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/stride_table.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"io"
 	"math/bits"
+	"net/netip"
 	"strconv"
 	"strings"
 )
@@ -32,6 +33,9 @@ type strideEntry[T any] struct {
 // The leaves of the binary tree are host routes (/8s). Each parent is a
 // successively larger prefix that encompasses its children (/7 through /0).
 type strideTable[T any] struct {
+	// prefix is the prefix represented by the 0/0 route of this strideTable. It
+	// is used in multi-level tables to support path compression.
+	prefix netip.Prefix
 	// entries is the nodes of the binary tree, laid out in a flattened array.
 	//
 	// The array indices are arranged by the prefixIndex function, such that the
@@ -76,7 +80,9 @@ func (t *strideTable[T]) deleteChild(idx int) {
 func (t *strideTable[T]) getOrCreateChild(addr uint8) *strideTable[T] {
 	idx := hostIndex(addr)
 	if t.entries[idx].child == nil {
-		t.entries[idx].child = new(strideTable[T])
+		t.entries[idx].child = &strideTable[T]{
+			prefix: childPrefixOf(t.prefix, addr),
+		}
 		t.refs++
 	}
 	return t.entries[idx].child
@@ -229,3 +235,29 @@ func formatPrefixTable(addr uint8, len int) string {
 	}
 	return fmt.Sprintf("%3d/%d", addr, len)
 }
+
+// childPrefixOf returns the child prefix of parent whose final byte
+// is stride. The parent prefix must be byte-aligned
+// (i.e. parent.Bits() must be a multiple of 8), and be no more
+// specific than /24 for IPv4 or /120 for IPv6.
+//
+// For example, childPrefixOf("192.168.0.0/16", 8) == "192.168.8.0/24".
+func childPrefixOf(parent netip.Prefix, stride uint8) netip.Prefix {
+	l := parent.Bits()
+	if l%8 != 0 {
+		panic("parent prefix is not 8-bit aligned")
+	}
+	if l >= parent.Addr().BitLen() {
+		panic("parent prefix cannot be extended further")
+	}
+	off := l / 8
+	if parent.Addr().Is4() {
+		bs := parent.Addr().As4()
+		bs[off] = stride
+		return netip.PrefixFrom(netip.AddrFrom4(bs), l+8)
+	} else {
+		bs := parent.Addr().As16()
+		bs[off] = stride
+		return netip.PrefixFrom(netip.AddrFrom16(bs), l+8)
+	}
+}

net/art/stride_table_test.go

@@ -7,6 +7,7 @@ import (
 	"bytes"
 	"fmt"
 	"math/rand"
+	"net/netip"
 	"sort"
 	"strings"
 	"testing"
@@ -100,7 +101,7 @@ func TestStrideTableInsertShuffled(t *testing.T) {
 		for _, route := range routes2 {
 			rt2.insert(route.addr, route.len, route.val)
 		}
-		if diff := cmp.Diff(rt, rt2, cmp.AllowUnexported(strideTable[int]{}, strideEntry[int]{})); diff != "" {
+		if diff := cmp.Diff(rt, rt2, cmpDiffOpts...); diff != "" {
 			t.Errorf("tables ended up different with different insertion order (-got+want):\n%s\n\nOrder 1: %v\nOrder 2: %v", diff, formatSlowEntriesShort(routes), formatSlowEntriesShort(routes2))
 		}
 
@@ -108,7 +109,7 @@ func TestStrideTableInsertShuffled(t *testing.T) {
 		for _, route := range routes2 {
 			rtZero2.insert(route.addr, route.len, &zero)
 		}
-		if diff := cmp.Diff(rtZero, rtZero2, cmp.AllowUnexported(strideTable[int]{}, strideEntry[int]{})); diff != "" {
+		if diff := cmp.Diff(rtZero, rtZero2, cmpDiffOpts...); diff != "" {
 			t.Errorf("tables with identical vals ended up different with different insertion order (-got+want):\n%s\n\nOrder 1: %v\nOrder 2: %v", diff, formatSlowEntriesShort(routes), formatSlowEntriesShort(routes2))
 		}
 	}
@@ -180,7 +181,7 @@ func TestStrideTableDeleteShuffle(t *testing.T) {
 		for _, route := range toDelete2 {
 			rt2.delete(route.addr, route.len)
 		}
-		if diff := cmp.Diff(rt, rt2, cmp.AllowUnexported(strideTable[int]{}, strideEntry[int]{})); diff != "" {
+		if diff := cmp.Diff(rt, rt2, cmpDiffOpts...); diff != "" {
 			t.Errorf("tables ended up different with different deletion order (-got+want):\n%s\n\nOrder 1: %v\nOrder 2: %v", diff, formatSlowEntriesShort(toDelete), formatSlowEntriesShort(toDelete2))
 		}
 
@@ -191,7 +192,7 @@ func TestStrideTableDeleteShuffle(t *testing.T) {
 		for _, route := range toDelete2 {
 			rtZero2.delete(route.addr, route.len)
 		}
-		if diff := cmp.Diff(rtZero, rtZero2, cmp.AllowUnexported(strideTable[int]{}, strideEntry[int]{})); diff != "" {
+		if diff := cmp.Diff(rtZero, rtZero2, cmpDiffOpts...); diff != "" {
 			t.Errorf("tables with identical vals ended up different with different deletion order (-got+want):\n%s\n\nOrder 1: %v\nOrder 2: %v", diff, formatSlowEntriesShort(toDelete), formatSlowEntriesShort(toDelete2))
 		}
 	}
@@ -382,3 +383,8 @@ func formatSlowEntriesShort[T any](ents []slowEntry[T]) string {
 	}
 	return "[" + strings.Join(ret, " ") + "]"
 }
+
+var cmpDiffOpts = []cmp.Option{
+	cmp.AllowUnexported(strideTable[int]{}, strideEntry[int]{}),
+	cmp.Comparer(func(a, b netip.Prefix) bool { return a == b }),
+}

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]
+	v4       strideTable[T]
-	v6 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 {