More helpful error message when --ip-alloc-range mismatched

ipam/allocator.go

@@ -613,6 +613,10 @@ func (alloc *Allocator) update(sender router.PeerName, msg []byte) error {
 	// shouldn't get updates for a empty Ring. But tolerate
 	// them just in case.
 	if data.Ring != nil {
+		if data.Ring.Range() != alloc.universe {
+			return fmt.Errorf("Incompatible IP allocation range %s; ours is %s",
+				data.Ring.Range().AsCIDRString(), alloc.universe.AsCIDRString())
+		}
 		err = alloc.ring.Merge(*data.Ring)
 		if !alloc.ring.Empty() {
 			alloc.pruneNicknames()

ipam/ring/ring.go

@@ -92,6 +92,10 @@ func New(start, end address.Address, peer router.PeerName) *Ring {
 	return ring
 }
 
+func (r *Ring) Range() address.Range {
+	return address.Range{Start: r.Start, End: r.End}
+}
+
 // Returns the distance between two tokens on this ring, dealing
 // with ranges which cross the origin
 func (r *Ring) distance(start, end address.Address) address.Offset {

net/address/address.go

@@ -23,6 +23,17 @@ func (r Range) String() string             { return fmt.Sprintf("[%s-%s)", r.Sta
 func (r Range) Overlaps(or Range) bool     { return !(r.Start >= or.End || r.End <= or.Start) }
 func (r Range) Contains(addr Address) bool { return addr >= r.Start && addr < r.End }
 
+func (r Range) AsCIDRString() string {
+	prefixLen := 32
+	for size := r.Size(); size > 1; size = size / 2 {
+		if size%2 != 0 { // Size not a power of two; cannot be expressed as a CIDR.
+			return r.String()
+		}
+		prefixLen--
+	}
+	return CIDR{Start: r.Start, PrefixLen: prefixLen}.String()
+}
+
 type CIDR struct {
 	Start     Address
 	PrefixLen int