Limit group requests to 3 at once

using a FIFO queue.

This is needed in order to lower the priority of getting group
state and prioritise everything else, namely initial sync.

It should be noted that this by no means guarantees that the
first incremental sync will happen sooner; the client could
end up doing some other requests first instead.

src/stores/GroupStore.js

@@ -27,6 +27,48 @@ function parseRoomsResponse(response) {
     return response.chunk.map((apiRoom) => groupRoomFromApiObject(apiRoom));
 }
 
+// The number of ongoing group requests
+let ongoingRequestCount = 0;
+
+// This has arbitrarily been set to a small number to lower the priority
+// of doing group-related requests because we care about other important
+// requests like hitting /sync.
+const LIMIT = 3; // Maximum number of ongoing group requests
+
+// FIFO queue of functions to call in the backlog
+const backlogQueue = [
+    // () => {...}
+];
+
+// Pull from the FIFO queue
+function checkBacklog() {
+    const item = backlogQueue.shift();
+    if (typeof item === 'function') item();
+}
+
+// Limit the maximum number of ongoing promises returned by fn to LIMIT and
+// use a FIFO queue to handle the backlog.
+function limitConcurrency(fn) {
+    return new Promise((resolve, reject) => {
+        const item = () => {
+            ongoingRequestCount++;
+            resolve();
+        };
+        if (ongoingRequestCount >= LIMIT) {
+            // Enqueue this request for later execution
+            backlogQueue.push(item);
+        } else {
+            item();
+        }
+    })
+    .then(fn)
+    .then((result) => {
+        ongoingRequestCount--;
+        checkBacklog();
+        return result;
+    });
+}
+
 /**
  * Stores the group summary for a room and provides an API to change it and
  * other useful group APIs that may have an effect on the group summary.
@@ -56,23 +98,24 @@ export default class GroupStore extends EventEmitter {
         this._fetchResourcePromise = {};
         this._resourceFetcher = {
             [GroupStore.STATE_KEY.Summary]: () => {
-                return MatrixClientPeg.get()
-                    .getGroupSummary(this.groupId);
+                return limitConcurrency(
+                    () => MatrixClientPeg.get().getGroupSummary(this.groupId),
+                );
             },
             [GroupStore.STATE_KEY.GroupRooms]: () => {
-                return MatrixClientPeg.get()
-                    .getGroupRooms(this.groupId)
-                    .then(parseRoomsResponse);
+                return limitConcurrency(
+                    () => MatrixClientPeg.get().getGroupRooms(this.groupId).then(parseRoomsResponse),
+                );
             },
             [GroupStore.STATE_KEY.GroupMembers]: () => {
-                return MatrixClientPeg.get()
-                    .getGroupUsers(this.groupId)
-                    .then(parseMembersResponse);
+                return limitConcurrency(
+                    () => MatrixClientPeg.get().getGroupUsers(this.groupId).then(parseMembersResponse),
+                );
             },
             [GroupStore.STATE_KEY.GroupInvitedMembers]: () => {
-                return MatrixClientPeg.get()
-                    .getGroupInvitedUsers(this.groupId)
-                    .then(parseMembersResponse);
+                return limitConcurrency(
+                    () => MatrixClientPeg.get().getGroupInvitedUsers(this.groupId).then(parseMembersResponse),
+                );
             },
         };