util: Add a simple template system for systemd unit files

Just the basics for what we need, nothing more. Not intended as a
general-purpose library for use elsewhere.

util/systemd_unit.go

@@ -0,0 +1,134 @@
+// Mgmt
+// Copyright (C) 2013-2024+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+//
+// Additional permission under GNU GPL version 3 section 7
+//
+// If you modify this program, or any covered work, by linking or combining it
+// with embedded mcl code and modules (and that the embedded mcl code and
+// modules which link with this program, contain a copy of their source code in
+// the authoritative form) containing parts covered by the terms of any other
+// license, the licensors of this program grant you additional permission to
+// convey the resulting work. Furthermore, the licensors of this program grant
+// the original author, James Shubin, additional permission to update this
+// additional permission if he deems it necessary to achieve the goals of this
+// additional permission.
+
+package util
+
+import (
+	"fmt"
+)
+
+// UnitData is the data struct used to build a systemd unit file. This isn't an
+// exhaustive representation of what's possible, but is meant to handle most
+// common cases. Alternatively we could have used the
+// github.com/coreos/go-systemd/v22/unit library, but it didn't provide much
+// value. More documentation on these fields can be seen at:
+// https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html
+type UnitData struct {
+	// Description field for the unit file.
+	Description string
+
+	// Documentation field for the unit file.
+	Documentation string
+
+	// After makes this happen after that target is ready. Usually you want
+	// "network.target" or "systemd-networkd.service" here I think.
+	After []string
+
+	// Type is the type of service to run, such as "oneshot".
+	Type string
+
+	// ExecStart is the main command to run.
+	ExecStart string
+
+	// RestartSec is the number of seconds to wait between restarts.
+	RestartSec string
+
+	// Restart is the restart policy. Usually you want "always".
+	Restart string
+
+	// RemainAfterExit can be set to true to make it look like the service
+	// is still active if it exits successfully.
+	RemainAfterExit bool
+
+	// StandardOutput specifies what happens with stdout.
+	StandardOutput string
+
+	// StandardError specifies what happens with stderr.
+	StandardError string
+
+	// WantedBy makes this a dependency of this target. Usually you want
+	// "multi-user.target" here.
+	WantedBy []string
+}
+
+// Template uses the UnitData to build a systemd unit file. This API is subject
+// to change, don't assume the output is stable either.
+// TODO: Should this return an error or not?
+func (obj *UnitData) Template() (string, error) {
+	data := ""
+	data += "[Unit]\n"
+	if obj.Description != "" {
+		data += fmt.Sprintf("Description=%s\n", obj.Description)
+	}
+	if obj.Documentation != "" {
+		data += fmt.Sprintf("Documentation=%s\n", obj.Documentation)
+	}
+	for _, x := range obj.After {
+		if x == "" {
+			continue
+		}
+		data += fmt.Sprintf("After=%s\n", x)
+	}
+
+	data += "\n"
+	data += "[Service]\n"
+	if obj.Type != "" {
+		data += fmt.Sprintf("Type=%s\n", obj.Type)
+	}
+	if obj.ExecStart != "" {
+		data += fmt.Sprintf("ExecStart=%s\n", obj.ExecStart)
+	}
+	if obj.RestartSec != "" {
+		data += fmt.Sprintf("RestartSec=%s\n", obj.RestartSec)
+	}
+	if obj.Restart != "" {
+		data += fmt.Sprintf("Restart=%s\n", obj.Restart)
+	}
+
+	if obj.RemainAfterExit {
+		data += "RemainAfterExit=yes\n"
+	}
+	if obj.StandardOutput != "" {
+		data += fmt.Sprintf("StandardOutput=%s\n", obj.StandardOutput)
+	}
+	if obj.StandardError != "" {
+		data += fmt.Sprintf("StandardError=%s\n", obj.StandardError)
+	}
+
+	data += "\n"
+	data += "[Install]\n"
+	for _, x := range obj.WantedBy {
+		if x == "" {
+			continue
+		}
+		data += fmt.Sprintf("WantedBy=%s\n", x)
+	}
+
+	return data, nil
+}