Fix incorrect calculation of free space

when granting a range that spans a token and inserting a new token, the
free space on the last token that we are handing over has to change.
Add unit test for this too.

ipam/address/address.go

@@ -52,3 +52,10 @@ func Subtract(a, b Address) Offset {
 	common.Assert(a >= b)
 	return Offset(a - b)
 }
+
+func Min(a, b Offset) Offset {
+	if a > b {
+		return b
+	}
+	return a
+}

ipam/ring/ring.go

@@ -167,7 +167,7 @@ func (r *Ring) GrantRangeToHost(start, end address.Address, peer router.PeerName
 	pos := preceedingPos + 1
 	for ; pos < len(r.Entries) && r.Entries.entry(pos).Token < end; pos++ {
 		entry := r.Entries.entry(pos)
-		entry.update(peer, entry.Free)
+		entry.update(peer, address.Min(entry.Free, r.distance(entry.Token, end)))
 	}
 
 	// There is never an entry with a token of r.End, as the end of

ipam/ring/ring_test.go

@@ -158,6 +158,17 @@ func TestGrantSplit(t *testing.T) {
 	wt.AssertEquals(t, ring1.Entries, entries{{Token: start, Peer: peer1name, Free: 10, Version: 1},
 		{Token: dot10, Peer: peer2name, Free: 235},
 		{Token: dot245, Peer: peer1name, Free: 10}})
+	ring1.assertInvariants()
+
+	// Grant range spanning a live token, and inserting a new token
+	ring1.Entries = []*entry{{Token: start, Peer: peer1name, Free: 10, Version: 2},
+		{Token: dot10, Peer: peer1name, Free: 118}, {Token: middle, Peer: peer1name, Free: 127}}
+	ring1.GrantRangeToHost(dot10, dot245, peer2name)
+	wt.AssertEquals(t, ring1.Entries, entries{{Token: start, Peer: peer1name, Free: 10, Version: 2},
+		{Token: dot10, Peer: peer2name, Free: 118, Version: 1},
+		{Token: middle, Peer: peer2name, Free: 117, Version: 1},
+		{Token: dot245, Peer: peer1name, Free: 10, Version: 0}})
+	ring1.assertInvariants()
 }
 
 func TestMergeSimple(t *testing.T) {