appc: add ippool type

As part of the conn25 work we will want to be able to keep track of a
pool of IP Addresses and know which have been used and which have not.

Fixes tailscale/corp#34247

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

appc/ippool.go

@@ -0,0 +1,61 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package appc
+
+import (
+	"errors"
+	"net/netip"
+
+	"go4.org/netipx"
+)
+
+// errPoolExhausted is returned when there are no more addresses to iterate over.
+var errPoolExhausted = errors.New("ip pool exhausted")
+
+// ippool allows for iteration over all the addresses within a netipx.IPSet.
+// netipx.IPSet has a Ranges call that returns the "minimum and sorted set of IP ranges that covers [the set]".
+// netipx.IPRange is "an inclusive range of IP addresses from the same address family.". So we can iterate over
+// all the addresses in the set by keeping a track of the last address we returned, calling Next on the last address
+// to get the new one, and if we run off the edge of the current range, starting on the next one.
+type ippool struct {
+	// ranges defines the addresses in the pool
+	ranges []netipx.IPRange
+	// last is internal tracking of which the last address provided was.
+	last netip.Addr
+	// rangeIdx is internal tracking of which netipx.IPRange from the IPSet we are currently on.
+	rangeIdx int
+}
+
+func newIPPool(ipset *netipx.IPSet) *ippool {
+	if ipset == nil {
+		return &ippool{}
+	}
+	return &ippool{ranges: ipset.Ranges()}
+}
+
+// next returns the next address from the set, or errPoolExhausted if we have
+// iterated over the whole set.
+func (ipp *ippool) next() (netip.Addr, error) {
+	if ipp.rangeIdx >= len(ipp.ranges) {
+		// ipset is empty or we have iterated off the end
+		return netip.Addr{}, errPoolExhausted
+	}
+	if !ipp.last.IsValid() {
+		// not initialized yet
+		ipp.last = ipp.ranges[0].From()
+		return ipp.last, nil
+	}
+	currRange := ipp.ranges[ipp.rangeIdx]
+	if ipp.last == currRange.To() {
+		// then we need to move to the next range
+		ipp.rangeIdx++
+		if ipp.rangeIdx >= len(ipp.ranges) {
+			return netip.Addr{}, errPoolExhausted
+		}
+		ipp.last = ipp.ranges[ipp.rangeIdx].From()
+		return ipp.last, nil
+	}
+	ipp.last = ipp.last.Next()
+	return ipp.last, nil
+}

appc/ippool_test.go

@@ -0,0 +1,60 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package appc
+
+import (
+	"errors"
+	"net/netip"
+	"testing"
+
+	"go4.org/netipx"
+	"tailscale.com/util/must"
+)
+
+func TestNext(t *testing.T) {
+	a := ippool{}
+	_, err := a.next()
+	if !errors.Is(err, errPoolExhausted) {
+		t.Fatalf("expected errPoolExhausted, got %v", err)
+	}
+
+	var isb netipx.IPSetBuilder
+	ipset := must.Get(isb.IPSet())
+	b := newIPPool(ipset)
+	_, err = b.next()
+	if !errors.Is(err, errPoolExhausted) {
+		t.Fatalf("expected errPoolExhausted, got %v", err)
+	}
+
+	isb.AddRange(netipx.IPRangeFrom(netip.MustParseAddr("192.168.0.0"), netip.MustParseAddr("192.168.0.2")))
+	isb.AddRange(netipx.IPRangeFrom(netip.MustParseAddr("200.0.0.0"), netip.MustParseAddr("200.0.0.0")))
+	isb.AddRange(netipx.IPRangeFrom(netip.MustParseAddr("201.0.0.0"), netip.MustParseAddr("201.0.0.1")))
+	ipset = must.Get(isb.IPSet())
+	c := newIPPool(ipset)
+	expected := []string{
+		"192.168.0.0",
+		"192.168.0.1",
+		"192.168.0.2",
+		"200.0.0.0",
+		"201.0.0.0",
+		"201.0.0.1",
+	}
+	for i, want := range expected {
+		addr, err := c.next()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if addr != netip.MustParseAddr(want) {
+			t.Fatalf("next call %d want: %s, got: %v", i, want, addr)
+		}
+	}
+	_, err = c.next()
+	if !errors.Is(err, errPoolExhausted) {
+		t.Fatalf("expected errPoolExhausted, got %v", err)
+	}
+	_, err = c.next()
+	if !errors.Is(err, errPoolExhausted) {
+		t.Fatalf("expected errPoolExhausted, got %v", err)
+	}
+}