BREAKING CHANGE: move Uncloud API socket to /run/uncloud/api/uncloud.sock, activate it by systemd socket unit

internal/daemon/daemon.go

@@ -2,24 +2,50 @@ package daemon
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"log/slog"
+	"net"
 
+	"github.com/coreos/go-systemd/activation"
 	systemd "github.com/coreos/go-systemd/daemon"
 	"github.com/psviderski/uncloud/internal/machine"
 )
 
+const systemdSocketUnit = "uncloud.socket"
+
 type Daemon struct {
 	machine *machine.Machine
 }
 
 func New(dataDir string) (*Daemon, error) {
+	listeners, err := activation.ListenersWithNames()
+	if err != nil {
+		return nil, fmt.Errorf("get systemd-activated sockets: %w", err)
+	}
+
+	listener, err := selectActivatedListener(listeners)
+	if err != nil {
+		return nil, err
+	}
+	if listener != nil {
+		slog.Info("Using systemd-activated API socket.", "addr", listener.Addr().String())
+	}
+
 	config := &machine.Config{
-		DataDir: dataDir,
+		DataDir:            dataDir,
+		ClusterAPISockPath: machine.DefaultClusterAPISockPath,
+		ClusterAPIListener: listener,
 	}
 	mach, err := machine.NewMachine(config)
 	if err != nil {
-		return nil, fmt.Errorf("init machine: %w", err)
+		initErr := fmt.Errorf("init machine: %w", err)
+		if listener != nil {
+			if closeErr := listener.Close(); closeErr != nil {
+				initErr = errors.Join(initErr, fmt.Errorf("close systemd-activated socket: %w", closeErr))
+			}
+		}
+		return nil, initErr
 	}
 
 	return &Daemon{
@@ -27,6 +53,40 @@ func New(dataDir string) (*Daemon, error) {
 	}, nil
 }
 
+// selectActivatedListener returns the systemd activated listener. If systemd supplied multiple listeners,
+// it returns the one associated with systemdSocketUnit.
+func selectActivatedListener(listeners map[string][]net.Listener) (net.Listener, error) {
+	var activated []net.Listener
+	for _, named := range listeners {
+		activated = append(activated, named...)
+	}
+
+	if len(activated) == 0 {
+		return nil, nil
+	}
+	if len(activated) == 1 {
+		return activated[0], nil
+	}
+
+	named, ok := listeners[systemdSocketUnit]
+	if !ok || len(named) != 1 {
+		return nil, fmt.Errorf("expected one systemd-activated socket named '%s', received %d",
+			systemdSocketUnit, len(named))
+	}
+
+	// Close all other unused listeners.
+	for name, unused := range listeners {
+		if name == systemdSocketUnit {
+			continue
+		}
+		for _, l := range unused {
+			_ = l.Close()
+		}
+	}
+
+	return named[0], nil
+}
+
 func (d *Daemon) Run(ctx context.Context) error {
 	slog.Info("Starting machine.")