change: drop untargeted pings when collapsing to a full

The ping ID in the URL is the only authentication on the answer, so
rescuing an untargeted ping for every node would let any node answer
for another. Rescue a ping only for the node it targets.

hscontrol/types/change/change.go

@@ -232,14 +232,15 @@ func HasFull(rs []Change) bool {
 
 // CollapseToFull returns what nodeID receives when a full update supersedes
 // changes: one [FullUpdate], then a ping-only [PingNode] for every
-// [Change.PingRequest] addressed to nodeID, in order. A full renders state at
+// [Change.PingRequest] targeted at nodeID, in order. A full renders state at
 // drain time, so it covers every state change; a ping is a one-shot command
-// the full cannot carry.
+// the full cannot carry. An untargeted ping is dropped: the ID in its URL is
+// all that authenticates the answer, so any node could answer for another.
 func CollapseToFull(nodeID types.NodeID, changes []Change) []Change {
 	out := []Change{FullUpdate()}
 
 	for _, c := range changes {
-		if c.PingRequest != nil && c.ShouldSendToNode(nodeID) {
+		if c.PingRequest != nil && c.TargetNode == nodeID {
 			out = append(out, PingNode(nodeID, c.PingRequest))
 		}
 	}

hscontrol/types/change/change_test.go

@@ -67,7 +67,8 @@ func TestChange_FullUpdateSubsumesAllButPing(t *testing.T) {
 			isCarried, ok := carried[field.Name]
 			require.True(t, ok, "field %s is not classified", field.Name)
 
-			var c Change
+			// Addressed to self, so only the field under test decides.
+			c := Change{TargetNode: self}
 
 			v := reflect.ValueOf(&c).Elem().FieldByIndex(field.Index)
 			setNonZero(t, v, self)
@@ -701,9 +702,9 @@ func TestCollapseToFullKeepsPings(t *testing.T) {
 			want:    []Change{FullUpdate(), PingNode(self, prA)},
 		},
 		{
-			name:    "untargeted ping is addressed to every node",
+			name:    "untargeted ping is dropped",
 			changes: []Change{{Reason: "broadcast ping", PingRequest: prA}},
-			want:    []Change{FullUpdate(), PingNode(self, prA)},
+			want:    []Change{FullUpdate()},
 		},
 		{
 			name:    "fulls never stack",