MSC2836: Threading - part one (#1589)

* Add mscs/hooks package, begin work for msc2836

* Flesh out hooks and add SQL schema

* Begin implementing core msc2836 logic

* Add test harness

* Linting

* Implement visibility checks; stub out APIs for tests

* Flesh out testing

* Flesh out walkThread a bit

* Persist the origin_server_ts as well

* Edges table instead of relationships

* Add nodes table for event metadata

* LEFT JOIN to extract origin_server_ts for children

* Add graph walking structs

* Implement walking algorithm

* Add more graph walking tests

* Add auto_join for local rooms

* Fix create table syntax on postgres

* Add relationship_room_id|servers to the unsigned section of events

* Persist the parent room_id/servers in edge metadata

Other events cannot assert the true room_id/servers for the
parent event, only make claims to them, hence why this is
edge metadata.

* guts to pass through room_id/servers

* Refactor msc2836 to allow handling from federation

* Add JoinedVia to PerformJoin responses

* Fix tests; review comments

internal/hooks/hooks.go

@@ -0,0 +1,74 @@
+// Copyright 2020 The Matrix.org Foundation C.I.C.
+//
+// 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 hooks exposes places in Dendrite where custom code can be executed, useful for MSCs.
+// Hooks can only be run in monolith mode.
+package hooks
+
+import "sync"
+
+const (
+	// KindNewEventPersisted is a hook which is called with *gomatrixserverlib.HeaderedEvent
+	// It is run when a new event is persisted in the roomserver.
+	// Usage:
+	//   hooks.Attach(hooks.KindNewEventPersisted, func(headeredEvent interface{}) { ... })
+	KindNewEventPersisted = "new_event_persisted"
+	// KindNewEventReceived is a hook which is called with *gomatrixserverlib.HeaderedEvent
+	// It is run before a new event is processed by the roomserver. This hook can be used
+	// to modify the event before it is persisted by adding data to `unsigned`.
+	// Usage:
+	//   hooks.Attach(hooks.KindNewEventReceived, func(headeredEvent interface{}) {
+	//     ev := headeredEvent.(*gomatrixserverlib.HeaderedEvent)
+	//     _ = ev.SetUnsignedField("key", "val")
+	//   })
+	KindNewEventReceived = "new_event_received"
+)
+
+var (
+	hookMap = make(map[string][]func(interface{}))
+	hookMu  = sync.Mutex{}
+	enabled = false
+)
+
+// Enable all hooks. This may slow down the server slightly. Required for MSCs to work.
+func Enable() {
+	enabled = true
+}
+
+// Run any hooks
+func Run(kind string, data interface{}) {
+	if !enabled {
+		return
+	}
+	cbs := callbacks(kind)
+	for _, cb := range cbs {
+		cb(data)
+	}
+}
+
+// Attach a hook
+func Attach(kind string, callback func(interface{})) {
+	if !enabled {
+		return
+	}
+	hookMu.Lock()
+	defer hookMu.Unlock()
+	hookMap[kind] = append(hookMap[kind], callback)
+}
+
+func callbacks(kind string) []func(interface{}) {
+	hookMu.Lock()
+	defer hookMu.Unlock()
+	return hookMap[kind]
+}