feat(state): add core support for Pebble Notices (#295)

This implements the core state aspects of Pebble Notices -- spec JU048.

The key user-visible methods are State.AddNotice to add a new notices, State.Notices and State.Notice to fetch and filter existing notices, and State.WaitNotices to wait (or long-poll) for notices as they occur.

internals/overlord/overlord_test.go

@@ -79,7 +79,15 @@ func (ovs *overlordSuite) TestNew(c *C) {
 }
 
 func (ovs *overlordSuite) TestNewWithGoodState(c *C) {
-	fakeState := []byte(fmt.Sprintf(`{"data":{"patch-level":%d,"patch-sublevel":%d,"patch-sublevel-last-version":%q,"some":"data"},"changes":null,"tasks":null,"last-change-id":0,"last-task-id":0,"last-lane-id":0}`, patch.Level, patch.Sublevel, cmd.Version))
+	fakeState := []byte(fmt.Sprintf(`{
+		"data": {"patch-level": %d, "patch-sublevel": %d, "patch-sublevel-last-version": %q, "some": "data"},
+		"changes": null,
+		"tasks": null,
+		"last-change-id": 0,
+		"last-task-id": 0,
+		"last-lane-id": 0,
+		"last-notice-id": 0
+	}`, patch.Level, patch.Sublevel, cmd.Version))
 	err := ioutil.WriteFile(ovs.statePath, fakeState, 0600)
 	c.Assert(err, IsNil)
 

internals/overlord/state/export_test.go

@@ -73,3 +73,9 @@ var (
 	ErrNoWarningExpireAfter = errNoWarningExpireAfter
 	ErrNoWarningRepeatAfter = errNoWarningRepeatAfter
 )
+
+// NumNotices returns the total number of notices, including expired ones that
+// haven't yet been pruned.
+func (s *State) NumNotices() int {
+	return len(s.notices)
+}