fix: FilePeers implies no Redis (#1251)

## Which problem is this PR solving?

- If someone specified FilePeers, then we are assuming they don't want
to run Redis.

## Short description of the changes

- Simplify logic; if FilePeers, no redis and only local pubsub.
- Update some of the documentation.

cmd/refinery/main.go

@@ -116,6 +116,7 @@ func main() {
 		os.Exit(1)
 	}
 
+	// set up the peer management and pubsub implementations
 	var peers peer.Peers
 	var pubsubber pubsub.PubSub
 	ptype, err := c.GetPeerManagementType()
@@ -124,24 +125,15 @@ func main() {
 	}
 	switch ptype {
 	case "file":
-		// we want to use the file peers object to ask if we have only one peer, so we need to instantiate it
-		// with a dummy metrics object. we'll replace it with the real one later.
-		peers = &peer.FilePeers{Cfg: c, Metrics: &metrics.NullMetrics{}}
-		// if we only have one, we can use the local pubsub implementation.
-		peerList, err := peers.GetPeers()
-		if err != nil {
-			panic(err)
-		}
-		if len(peerList) == 1 {
-			pubsubber = &pubsub.LocalPubSub{}
-		} else {
-			pubsubber = &pubsub.GoRedisPubSub{}
-		}
-		// now erase the peers Metrics object so that it will get injected with the right one later
-		peers.(*peer.FilePeers).Metrics = nil
+		// In the case of file peers, we do not use Redis for anything, including pubsub, so
+		// we use the local pubsub implementation. Even if we have multiple peers, these
+		// peers cannot communicate using pubsub.
+		peers = &peer.FilePeers{}
+		pubsubber = &pubsub.LocalPubSub{}
 	case "redis":
-		pubsubber = &pubsub.GoRedisPubSub{}
+		// if we're using redis, we need to set it up for both peers and pubsub
 		peers = &peer.RedisPubsubPeers{}
+		pubsubber = &pubsub.GoRedisPubSub{}
 	default:
 		// this should have been caught by validation
 		panic("invalid config option 'PeerManagement.Type'")

config/metadata/configMeta.yaml

@@ -835,12 +835,15 @@ groups:
           Peer management is the mechanism by which Refinery locates its peers.
 
           `file` means that Refinery gets its peer list from the Peers list in
-          this config file.
+          this config file. It also prevents Refinery from using a publish/subscribe
+          mechanism to propagate peer lists, stress levels, and configuration changes.
 
           `redis` means that Refinery uses a Publish/Subscribe mechanism,
-          implemented on Redis, to propagate peer lists much more quickly than
-          the legacy mechanism. This is the recommended setting, especially for
-          new installations.
+          implemented on Redis, to propagate peer lists, stress levels, and
+          notification of configuration changes much more quickly than the
+          legacy mechanism. This is the recommended setting, especially for new
+          installations. If this is specified, fields in `RedisPeerManagement`
+          must also be set.
 
       - name: Identifier
         v1group: PeerManagement