Split out notifying /sync requests and calculating sync responses (#96)

* Split out notifying /sync requests and calculating sync responses

The logic for notifying /sync requests is about to get really
complicated as we optimise when to wake up requests, so split
out that code into a separate struct to isolate it and make
it easier to unit test.

src/github.com/matrix-org/dendrite/syncapi/sync/notifier.go

@@ -0,0 +1,67 @@
+// Copyright 2017 Vector Creations Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sync
+
+import (
+	"sync"
+
+	"github.com/matrix-org/dendrite/syncapi/types"
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+// Notifier will wake up sleeping requests in the request pool when there
+// is some new data. It does not tell requests what that data is, only the
+// stream position which they can use to get at it.
+type Notifier struct {
+	// The latest sync stream position: guarded by 'cond'.
+	currPos types.StreamPosition
+	// A condition variable to notify all waiting goroutines of a new sync stream position
+	cond *sync.Cond
+}
+
+// NewNotifier creates a new notifier set to the given stream position.
+func NewNotifier(pos types.StreamPosition) *Notifier {
+	return &Notifier{
+		pos,
+		sync.NewCond(&sync.Mutex{}),
+	}
+}
+
+// OnNewEvent is called when a new event is received from the room server. Must only be
+// called from a single goroutine, to avoid races between updates which could set the
+// current position in the stream incorrectly.
+func (n *Notifier) OnNewEvent(ev *gomatrixserverlib.Event, pos types.StreamPosition) {
+	// update the current position in a guard and then notify all /sync streams
+	n.cond.L.Lock()
+	n.currPos = pos
+	n.cond.L.Unlock()
+
+	n.cond.Broadcast() // notify ALL waiting goroutines
+}
+
+// WaitForEvents blocks until there are new events for this request.
+func (n *Notifier) WaitForEvents(req syncRequest) types.StreamPosition {
+	// In a guard, check if the /sync request should block, and block it until we get a new position
+	n.cond.L.Lock()
+	currentPos := n.currPos
+	for req.since == currentPos {
+		// we need to wait for a new event.
+		// TODO: This waits for ANY new event, we need to only wait for events which we care about.
+		n.cond.Wait() // atomically unlocks and blocks goroutine, then re-acquires lock on unblock
+		currentPos = n.currPos
+	}
+	n.cond.L.Unlock()
+	return currentPos
+}