Update comments from review

serverkeyapi/internal/api.go

@@ -157,11 +157,17 @@ func (s *ServerKeyAPI) handleDatabaseKeys(
 
 	// We successfully got some keys. Add them to the results.
 	for req, res := range dbResults {
+		// The key we've retrieved from the database/cache might
+		// have passed its validity period, but right now, it's
+		// the best thing we've got, and it might be sufficient to
+		// verify a past event.
 		results[req] = res
 
 		// If the key is valid right now then we can also remove it
 		// from the request list as we don't need to fetch it again
-		// in that case.
+		// in that case. If the key isn't valid right now, then by
+		// leaving it in the 'requests' map, we'll try to update the
+		// key using the fetchers in handleFetcherKeys.
 		if res.WasValidAt(now, true) {
 			delete(requests, req)
 		}
@@ -169,7 +175,7 @@ func (s *ServerKeyAPI) handleDatabaseKeys(
 	return nil
 }
 
-// handleDatabaseKeys handles cases where a fetcher can satisfy
+// handleFetcherKeys handles cases where a fetcher can satisfy
 // the remaining requests.
 func (s *ServerKeyAPI) handleFetcherKeys(
 	ctx context.Context,

serverkeyapi/serverkeyapi_test.go

@@ -21,12 +21,12 @@ import (
 )
 
 type server struct {
-	name      gomatrixserverlib.ServerName
-	validity  time.Duration
-	config    *config.Dendrite
-	fedclient *gomatrixserverlib.FederationClient
-	cache     *caching.Caches
-	api       api.ServerKeyInternalAPI
+	name      gomatrixserverlib.ServerName        // server name
+	validity  time.Duration                       // key validity duration from now
+	config    *config.Dendrite                    // skeleton config, from TestMain
+	fedclient *gomatrixserverlib.FederationClient // uses MockRoundTripper
+	cache     *caching.Caches                     // server-specific cache
+	api       api.ServerKeyInternalAPI            // server-specific server key API
 }
 
 func (s *server) renew() {