feat: close transports that implement io.Closer

This way, transports with shared resources (e.g., reused sockets) can clean them
up.

fixes libp2p/go-libp2p#999

swarm.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -176,6 +177,20 @@ func (s *Swarm) teardown() error {
 	// Wait for everything to finish.
 	s.refs.Wait()
 
+	// Now close out any transports (if necessary).
+	s.transports.Lock()
+	transports := s.transports.m
+	s.transports.m = nil
+	s.transports.Unlock()
+
+	for t := range transports {
+		if closer, ok := t.(io.Closer); ok {
+			if err := closer.Close(); err != nil {
+				log.Errorf("error when closing down transport: %s", err)
+			}
+		}
+	}
+
 	return nil
 }