Fix write error handling

Fixes https://github.com/anacrolix/torrent/issues/798.

Prior to this fix, it looks like the writer would just keep writing chunks of the front buffer (incorrectly if there was an error), until presumably the writer would be killed by read hangup elsewhere.

peer-conn-msg-writer.go

@@ -104,6 +104,9 @@ func (cn *peerConnMsgWriter) run(keepAliveTimeout time.Duration) {
 			if err == nil && n != len(next) {
 				panic("expected full write")
 			}
+			if err != nil {
+				break
+			}
 		}
 		if err != nil {
 			cn.logger.WithDefaultLevel(log.Debug).Printf("error writing: %v", err)

tests/issue-798/main.go

@@ -0,0 +1,18 @@
+package main
+
+import (
+	"fmt"
+	"github.com/anacrolix/torrent"
+)
+
+func main() {
+	config := torrent.NewDefaultClientConfig()
+	config.DataDir = "./output"
+	c, _ := torrent.NewClient(config)
+	defer c.Close()
+	t, _ := c.AddMagnet("magnet:?xt=urn:btih:99c82bb73505a3c0b453f9fa0e881d6e5a32a0c1&tr=https%3A%2F%2Ftorrent.ubuntu.com%2Fannounce&tr=https%3A%2F%2Fipv6.torrent.ubuntu.com%2Fannounce")
+	<-t.GotInfo()
+	fmt.Println("start downloading")
+	t.DownloadAll()
+	c.WaitAll()
+}