change Err field in PeerStatus to string

The main use for this status is to send update messages through the WS,
in the form of JSON. Marshalling a JSON field into an error object
doesn't work in Go, so it's better to send the error message as string.
See golang/go#5161

client-peerconn_test.go

@@ -1,7 +1,6 @@
 package torrent
 
 import (
-	"errors"
 	"io"
 	"os"
 	"testing"
@@ -37,7 +36,7 @@ func TestPeerConnObserverReadStatusOk(t *testing.T) {
 
 	status := readChannelTimeout(t, cfg.Observers.Peers.PeerStatus, 500*time.Millisecond).(PeerStatus)
 	require.True(t, status.Ok)
-	require.Nil(t, status.Err)
+	require.Equal(t, "", status.Err)
 }
 
 func TestPeerConnObserverReadStatusErr(t *testing.T) {
@@ -55,13 +54,13 @@ func TestPeerConnObserverReadStatusErr(t *testing.T) {
 
 	go func() {
 		cfg.Observers.Peers.PeerStatus <- PeerStatus{
-			Err: errors.New("test error"),
+			Err: "test error",
 		}
 	}()
 
 	status := readChannelTimeout(t, cfg.Observers.Peers.PeerStatus, 500*time.Millisecond).(PeerStatus)
 	require.False(t, status.Ok)
-	require.EqualError(t, status.Err, "test error")
+	require.Equal(t, status.Err, "test error")
 }
 
 func TestPeerConnEstablished(t *testing.T) {
@@ -92,13 +91,13 @@ func TestPeerConnEstablished(t *testing.T) {
 	// FIXME converting [20]byte to string is not enough to pass the test
 	// require.Equal(t, "12345123451234512345", fmt.Sprintf("%+q", status.Id))
 	require.True(t, status.Ok)
-	require.Nil(t, status.Err)
+	require.Equal(t, "", status.Err)
 
 	// Peer conn is dropped after transfer is finished. This is the next update we receive.
 	status = readChannelTimeout(t, obs.Peers.PeerStatus, 500*time.Millisecond).(PeerStatus)
 	// TODO a check on PeerID
 	require.False(t, status.Ok)
-	require.Nil(t, status.Err)
+	require.Equal(t, "", status.Err)
 }
 
 type ConfigureClient struct {

client.go

@@ -1095,11 +1095,19 @@ func (cl *Client) runHandshookConn(c *PeerConn, t *Torrent) error {
 
 	// TODO here we could send an update to say the PeerConn state connected is true
 	// perhaps could also use c.pex.IsEnabled() or something similar
-	c.UpdatePeerConnStatus(PeerStatus{c.PeerID, true, nil})
+	c.UpdatePeerConnStatus(PeerStatus{
+		Id: c.PeerID,
+		Ok: true,
+	})
 
 	err := c.mainReadLoop()
 	if err != nil {
-		c.UpdatePeerConnStatus(PeerStatus{c.PeerID, false, err})
+		// c.UpdatePeerConnStatus(PeerStatus{c.PeerID, false, err})
+		c.UpdatePeerConnStatus(PeerStatus{
+			Id:  c.PeerID,
+			Ok:  false,
+			Err: fmt.Sprintf("%s", err),
+		})
 		return fmt.Errorf("main read loop: %w", err)
 	}
 	return nil

peerconn.go

@@ -35,7 +35,7 @@ import (
 type PeerStatus struct {
 	Id  PeerID
 	Ok  bool
-	Err error
+	Err string // see https://github.com/golang/go/issues/5161
 }
 
 type PeerObserver struct {

torrent.go

@@ -1596,7 +1596,10 @@ func (t *Torrent) dropConnection(c *PeerConn) {
 	t.cl.event.Broadcast()
 	c.close()
 
-	c.UpdatePeerConnStatus(PeerStatus{c.PeerID, false, nil})
+	c.UpdatePeerConnStatus(PeerStatus{
+		Id: c.PeerID,
+		Ok: false,
+	})
 	t.logger.WithDefaultLevel(log.Debug).Printf("dropping connection to %+q, sent peerconn update", c.PeerID)
 
 	if t.deletePeerConn(c) {