Track packets dropped by full device TX queue

QDisc/LinkEndpoint may drop packets if the device's send/transmit queue
is full.

BUG: https://fxbug.dev/98974
PiperOrigin-RevId: 448570489

pkg/sentry/socket/netstack/netstack.go

@@ -88,6 +88,7 @@ var Metrics = tcpip.Stats{
 			Packets: mustCreateMetric("/netstack/nic/tx/packets", "Number of packets transmitted."),
 			Bytes:   mustCreateMetric("/netstack/nic/tx/bytes", "Number of bytes transmitted."),
 		},
+		TxPacketsDroppedNoBufferSpace: mustCreateMetric("/netstack/nic/tx_packets_dropped_no_buffer_space", "Number of TX packets dropped as a result of no buffer space errors."),
 		Rx: tcpip.NICPacketStats{
 			Packets: mustCreateMetric("/netstack/nic/rx/packets", "Number of packets received."),
 			Bytes:   mustCreateMetric("/netstack/nic/rx/bytes", "Number of bytes received."),

pkg/tcpip/stack/nic.go

@@ -394,6 +394,9 @@ func (n *nic) writePacket(pkt *PacketBuffer) tcpip.Error {
 
 func (n *nic) writeRawPacket(pkt *PacketBuffer) tcpip.Error {
 	if err := n.qDisc.WritePacket(pkt); err != nil {
+		if _, ok := err.(*tcpip.ErrNoBufferSpace); ok {
+			n.stats.txPacketsDroppedNoBufferSpace.Increment()
+		}
 		return err
 	}
 

pkg/tcpip/stack/nic_stats.go

@@ -56,6 +56,7 @@ type multiCounterNICStats struct {
 	unknownL4ProtocolRcvdPacketCounts tcpip.MultiIntegralStatCounterMap
 	malformedL4RcvdPackets            tcpip.MultiCounterStat
 	tx                                multiCounterNICPacketStats
+	txPacketsDroppedNoBufferSpace     tcpip.MultiCounterStat
 	rx                                multiCounterNICPacketStats
 	disabledRx                        multiCounterNICPacketStats
 	neighbor                          multiCounterNICNeighborStats
@@ -66,6 +67,7 @@ func (m *multiCounterNICStats) init(a, b *tcpip.NICStats) {
 	m.unknownL4ProtocolRcvdPacketCounts.Init(a.UnknownL4ProtocolRcvdPacketCounts, b.UnknownL4ProtocolRcvdPacketCounts)
 	m.malformedL4RcvdPackets.Init(a.MalformedL4RcvdPackets, b.MalformedL4RcvdPackets)
 	m.tx.init(&a.Tx, &b.Tx)
+	m.txPacketsDroppedNoBufferSpace.Init(a.TxPacketsDroppedNoBufferSpace, b.TxPacketsDroppedNoBufferSpace)
 	m.rx.init(&a.Rx, &b.Rx)
 	m.disabledRx.init(&a.DisabledRx, &b.DisabledRx)
 	m.neighbor.init(&a.Neighbor, &b.Neighbor)

pkg/tcpip/tcpip.go

@@ -2103,6 +2103,13 @@ type NICStats struct {
 	// Tx contains statistics about transmitted packets.
 	Tx NICPacketStats
 
+	// TxPacketsDroppedNoBufferSpace is the number of packets dropepd due to the
+	// NIC not having enough buffer space to send the packet.
+	//
+	// Packets may be dropped with a no buffer space error when the device TX
+	// queue is full.
+	TxPacketsDroppedNoBufferSpace *StatCounter
+
 	// Rx contains statistics about received packets.
 	Rx NICPacketStats
 

pkg/tcpip/transport/datagram_test.go

@@ -468,7 +468,29 @@ func TestDeviceReturnErrNoBufferSpace(t *testing.T) {
 						t.Fatalf("ep.Connect(%#v): %s", to, err)
 					}
 
-					ops := ep.SocketOptions()
+					stackTxPacketsDroppedNoBufferSpace := s.Stats().NICs.TxPacketsDroppedNoBufferSpace
+
+					nicsInfo := s.NICInfo()
+					nicInfo, ok := nicsInfo[nicID]
+					if !ok {
+						t.Fatalf("expected NICInfo for nicID=%d; got s.NICInfo() = %#v", nicID, nicsInfo)
+					}
+					nicTxPacketsDroppedNoBufferSpace := nicInfo.Stats.TxPacketsDroppedNoBufferSpace
+
+					checkStats := func(want uint64) {
+						t.Helper()
+
+						if got := stackTxPacketsDroppedNoBufferSpace.Value(); got != want {
+							t.Errorf("got stackTxPacketsDroppedNoBufferSpace.Value() = %d, want = %d", got, want)
+						}
+						if got := nicTxPacketsDroppedNoBufferSpace.Value(); got != want {
+							t.Errorf("got nicTxPacketsDroppedNoBufferSpace.Value() = %d, want = %d", got, want)
+						}
+					}
+
+					droppedPkts := uint64(0)
+					checkStats(droppedPkts)
+
 					checkWrite := func(netProto tcpip.NetworkProtocolNumber) {
 						t.Helper()
 
@@ -481,8 +503,12 @@ func TestDeviceReturnErrNoBufferSpace(t *testing.T) {
 						if n, err := ep.Write(&r, tcpip.WriteOptions{}); err != wantErr {
 							t.Fatalf("got Write(...) = (%d, %s), want = (_, %s)", n, err, wantErr)
 						}
+
+						droppedPkts++
+						checkStats(droppedPkts)
 					}
 
+					ops := ep.SocketOptions()
 					ops.SetIPv4RecvError(true)
 					checkWrite(ipv4.ProtocolNumber)