scion/ping: wait for send go routine to terminate (#4392)

Wait for the ping sender go routine to terminate before we return the
stats. With bad luck, the receiver go routine receives all the replies
before the sender go routine has terminated. Because only the sender
go routine increments the stats.Sent count, we end up with wrong stats
that claim more packets were received then sent.

The race condition is that `WriteTo` in the last `pinger.send` succeeds, but then the writer go-routine is not scheduled to increment the `stats.Sent` counter before these stats are returned from `pinger.Ping`.

scion/ping/ping.go

@@ -20,6 +20,7 @@ import (
 	"encoding/binary"
 	"math/rand"
 	"net"
+	"sync"
 	"time"
 
 	"github.com/scionproto/scion/pkg/addr"
@@ -178,8 +179,12 @@ func (p *pinger) Ping(ctx context.Context, remote *snet.UDPAddr) (Stats, error)
 		p.drain(ctx)
 	}()
 
+	var wg sync.WaitGroup
+	wg.Add(1)
+
 	go func() {
 		defer log.HandlePanic()
+		defer wg.Done()
 		for i := uint16(0); i < p.attempts; i++ {
 			if err := p.send(remote); err != nil {
 				errSend <- serrors.WrapStr("sending", err)
@@ -210,6 +215,7 @@ func (p *pinger) Ping(ctx context.Context, remote *snet.UDPAddr) (Stats, error)
 			p.receive(reply)
 		}
 	}
+	wg.Wait()
 	return p.stats, nil
 }