net/art: make each strideTable track the IP prefix it represents

This is a prerequisite for path compression, so that insert/delete
can determine when compression occurred.

Updates #7781

Signed-off-by: David Anderson <danderson@tailscale.com>

net/art/table.go

@@ -18,17 +18,27 @@ import (
 	"io"
 	"net/netip"
 	"strings"
+	"sync"
 )
 
 // Table is an IPv4 and IPv6 routing table.
 type Table[T any] struct {
-	v4 strideTable[T]
-	v6 strideTable[T]
+	v4       strideTable[T]
+	v6       strideTable[T]
+	initOnce sync.Once
+}
+
+func (t *Table[T]) init() {
+	t.initOnce.Do(func() {
+		t.v4.prefix = netip.PrefixFrom(netip.IPv4Unspecified(), 0)
+		t.v6.prefix = netip.PrefixFrom(netip.IPv6Unspecified(), 0)
+	})
 }
 
 // Get does a route lookup for addr and returns the associated value, or nil if
 // no route matched.
 func (t *Table[T]) Get(addr netip.Addr) *T {
+	t.init()
 	st := &t.v4
 	if addr.Is6() {
 		st = &t.v6
@@ -58,6 +68,7 @@ func (t *Table[T]) Get(addr netip.Addr) *T {
 // Insert adds pfx to the table, with value val.
 // If pfx is already present in the table, its value is set to val.
 func (t *Table[T]) Insert(pfx netip.Prefix, val *T) {
+	t.init()
 	if val == nil {
 		panic("Table.Insert called with nil value")
 	}
@@ -85,6 +96,7 @@ func (t *Table[T]) Insert(pfx netip.Prefix, val *T) {
 
 // Delete removes pfx from the table, if it is present.
 func (t *Table[T]) Delete(pfx netip.Prefix) {
+	t.init()
 	st := &t.v4
 	if pfx.Addr().Is6() {
 		st = &t.v6
@@ -141,6 +153,7 @@ func (t *Table[T]) Delete(pfx netip.Prefix) {
 // debugSummary prints the tree of allocated strideTables in t, with each
 // strideTable's refcount.
 func (t *Table[T]) debugSummary() string {
+	t.init()
 	var ret bytes.Buffer
 	fmt.Fprintf(&ret, "v4: ")
 	strideSummary(&ret, &t.v4, 0)
@@ -150,7 +163,7 @@ func (t *Table[T]) debugSummary() string {
 }
 
 func strideSummary[T any](w io.Writer, st *strideTable[T], indent int) {
-	fmt.Fprintf(w, "%d refs\n", st.refs)
+	fmt.Fprintf(w, "%s: %d refs\n", st.prefix, st.refs)
 	indent += 2
 	for i := firstHostIndex; i <= lastHostIndex; i++ {
 		if child := st.entries[i].child; child != nil {