Reuse epoch-verified record snapshots and avoid empty push payloads

app/controllers/concerns/authentication/session_lookup.rb

@@ -1,7 +1,12 @@
 module Authentication::SessionLookup
   def find_session_by_cookie
     if token = cookies.signed[:session_token]
-      Session.find_by(token: token)
+      version = @response_cache_version if @response_cache_version && (request.get? || request.head?)
+      session, user = RecordCache.fetch([ "session", Digest::SHA256.hexdigest(token.to_s) ], version) do
+        session = Session.find_by(token: token)
+        [ session, session&.user ]
+      end
+      session&.tap { |record| record.user = user }
     end
   end
 end

app/controllers/concerns/cached_responses.rb

@@ -35,6 +35,10 @@ module CachedResponses
   end
 
   private
+    def read_record_cache_version
+      @response_cache_version if request.get? || request.head?
+    end
+
     def capture_response_cache_version
       # Capture for native HTML/JSON/stream renders too, even with page reuse off.
       # Detached renderers do not run callbacks and therefore render uncached.
@@ -81,6 +85,10 @@ module CachedResponses
     def cache_completed_response(key, original_session, encoding)
       if response.status == 200 && response.media_type == "text/html" && session.to_hash == original_session && !response.headers["Content-Encoding"]
         body = encoding == "gzip" ? Zlib.gzip(response.body) : response.body
+        unless body.empty? || response.headers["ETag"] || response.headers["Last-Modified"]
+          # Match Rack::ETag once, rather than hashing the same bytes on each hit.
+          response.headers["ETag"] = %(W/"#{Digest::SHA256.hexdigest(body).byteslice(0, 32)}")
+        end
         response.headers["Content-Encoding"] = "gzip" if encoding == "gzip"
         response.headers["Vary"] = (response.headers["Vary"].to_s.split(/,\s*/) | [ "Accept-Encoding" ]).join(", ")
         headers = response.headers.slice(*CACHE_HEADERS).to_h.freeze

app/controllers/concerns/room_scoped.rb

@@ -7,7 +7,10 @@ module RoomScoped
 
   private
     def set_room
-      @membership = Current.user.memberships.find_by!(room_id: params[:room_id])
-      @room = @membership.room
+      @membership, @room = RecordCache.fetch([ "membership", Current.user.id, params[:room_id] ], read_record_cache_version) do
+        membership = Current.user.memberships.find_by!(room_id: params[:room_id])
+        [ membership, membership.room ]
+      end
+      @membership.room = @room
     end
 end

app/controllers/rooms_controller.rb

@@ -21,7 +21,10 @@ class RoomsController < ApplicationController
 
   private
     def set_room
-      if room = room_scope.find_by(id: params[:room_id] || params[:id])
+      room = RecordCache.fetch([ "room", self.class.name, Current.user.id, params[:room_id] || params[:id] ], read_record_cache_version) do
+        [ room_scope.find_by(id: params[:room_id] || params[:id]) ]
+      end.first
+      if room
         @room = room
       else
         redirect_to root_url, alert: "Room not found or inaccessible"