netstat: model legacy TcpExt counters removed from recent kernels

proc_netstat.go

@@ -146,6 +146,20 @@ type TcpExt struct { // nolint:revive
 	TCPMTUPFail               *float64
 	TCPMTUPSuccess            *float64
 	TCPWqueueTooBig           *float64
+
+	// Legacy TcpExt counters dropped from recent kernels' TcpExt table;
+	// modeled so consumers retain metric parity with older kernels that
+	// still print them in /proc/net/netstat.
+	TCPLoss                   *float64
+	PAWSPassive               *float64
+	TCPForwardRetrans         *float64
+	TCPSchedulerFailed        *float64
+	TCPPrequeued              *float64
+	TCPDirectCopyFromBacklog  *float64
+	TCPDirectCopyFromPrequeue *float64
+	TCPPrequeueDropped        *float64
+	TCPFACKReorder            *float64
+	TCPHPHitsToUser           *float64
 }
 
 type IpExt struct { // nolint:revive
@@ -396,6 +410,26 @@ func parseProcNetstat(r io.Reader, fileName string) (ProcNetstat, error) {
 					procNetstat.TCPMTUPSuccess = &value
 				case "TCPWqueueTooBig":
 					procNetstat.TCPWqueueTooBig = &value
+				case "TCPLoss":
+					procNetstat.TCPLoss = &value
+				case "PAWSPassive":
+					procNetstat.PAWSPassive = &value
+				case "TCPForwardRetrans":
+					procNetstat.TCPForwardRetrans = &value
+				case "TCPSchedulerFailed":
+					procNetstat.TCPSchedulerFailed = &value
+				case "TCPPrequeued":
+					procNetstat.TCPPrequeued = &value
+				case "TCPDirectCopyFromBacklog":
+					procNetstat.TCPDirectCopyFromBacklog = &value
+				case "TCPDirectCopyFromPrequeue":
+					procNetstat.TCPDirectCopyFromPrequeue = &value
+				case "TCPPrequeueDropped":
+					procNetstat.TCPPrequeueDropped = &value
+				case "TCPFACKReorder":
+					procNetstat.TCPFACKReorder = &value
+				case "TCPHPHitsToUser":
+					procNetstat.TCPHPHitsToUser = &value
 				}
 			case "IpExt":
 				switch key {

proc_netstat_test.go

@@ -14,6 +14,7 @@
 package procfs
 
 import (
+	"strings"
 	"testing"
 )
 
@@ -48,3 +49,58 @@ func TestProcNetstat(t *testing.T) {
 		}
 	}
 }
+
+// TestParseProcNetstatLegacyFields covers TcpExt counters that are no longer
+// part of the kernel's TcpExt table but were still printed by older kernels.
+func TestParseProcNetstatLegacyFields(t *testing.T) {
+	payload := `TcpExt: SyncookiesSent PAWSPassive TCPPrequeued TCPDirectCopyFromBacklog TCPDirectCopyFromPrequeue TCPPrequeueDropped TCPLoss TCPFACKReorder TCPForwardRetrans TCPHPHitsToUser TCPSchedulerFailed
+TcpExt: 1 2 3 4 5 6 7 8 9 10 11
+IpExt: InNoRoutes OutOctets
+IpExt: 12 13`
+
+	procNetstat, err := parseProcNetstat(strings.NewReader(payload), "net/netstat")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, test := range []struct {
+		name string
+		want float64
+		have float64
+	}{
+		{name: "TcpExt:SyncookiesSent", want: 1, have: *procNetstat.SyncookiesSent},
+		{name: "TcpExt:PAWSPassive", want: 2, have: *procNetstat.PAWSPassive},
+		{name: "TcpExt:TCPPrequeued", want: 3, have: *procNetstat.TCPPrequeued},
+		{name: "TcpExt:TCPDirectCopyFromBacklog", want: 4, have: *procNetstat.TCPDirectCopyFromBacklog},
+		{name: "TcpExt:TCPDirectCopyFromPrequeue", want: 5, have: *procNetstat.TCPDirectCopyFromPrequeue},
+		{name: "TcpExt:TCPPrequeueDropped", want: 6, have: *procNetstat.TCPPrequeueDropped},
+		{name: "TcpExt:TCPLoss", want: 7, have: *procNetstat.TCPLoss},
+		{name: "TcpExt:TCPFACKReorder", want: 8, have: *procNetstat.TCPFACKReorder},
+		{name: "TcpExt:TCPForwardRetrans", want: 9, have: *procNetstat.TCPForwardRetrans},
+		{name: "TcpExt:TCPHPHitsToUser", want: 10, have: *procNetstat.TCPHPHitsToUser},
+		{name: "TcpExt:TCPSchedulerFailed", want: 11, have: *procNetstat.TCPSchedulerFailed},
+		{name: "IpExt:InNoRoutes", want: 12, have: *procNetstat.InNoRoutes},
+		{name: "IpExt:OutOctets", want: 13, have: *procNetstat.OutOctets},
+	} {
+		if test.want != test.have {
+			t.Errorf("want %s %f, have %f", test.name, test.want, test.have)
+		}
+	}
+
+	modern := `TcpExt: SyncookiesSent
+TcpExt: 1
+IpExt: InNoRoutes
+IpExt: 12`
+
+	pn, err := parseProcNetstat(strings.NewReader(modern), "net/netstat")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if pn.SyncookiesSent == nil || *pn.SyncookiesSent != 1 {
+		t.Error("want TcpExt:SyncookiesSent 1")
+	}
+	if pn.TCPPrequeued != nil {
+		t.Error("TCPPrequeued should be nil when the kernel does not report it")
+	}
+}