refactor: Extract watchdog module.

We now have 100% code coverage on this somewhat
fiddly code.

We also break activity's dependency on server_events.

docs/subsystems/hashchange-system.md

@@ -86,8 +86,8 @@ reload itself:
   garbage-collected by the server, meaning the browser can no longer
   get real-time updates altogether.  In this case, the browser
   auto-reloads immediately in order to reconnect.  We have coded an
-  unsuspend trigger (based on some clever time logic) that ensures we
-  check immediately when a client unsuspends; grep for `unsuspend` to
+  unsuspend callback (based on some clever time logic) that ensures we
+  check immediately when a client unsuspends; grep for `watchdog` to
   see the code.
 * If a new version of the server has been deployed, we want to reload
   the browser so that it will start running the latest code.  However,

frontend_tests/node_tests/activity.js

@@ -51,12 +51,12 @@ mock_esm("../../static/js/resize", {
 mock_esm("../../static/js/scroll_util", {
     scroll_element_into_container: () => {},
 });
-mock_esm("../../static/js/server_events", {
-    check_for_unsuspend() {},
-});
 mock_esm("../../static/js/stream_popover", {
     show_streamlist_sidebar() {},
 });
+mock_esm("../../static/js/watchdog", {
+    check_for_unsuspend() {},
+});
 set_global("document", _document);
 
 const huddle_data = zrequire("huddle_data");

frontend_tests/node_tests/watchdog.js

@@ -0,0 +1,63 @@
+"use strict";
+
+const {strict: assert} = require("assert");
+
+const MockDate = require("mockdate");
+
+const {set_global, zrequire} = require("../zjsunit/namespace");
+const {run_test} = require("../zjsunit/test");
+
+let time = 0;
+let checker;
+MockDate.set(time);
+
+function advance_secs(secs) {
+    time += secs * 1000;
+    MockDate.set(time);
+}
+
+set_global("setInterval", (f, interval) => {
+    checker = f;
+    assert.equal(interval, 5000);
+});
+
+const watchdog = zrequire("watchdog");
+
+run_test("basics", () => {
+    // Test without callbacks first.
+    checker();
+    advance_secs(5);
+    checker();
+
+    let num_times_called_back = 0;
+
+    function callback() {
+        num_times_called_back += 1;
+    }
+
+    watchdog.on_unsuspend(callback);
+
+    // Simulate healthy operation.
+    advance_secs(5);
+    checker();
+    assert.equal(num_times_called_back, 0);
+
+    // Simulate machine going to sleep.
+    advance_secs(21);
+    checker();
+    assert.equal(num_times_called_back, 1);
+
+    // Simulate healthy operations resume, and
+    // explicitly call check_for_unsuspend.
+    num_times_called_back = 0;
+    advance_secs(5);
+    watchdog.check_for_unsuspend();
+    assert.equal(num_times_called_back, 0);
+
+    // Simulate another suspension.
+    advance_secs(21);
+    watchdog.check_for_unsuspend();
+    assert.equal(num_times_called_back, 1);
+});
+
+MockDate.reset();

static/js/activity.js

@@ -12,9 +12,9 @@ import * as people from "./people";
 import * as pm_list from "./pm_list";
 import * as popovers from "./popovers";
 import * as presence from "./presence";
-import * as server_events from "./server_events";
 import {UserSearch} from "./user_search";
 import * as user_status from "./user_status";
+import * as watchdog from "./watchdog";
 
 export let user_cursor;
 export let user_filter;
@@ -195,7 +195,7 @@ export function send_presence_to_server(want_redraw) {
         return;
     }
 
-    server_events.check_for_unsuspend();
+    watchdog.check_for_unsuspend();
 
     channel.post({
         url: "/json/users/me/presence",

static/js/server_events.js

@@ -11,6 +11,7 @@ import * as reload_state from "./reload_state";
 import * as sent_messages from "./sent_messages";
 import * as server_events_dispatch from "./server_events_dispatch";
 import * as ui_report from "./ui_report";
+import * as watchdog from "./watchdog";
 
 // Docs: https://zulip.readthedocs.io/en/latest/subsystems/events-system.html
 
@@ -302,26 +303,8 @@ export function home_view_loaded() {
     $(document).trigger("home_view_loaded.zulip");
 }
 
-let watchdog_time = Date.now();
-
-export function check_for_unsuspend() {
-    const new_time = Date.now();
-    if (new_time - watchdog_time > 20000) {
-        // 20 seconds.
-        // Defensively reset watchdog_time here in case there's an
-        // exception in one of the event handlers
-        watchdog_time = new_time;
-        // Our app's JS wasn't running, which probably means the machine was
-        // asleep.
-        $(document).trigger("unsuspend");
-    }
-    watchdog_time = new_time;
-}
-
-setInterval(check_for_unsuspend, 5000);
-
 export function initialize() {
-    $(document).on("unsuspend", () => {
+    watchdog.on_unsuspend(() => {
         // Immediately poll for new events on unsuspend
         blueslip.log("Restarting get_events due to unsuspend");
         get_events_failures = 0;

static/js/watchdog.js

@@ -0,0 +1,34 @@
+const unsuspend_callbacks = [];
+let watchdog_time = Date.now();
+
+/*
+    Our watchdog code checks every 5 seconds to make sure that we
+    haven't gone 20 seconds since the last "5-second-ago" check.
+    This sounds confusing, but it is just is a way to detect that
+    the machine has gone to sleep.
+
+    When we detect the condition we call back to server_events code
+    to reset ourselves accordingly.
+*/
+
+export function check_for_unsuspend() {
+    const new_time = Date.now();
+    if (new_time - watchdog_time > 20000) {
+        // 20 seconds.
+        // Defensively reset watchdog_time here in case there's an
+        // exception in one of the event handlers
+        watchdog_time = new_time;
+        // Our app's JS wasn't running, which probably means the machine was
+        // asleep.
+        for (const callback of unsuspend_callbacks) {
+            callback();
+        }
+    }
+    watchdog_time = new_time;
+}
+
+export function on_unsuspend(f) {
+    unsuspend_callbacks.push(f);
+}
+
+setInterval(check_for_unsuspend, 5000);