quic: pass the connection ID length into 1-RTT packet parsing

1-RTT packets contain a variable-length connection ID field, but
no indication of the length of the connection ID.  The recipient
of the packet has chosen the connection ID, and is expected to
either choose a consistent length or encode the length in the
connection ID.

Change the parse1RTTPacket function to take the connection ID
length as an input, rather than assuming that all 1-RTT packets
contain our hardcoded connection ID length.

This permits using parse1RTTPacket in tests which may create
and parse packets using other lengths.

For golang/go#58547

Change-Id: I9d09e4a0041051be1604c9146f6db9ca959ad696
Reviewed-on: https://go-review.googlesource.com/c/net/+/504856
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

internal/quic/packet_codec_test.go

@@ -185,7 +185,7 @@ func TestRoundtripEncodeShortPacket(t *testing.T) {
 			w.b = append(w.b, test.payload...)
 			w.finish1RTTPacket(test.num, 0, connID, test.k)
 			pkt := w.datagram()
-			p, n := parse1RTTPacket(pkt, test.k, 0)
+			p, n := parse1RTTPacket(pkt, test.k, connIDLen, 0)
 			if n != len(pkt) {
 				t.Errorf("parse1RTTPacket: n=%v, want %v", n, len(pkt))
 			}

internal/quic/packet_parser.go

@@ -146,9 +146,9 @@ func skipLongHeaderPacket(pkt []byte) int {
 //
 // On input, pkt contains a short header packet, k the decryption keys for the packet,
 // and pnumMax the largest packet number seen in the number space of this packet.
-func parse1RTTPacket(pkt []byte, k keys, pnumMax packetNumber) (p shortPacket, n int) {
+func parse1RTTPacket(pkt []byte, k keys, dstConnIDLen int, pnumMax packetNumber) (p shortPacket, n int) {
 	var err error
-	p.payload, p.num, err = k.unprotect(pkt, 1+connIDLen, pnumMax)
+	p.payload, p.num, err = k.unprotect(pkt, 1+dstConnIDLen, pnumMax)
 	if err != nil {
 		return shortPacket{}, -1
 	}