Add logic for determining when device lists have changed due to membership changes (#1220)

* Flesh out structure for handling device list updates for room membership changes

* First cut untested algorithm

* Add tests for determining changed/left device lists

* Linting

* Unbreak tests

* Sigh.. linting

internal/consumers.go

@@ -58,11 +58,17 @@ var ErrShutdown = fmt.Errorf("shutdown")
 // Returns nil once all the goroutines are started.
 // Returns an error if it can't start consuming for any of the partitions.
 func (c *ContinualConsumer) Start() error {
+	_, err := c.StartOffsets()
+	return err
+}
+
+// StartOffsets is the same as Start but returns the loaded offsets as well.
+func (c *ContinualConsumer) StartOffsets() ([]sqlutil.PartitionOffset, error) {
 	offsets := map[int32]int64{}
 
 	partitions, err := c.Consumer.Partitions(c.Topic)
 	if err != nil {
-		return err
+		return nil, err
 	}
 	for _, partition := range partitions {
 		// Default all the offsets to the beginning of the stream.
@@ -71,7 +77,7 @@ func (c *ContinualConsumer) Start() error {
 
 	storedOffsets, err := c.PartitionStore.PartitionOffsets(context.TODO(), c.Topic)
 	if err != nil {
-		return err
+		return nil, err
 	}
 	for _, offset := range storedOffsets {
 		// We've already processed events from this partition so advance the offset to where we got to.
@@ -87,7 +93,7 @@ func (c *ContinualConsumer) Start() error {
 			for _, p := range partitionConsumers {
 				p.Close() // nolint: errcheck
 			}
-			return err
+			return nil, err
 		}
 		partitionConsumers = append(partitionConsumers, pc)
 	}
@@ -95,7 +101,7 @@ func (c *ContinualConsumer) Start() error {
 		go c.consumePartition(pc)
 	}
 
-	return nil
+	return storedOffsets, nil
 }
 
 // consumePartition consumes the room events for a single partition of the kafkaesque stream.