cmd/natc: move address storage behind an interface

Adds IPPool and moves all IP address management concerns behind that.

Updates #14667

Signed-off-by: Fran Bull <fran@tailscale.com>

cmd/natc/ippool/ipx_test.go

@@ -0,0 +1,150 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package ippool
+
+import (
+	"math"
+	"math/big"
+	"net/netip"
+	"testing"
+
+	"go4.org/netipx"
+	"tailscale.com/util/must"
+)
+
+func TestV4ToNum(t *testing.T) {
+	cases := []struct {
+		addr netip.Addr
+		num  uint32
+	}{
+		{netip.MustParseAddr("0.0.0.0"), 0},
+		{netip.MustParseAddr("255.255.255.255"), 0xffffffff},
+		{netip.MustParseAddr("8.8.8.8"), 0x08080808},
+		{netip.MustParseAddr("192.168.0.1"), 0xc0a80001},
+		{netip.MustParseAddr("10.0.0.1"), 0x0a000001},
+		{netip.MustParseAddr("172.16.0.1"), 0xac100001},
+		{netip.MustParseAddr("100.64.0.1"), 0x64400001},
+	}
+
+	for _, tc := range cases {
+		num := v4ToNum(tc.addr)
+		if num != tc.num {
+			t.Errorf("addrNum(%v) = %d, want %d", tc.addr, num, tc.num)
+		}
+		if numToV4(num) != tc.addr {
+			t.Errorf("numToV4(%d) = %v, want %v", num, numToV4(num), tc.addr)
+		}
+	}
+
+	func() {
+		defer func() {
+			if r := recover(); r == nil {
+				t.Fatal("expected panic")
+			}
+		}()
+
+		v4ToNum(netip.MustParseAddr("::1"))
+	}()
+}
+
+func TestAddrIndex(t *testing.T) {
+	builder := netipx.IPSetBuilder{}
+	builder.AddRange(netipx.MustParseIPRange("10.0.0.1-10.0.0.5"))
+	builder.AddRange(netipx.MustParseIPRange("192.168.0.1-192.168.0.10"))
+	ipset := must.Get(builder.IPSet())
+
+	indexCases := []struct {
+		addr  netip.Addr
+		index int
+	}{
+		{netip.MustParseAddr("10.0.0.1"), 0},
+		{netip.MustParseAddr("10.0.0.2"), 1},
+		{netip.MustParseAddr("10.0.0.3"), 2},
+		{netip.MustParseAddr("10.0.0.4"), 3},
+		{netip.MustParseAddr("10.0.0.5"), 4},
+		{netip.MustParseAddr("192.168.0.1"), 5},
+		{netip.MustParseAddr("192.168.0.5"), 9},
+		{netip.MustParseAddr("192.168.0.10"), 14},
+		{netip.MustParseAddr("172.16.0.1"), -1}, // Not in set
+	}
+
+	for _, tc := range indexCases {
+		index := indexOfAddr(tc.addr, ipset)
+		if index != tc.index {
+			t.Errorf("indexOfAddr(%v) = %d, want %d", tc.addr, index, tc.index)
+		}
+		if tc.index == -1 {
+			continue
+		}
+		addr := addrAtIndex(tc.index, ipset)
+		if addr != tc.addr {
+			t.Errorf("addrAtIndex(%d) = %v, want %v", tc.index, addr, tc.addr)
+		}
+	}
+}
+
+func TestAllocAddr(t *testing.T) {
+	builder := netipx.IPSetBuilder{}
+	builder.AddRange(netipx.MustParseIPRange("10.0.0.1-10.0.0.5"))
+	builder.AddRange(netipx.MustParseIPRange("192.168.0.1-192.168.0.10"))
+	ipset := must.Get(builder.IPSet())
+
+	allocated := new(big.Int)
+	for range 15 {
+		addr := allocAddr(ipset, allocated)
+		if !addr.IsValid() {
+			t.Errorf("allocAddr() = invalid, want valid")
+		}
+		if !ipset.Contains(addr) {
+			t.Errorf("allocAddr() = %v, not in set", addr)
+		}
+	}
+	addr := allocAddr(ipset, allocated)
+	if addr.IsValid() {
+		t.Errorf("allocAddr() = %v, want invalid", addr)
+	}
+	wantAddr := netip.MustParseAddr("10.0.0.2")
+	allocated.SetBit(allocated, indexOfAddr(wantAddr, ipset), 0)
+	addr = allocAddr(ipset, allocated)
+	if addr != wantAddr {
+		t.Errorf("allocAddr() = %v, want %v", addr, wantAddr)
+	}
+}
+
+func TestLeastZeroBit(t *testing.T) {
+	cases := []struct {
+		num  uint
+		want int
+	}{
+		{math.MaxUint, -1},
+		{0, 0},
+		{0b01, 1},
+		{0b11, 2},
+		{0b111, 3},
+		{math.MaxUint, -1},
+		{math.MaxUint - 1, 0},
+	}
+	if math.MaxUint == math.MaxUint64 {
+		cases = append(cases, []struct {
+			num  uint
+			want int
+		}{
+			{math.MaxUint >> 1, 63},
+		}...)
+	} else {
+		cases = append(cases, []struct {
+			num  uint
+			want int
+		}{
+			{math.MaxUint >> 1, 31},
+		}...)
+	}
+
+	for _, tc := range cases {
+		got := leastZeroBit(tc.num)
+		if got != tc.want {
+			t.Errorf("leastZeroBit(%b) = %d, want %d", tc.num, got, tc.want)
+		}
+	}
+}