quic: log packet_dropped events

Log unparsable or otherwise discarded packets.

For golang/go#58547

Change-Id: Ief64174d91c93691bd524515aa6518e487543ced
Reviewed-on: https://go-review.googlesource.com/c/net/+/564017
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: neild

internal/quic/conn_recv.go

@@ -8,6 +8,7 @@ package quic
 
 import (
 	"bytes"
+	"context"
 	"encoding/binary"
 	"errors"
 	"time"
@@ -56,9 +57,16 @@ func (c *Conn) handleDatagram(now time.Time, dgram *datagram) {
 			if len(buf) == len(dgram.b) && len(buf) > statelessResetTokenLen {
 				var token statelessResetToken
 				copy(token[:], buf[len(buf)-len(token):])
-				c.handleStatelessReset(now, token)
+				if c.handleStatelessReset(now, token) {
+					return
+				}
 			}
 			// Invalid data at the end of a datagram is ignored.
+			if c.logEnabled(QLogLevelPacket) {
+				c.log.LogAttrs(context.Background(), QLogLevelPacket,
+					"connectivity:packet_dropped",
+				)
+			}
 			break
 		}
 		c.idleHandlePacketReceived(now)
@@ -562,10 +570,11 @@ func (c *Conn) handleHandshakeDoneFrame(now time.Time, space numberSpace, payloa
 
 var errStatelessReset = errors.New("received stateless reset")
 
-func (c *Conn) handleStatelessReset(now time.Time, resetToken statelessResetToken) {
+func (c *Conn) handleStatelessReset(now time.Time, resetToken statelessResetToken) (valid bool) {
 	if !c.connIDState.isValidStatelessResetToken(resetToken) {
-		return
+		return false
 	}
 	c.setFinalError(errStatelessReset)
 	c.enterDraining(now)
+	return true
 }

internal/quic/qlog_test.go

@@ -7,6 +7,7 @@
 package quic
 
 import (
+	"bytes"
 	"encoding/hex"
 	"encoding/json"
 	"fmt"
@@ -230,6 +231,27 @@ func TestQLogLoss(t *testing.T) {
 	})
 }
 
+func TestQLogPacketDropped(t *testing.T) {
+	qr := &qlogRecord{}
+	tc := newTestConn(t, clientSide, permissiveTransportParameters, qr.config)
+	tc.handshake()
+
+	// A garbage-filled datagram with a DCID matching this connection.
+	dgram := bytes.Join([][]byte{
+		{headerFormShort | fixedBit},
+		testLocalConnID(0),
+		make([]byte, 100),
+		[]byte{1, 2, 3, 4}, // random data, to avoid this looking like a stateless reset
+	}, nil)
+	tc.endpoint.write(&datagram{
+		b: dgram,
+	})
+
+	qr.wantEvents(t, jsonEvent{
+		"name": "connectivity:packet_dropped",
+	})
+}
+
 type nopCloseWriter struct {
 	io.Writer
 }