From f2163c31c64b57400879c51c75759564f98149f5 Mon Sep 17 00:00:00 2001
From: Mark Haines <mjark@negativecurvature.net>
Date: Wed, 15 Mar 2017 13:47:12 +0000
Subject: [PATCH] Implement state conflict resolution (#37)

* Implement state conflict resolution
---
 .../dendrite/roomserver/input/state.go        | 96 ++++++++++++++++++-
 1 file changed, 93 insertions(+), 3 deletions(-)

diff --git a/src/github.com/matrix-org/dendrite/roomserver/input/state.go b/src/github.com/matrix-org/dendrite/roomserver/input/state.go
index 590bc267..d917180a 100644
--- a/src/github.com/matrix-org/dendrite/roomserver/input/state.go
+++ b/src/github.com/matrix-org/dendrite/roomserver/input/state.go
@@ -6,6 +6,7 @@ import (
 	"github.com/matrix-org/dendrite/roomserver/types"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
+	"sort"
 )
 
 // calculateAndStoreState calculates a snapshot of the state of a room before an event.
@@ -99,7 +100,16 @@ func calculateAndStoreStateAfterManyEvents(db RoomEventDatabase, roomNID types.R
 	if len(conflicts) > 0 {
 		// 5) There are conflicting state events, for each conflict workout
 		// what the appropriate state event is.
-		resolved, err := resolveConflicts(db, combined, conflicts)
+
+		// Work out which entries aren't conflicted.
+		var notConflicted []types.StateEntry
+		for _, entry := range combined {
+			if _, ok := stateEntryMap(conflicts).lookup(entry.StateKeyTuple); !ok {
+				notConflicted = append(notConflicted, entry)
+			}
+		}
+
+		resolved, err := resolveConflicts(db, notConflicted, conflicts)
 		if err != nil {
 			return 0, err
 		}
@@ -114,8 +124,88 @@ func calculateAndStoreStateAfterManyEvents(db RoomEventDatabase, roomNID types.R
 	return db.AddState(roomNID, nil, state)
 }
 
-func resolveConflicts(db RoomEventDatabase, combined, conflicted []types.StateEntry) ([]types.StateEntry, error) {
-	panic(fmt.Errorf("Not implemented"))
+// loadStateEvents loads the matrix events for a list of state entries.
+// Returns a list of state events in no particular order and a map from string event ID back to state entry.
+// The map can be used to recover which numeric state entry a given event is for.
+// Returns an error if there was a problem talking to the database.
+func loadStateEvents(db RoomEventDatabase, entries []types.StateEntry) ([]gomatrixserverlib.Event, map[string]types.StateEntry, error) {
+	eventNIDs := make([]types.EventNID, len(entries))
+	for i := range entries {
+		eventNIDs[i] = entries[i].EventNID
+	}
+	events, err := db.Events(eventNIDs)
+	if err != nil {
+		return nil, nil, err
+	}
+	eventIDMap := map[string]types.StateEntry{}
+	result := make([]gomatrixserverlib.Event, len(entries))
+	for i := range entries {
+		event, ok := eventMap(events).lookup(entries[i].EventNID)
+		if !ok {
+			panic(fmt.Errorf("Corrupt DB: Missing event numeric ID %d", entries[i].EventNID))
+		}
+		result[i] = event.Event
+		eventIDMap[event.Event.EventID()] = entries[i]
+	}
+	return result, eventIDMap, nil
+}
+
+// resolveConflicts resolves a list of conflicted state entries. It takes two lists.
+// The first is a list of all state entries that are not conflicted.
+// The second is a list of all state entries that are conflicted
+// A state entry is conflicted when there is more than one numeric event ID for the same state key tuple.
+// Returns a list that combines the entries without conflicts with the result of state resolution for the entries with conflicts.
+// The returned list is sorted by state key tuple.
+// Returns an error if there was a problem talking to the database.
+func resolveConflicts(db RoomEventDatabase, notConflicted, conflicted []types.StateEntry) ([]types.StateEntry, error) {
+
+	// Load the conflicted events
+	conflictedEvents, eventIDMap, err := loadStateEvents(db, conflicted)
+	if err != nil {
+		return nil, err
+	}
+
+	// Work out which auth events we need to load.
+	needed := gomatrixserverlib.StateNeededForAuth(conflictedEvents)
+
+	// Find the numeric IDs for the necessary state keys.
+	var neededStateKeys []string
+	neededStateKeys = append(neededStateKeys, needed.Member...)
+	neededStateKeys = append(neededStateKeys, needed.ThirdPartyInvite...)
+	stateKeyNIDMap, err := db.EventStateKeyNIDs(neededStateKeys)
+	if err != nil {
+		return nil, err
+	}
+
+	// Load the necessary auth events.
+	tuplesNeeded := stateKeyTuplesNeeded(stateKeyNIDMap, needed)
+	var authEntries []types.StateEntry
+	for _, tuple := range tuplesNeeded {
+		if eventNID, ok := stateEntryMap(notConflicted).lookup(tuple); ok {
+			authEntries = append(authEntries, types.StateEntry{tuple, eventNID})
+		}
+	}
+	authEvents, _, err := loadStateEvents(db, authEntries)
+	if err != nil {
+		return nil, err
+	}
+
+	// Resolve the conflicts.
+	fmt.Println("Resolving", len(conflicted), "conflicts with", len(authEvents), "authEvents")
+	resolvedEvents := gomatrixserverlib.ResolveStateConflicts(conflictedEvents, authEvents)
+
+	// Map from the full events back to numeric state entries.
+	for _, resolvedEvent := range resolvedEvents {
+		entry, ok := eventIDMap[resolvedEvent.EventID()]
+		if !ok {
+			panic(fmt.Errorf("Missing state entry for event ID %q", resolvedEvent.EventID()))
+		}
+		notConflicted = append(notConflicted, entry)
+	}
+
+	// Sort the result so it can be searched.
+	sort.Sort(stateEntrySorter(notConflicted))
+	return notConflicted, nil
 }
 
 // findDuplicateStateKeys finds the state entries where the state key tuple appears more than once in a sorted list.