prober: remove DERP pub key copying overheads in qd and non-tun measures (#14659)

Updates tailscale/corp#25883

Signed-off-by: Jordan Whited <jordan@tailscale.com>

prober/derp.go

@@ -431,6 +431,7 @@ func runDerpProbeQueuingDelayContinously(ctx context.Context, from, to *tailcfg.
 		t := time.NewTicker(time.Second / time.Duration(packetsPerSecond))
 		defer t.Stop()
 
+		toDERPPubKey := toc.SelfPublicKey()
 		seq := uint64(0)
 		for {
 			select {
@@ -446,7 +447,7 @@ func runDerpProbeQueuingDelayContinously(ctx context.Context, from, to *tailcfg.
 				txRecordsMu.Unlock()
 				binary.BigEndian.PutUint64(pkt, seq)
 				seq++
-				if err := fromc.Send(toc.SelfPublicKey(), pkt); err != nil {
+				if err := fromc.Send(toDERPPubKey, pkt); err != nil {
 					sendErrC <- fmt.Errorf("sending packet %w", err)
 					return
 				}
@@ -460,6 +461,7 @@ func runDerpProbeQueuingDelayContinously(ctx context.Context, from, to *tailcfg.
 	go func() {
 		defer wg.Done()
 		defer close(recvFinishedC) // to break out of 'select' below.
+		fromDERPPubKey := fromc.SelfPublicKey()
 		for {
 			m, err := toc.Recv()
 			if err != nil {
@@ -469,7 +471,7 @@ func runDerpProbeQueuingDelayContinously(ctx context.Context, from, to *tailcfg.
 			switch v := m.(type) {
 			case derp.ReceivedPacket:
 				now := time.Now()
-				if v.Source != fromc.SelfPublicKey() {
+				if v.Source != fromDERPPubKey {
 					recvFinishedC <- fmt.Errorf("got data packet from unexpected source, %v", v.Source)
 					return
 				}
@@ -767,9 +769,10 @@ func runDerpProbeNodePair(ctx context.Context, from, to *tailcfg.DERPNode, fromc
 	// Send the packets.
 	sendc := make(chan error, 1)
 	go func() {
+		toDERPPubKey := toc.SelfPublicKey()
 		for idx, pkt := range pkts {
 			inFlight.AcquireContext(ctx)
-			if err := fromc.Send(toc.SelfPublicKey(), pkt); err != nil {
+			if err := fromc.Send(toDERPPubKey, pkt); err != nil {
 				sendc <- fmt.Errorf("sending packet %d: %w", idx, err)
 				return
 			}
@@ -781,6 +784,7 @@ func runDerpProbeNodePair(ctx context.Context, from, to *tailcfg.DERPNode, fromc
 	go func() {
 		defer close(recvc) // to break out of 'select' below.
 		idx := 0
+		fromDERPPubKey := fromc.SelfPublicKey()
 		for {
 			m, err := toc.Recv()
 			if err != nil {
@@ -790,7 +794,7 @@ func runDerpProbeNodePair(ctx context.Context, from, to *tailcfg.DERPNode, fromc
 			switch v := m.(type) {
 			case derp.ReceivedPacket:
 				inFlight.Release()
-				if v.Source != fromc.SelfPublicKey() {
+				if v.Source != fromDERPPubKey {
 					recvc <- fmt.Errorf("got data packet %d from unexpected source, %v", idx, v.Source)
 					return
 				}
@@ -925,7 +929,7 @@ func derpProbeBandwidthTUN(ctx context.Context, transferTimeSeconds, totalBytesT
 
 		destinationAddrBytes := destinationAddr.AsSlice()
 		scratch := make([]byte, 4)
-		toPubDERPKey := toc.SelfPublicKey()
+		toDERPPubKey := toc.SelfPublicKey()
 		for {
 			n, err := dev.Read(bufs, sizes, tunStartOffset)
 			if err != nil {
@@ -954,7 +958,7 @@ func derpProbeBandwidthTUN(ctx context.Context, transferTimeSeconds, totalBytesT
 				copy(pkt[12:16], pkt[16:20])
 				copy(pkt[16:20], scratch)
 
-				if err := fromc.Send(toPubDERPKey, pkt); err != nil {
+				if err := fromc.Send(toDERPPubKey, pkt); err != nil {
 					tunReadErrC <- err
 					return
 				}