Paginate using clock-value & message-id instead of skip/limit

Paginating using the count of loaded messages might result in some
messages being skipped and not being loaded in the database, in case of
out-of-order messages received.

This commit changes the behavior to sort by `clock-value` and
`message-id`, which gives a consistent sorting.

The initial idea was to use a cursor `clock-value-message-id` and
iterate on that, but realm does not support filtering on string (</>),
so instead we keep track of messages with identical clock-value and
exclude those in the next page query.

The change might result in pages that have duplicates (so messages needs
to be deduped), but won't result in skipped messages.

src/status_im/chat/models/loading.cljs

@@ -111,9 +111,10 @@
     get-stored-user-statuses :get-stored-user-statuses
     get-referenced-messages :get-referenced-messages :as cofx}]
   (when-not (get-in db [:chats current-chat-id :all-loaded?])
-    (let [loaded-count               (count (get-in db [:chats current-chat-id :messages]))
+    (let [previous-pagination-info   (get-in db [:chats current-chat-id :pagination-info])
           {:keys [messages
-                  all-loaded?]}      (get-stored-messages current-chat-id loaded-count)
+                  pagination-info
+                  all-loaded?]}      (get-stored-messages current-chat-id previous-pagination-info)
           already-loaded-messages    (get-in db [:chats current-chat-id :messages])
           ;; We remove those messages that are already loaded, as we might get some duplicates
           new-messages               (remove (comp already-loaded-messages :message-id)
@@ -132,6 +133,7 @@
                          (update-in [:chats current-chat-id :message-statuses] merge new-statuses)
                          (update-in [:chats current-chat-id :referenced-messages]
                                     #(into (apply dissoc % new-message-ids) referenced-messages))
+                         (assoc-in [:chats current-chat-id :pagination-info] pagination-info)
                          (assoc-in [:chats current-chat-id :all-loaded?]
                                    all-loaded?))}
                 (chat-model/update-chats-unviewed-messages-count

src/status_im/data_store/messages.cljs

@@ -1,5 +1,7 @@
 (ns status-im.data-store.messages
   (:require [re-frame.core :as re-frame]
+            [clojure.set :as clojure.set]
+            [clojure.string :as string]
             [status-im.constants :as constants]
             [status-im.data-store.realm.core :as core]
             [status-im.utils.core :as utils]
@@ -11,15 +13,36 @@
         (update :message-type keyword)
         (assoc :content parsed-content))))
 
+(defn- exclude-messages [query message-ids]
+  (let [string-queries (map #(str "message-id != \"" % "\"") message-ids)]
+    (core/filtered query (string/join " AND " string-queries))))
+
 (defn- get-by-chat-id
   ([chat-id]
-   (get-by-chat-id chat-id 0))
-  ([chat-id from]
-   (let [messages (-> (core/get-by-field @core/account-realm :message :chat-id chat-id)
-                      (core/sorted :timestamp :desc)
-                      (core/page from (+ from constants/default-number-of-messages))
-                      (core/all-clj :message))]
+   (get-by-chat-id chat-id nil))
+  ([chat-id {:keys [last-clock-value message-ids]}]
+   (let [messages (cond-> (core/get-by-field @core/account-realm :message :chat-id chat-id)
+                    :always (core/multi-field-sorted [["clock-value" true] ["message-id" true]])
+                    last-clock-value    (core/filtered (str "clock-value <= \"" last-clock-value "\""))
+                    (seq message-ids)   (exclude-messages message-ids)
+                    :always (core/page 0  constants/default-number-of-messages)
+                    :always (core/all-clj :message))
+         clock-value (-> messages last :clock-value)
+         new-message-ids (->> messages
+                              (filter #(= clock-value (:clock-value %)))
+                              (map :message-id)
+                              (into #{}))]
      {:all-loaded? (> constants/default-number-of-messages (count messages))
+      ;; We paginate using clock-value + message-id to break ties, we need
+      ;; to exclude previously loaded messages with identical clock value
+      ;; otherwise we might fetch exactly the same page if all the messages
+      ;; in a page have the same clock-value. The initial idea was to use a
+      ;; cursor clock-value-message-id but realm does not support </> operators
+      ;; on strings
+      :pagination-info {:last-clock-value clock-value
+                        :message-ids (if (= clock-value last-clock-value)
+                                       (clojure.set/union message-ids new-message-ids)
+                                       new-message-ids)}
       :messages    (keep transform-message messages)})))
 
 (defn get-message-id-by-old [old-message-id]

src/status_im/data_store/realm/core.cljs

@@ -307,6 +307,9 @@
                                        false
                                        true)))
 
+(defn multi-field-sorted [results fields]
+  (.sorted results (clj->js fields)))
+
 (defn page [results from to]
   (js/Array.prototype.slice.call results from to))