feature/conn25: add the ability to return addresses to the IP Pools

This will be used as part of the address assignment expiry work.

Updates tailscale/corp#39975

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

feature/conn25/ippool.go

@@ -8,17 +8,24 @@ import (
 	"net/netip"
 
 	"go4.org/netipx"
+	"tailscale.com/util/set"
 )
 
 // 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.
+// errNotOurAddress is returned if a provided address is not from our pool
+var errNotOurAddress = errors.New("not our address")
+
+// errAddrExists is returned if a returned address is already in the returned pool.
+var errAddrExists = errors.New("address already returned")
+
+// ipSetIterator allows for round robin 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 {
+// to get the new one, and if we run off the edge of the current range, starting on the next one, or back at the beginning.
+type ipSetIterator struct {
 	// ranges defines the addresses in the pool
 	ranges []netipx.IPRange
 	// last is internal tracking of which the last address provided was.
@@ -27,35 +34,75 @@ type ippool struct {
 	rangeIdx int
 }
 
+// next returns the next address from the set.
+func (ipsi *ipSetIterator) next() (netip.Addr, error) {
+	if len(ipsi.ranges) == 0 {
+		// ipset is empty
+		return netip.Addr{}, errPoolExhausted
+	}
+	if !ipsi.last.IsValid() {
+		// not initialized yet
+		ipsi.last = ipsi.ranges[0].From()
+		return ipsi.last, nil
+	}
+	currRange := ipsi.ranges[ipsi.rangeIdx]
+	if ipsi.last == currRange.To() {
+		// then we need to move to the next range
+		ipsi.rangeIdx++
+		if ipsi.rangeIdx >= len(ipsi.ranges) {
+			// back to the beginning
+			ipsi.rangeIdx = 0
+		}
+		ipsi.last = ipsi.ranges[ipsi.rangeIdx].From()
+		return ipsi.last, nil
+	}
+	ipsi.last = ipsi.last.Next()
+	return ipsi.last, nil
+}
+
 func newIPPool(ipset *netipx.IPSet) *ippool {
 	if ipset == nil {
 		return &ippool{}
 	}
-	return &ippool{ranges: ipset.Ranges()}
+	return &ippool{
+		ipSet:         ipset,
+		ipSetIterator: &ipSetIterator{ranges: ipset.Ranges()},
+		inUse:         &set.Set[netip.Addr]{},
+	}
+}
+
+type ippool struct {
+	ipSet         *netipx.IPSet
+	ipSetIterator *ipSetIterator
+	inUse         *set.Set[netip.Addr]
 }
 
-// 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
+	a, err := ipp.ipSetIterator.next()
+	if err != nil {
+		return netip.Addr{}, err
 	}
-	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) {
+	startedAt := a
+	for ipp.inUse.Contains(a) {
+		a, err = ipp.ipSetIterator.next()
+		if err != nil {
+			return a, err
+		}
+		if a == startedAt {
 			return netip.Addr{}, errPoolExhausted
 		}
-		ipp.last = ipp.ranges[ipp.rangeIdx].From()
-		return ipp.last, nil
 	}
-	ipp.last = ipp.last.Next()
-	return ipp.last, nil
+	ipp.inUse.Add(a)
+	return a, nil
+}
+
+func (ipp *ippool) returnAddr(a netip.Addr) error {
+	if !ipp.ipSet.Contains(a) {
+		return errNotOurAddress
+	}
+	if !ipp.inUse.Contains(a) {
+		return errAddrExists
+	}
+	ipp.inUse.Delete(a)
+	return nil
 }