lib/discover: Don't leak goroutines (#5950)

lib/beacon/beacon.go

@@ -7,9 +7,13 @@
 package beacon
 
 import (
+	"fmt"
 	"net"
+	"time"
 
 	"github.com/thejerf/suture"
+
+	"github.com/syncthing/syncthing/lib/util"
 )
 
 type recv struct {
@@ -19,7 +23,93 @@ type recv struct {
 
 type Interface interface {
 	suture.Service
+	fmt.Stringer
 	Send(data []byte)
 	Recv() ([]byte, net.Addr)
 	Error() error
 }
+
+type cast struct {
+	*suture.Supervisor
+	name    string
+	reader  util.ServiceWithError
+	writer  util.ServiceWithError
+	outbox  chan recv
+	inbox   chan []byte
+	stopped chan struct{}
+}
+
+// newCast creates a base object for multi- or broadcasting. Afterwards the
+// caller needs to set reader and writer with the addReader and addWriter
+// methods to get a functional implementation of Interface.
+func newCast(name string) *cast {
+	return &cast{
+		Supervisor: suture.New(name, suture.Spec{
+			// Don't retry too frenetically: an error to open a socket or
+			// whatever is usually something that is either permanent or takes
+			// a while to get solved...
+			FailureThreshold: 2,
+			FailureBackoff:   60 * time.Second,
+			// Only log restarts in debug mode.
+			Log: func(line string) {
+				l.Debugln(line)
+			},
+			PassThroughPanics: true,
+		}),
+		name:    name,
+		inbox:   make(chan []byte),
+		outbox:  make(chan recv, 16),
+		stopped: make(chan struct{}),
+	}
+}
+
+func (c *cast) addReader(svc func(chan struct{}) error) {
+	c.reader = c.createService(svc, "reader")
+	c.Add(c.reader)
+}
+
+func (c *cast) addWriter(svc func(stop chan struct{}) error) {
+	c.writer = c.createService(svc, "writer")
+	c.Add(c.writer)
+}
+
+func (c *cast) createService(svc func(chan struct{}) error, suffix string) util.ServiceWithError {
+	return util.AsServiceWithError(func(stop chan struct{}) error {
+		l.Debugln("Starting", c.name, suffix)
+		err := svc(stop)
+		l.Debugf("Stopped %v %v: %v", c.name, suffix, err)
+		return err
+	})
+}
+
+func (c *cast) Stop() {
+	c.Supervisor.Stop()
+	close(c.stopped)
+}
+
+func (c *cast) String() string {
+	return fmt.Sprintf("%s@%p", c.name, c)
+}
+
+func (c *cast) Send(data []byte) {
+	select {
+	case c.inbox <- data:
+	case <-c.stopped:
+	}
+}
+
+func (c *cast) Recv() ([]byte, net.Addr) {
+	select {
+	case recv := <-c.outbox:
+		return recv.data, recv.src
+	case <-c.stopped:
+	}
+	return nil, nil
+}
+
+func (c *cast) Error() error {
+	if err := c.reader.Error(); err != nil {
+		return err
+	}
+	return c.writer.Error()
+}