cmd/daemonTCP/main.go
new file mode 100644
index 00000000..e667d8a0
--- /dev/null
+++ b/lib/systemd/testdata/cmd/daemonTCP/main.go
@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: BSD-3-Clause
+// SPDX-FileCopyrightText: 2026 M. Shulhan <ms@kilabit.info>
+
+package main
+
+import (
+	"fmt"
+	"log"
+
+	"git.sr.ht/~shulhan/pakakeh.go/lib/systemd"
+)
+
+func main() {
+	lns, err := systemd.Listeners(true)
+	if err != nil {
+		log.Fatal(err)
+	}
+	for _, ln := range lns {
+		fmt.Printf(`listener network address: %s`, ln.Addr())
+	}
+}