stream_topic_history_util: Add retry mechanism to get_server_history.

Earlier, in `get_server_history` the API call to fetch stream-topic
history didn't have a retry logic.

This commit adds a retry mechanism.

If the request to fetch stream topic history fails, we now retry
the request up to a fixed number of times before giving up.
This improves UX in case of temporary network issues.

web/src/stream_topic_history_util.ts

@@ -3,6 +3,7 @@ import {z} from "zod";
 
 import * as channel from "./channel.ts";
 import * as stream_topic_history from "./stream_topic_history.ts";
+import * as util from "./util.ts";
 
 const stream_topic_history_response_schema = z.object({
     topics: z.array(
@@ -15,6 +16,38 @@ const stream_topic_history_response_schema = z.object({
 
 const pending_on_success_callbacks = new Map<number, (() => void)[]>();
 
+export const MAX_RETRIES = 5;
+
+function fetch_channel_history_with_retry(stream_id: number, attempt = 1): void {
+    if (attempt > MAX_RETRIES) {
+        pending_on_success_callbacks.delete(stream_id);
+        stream_topic_history.remove_request_pending_for(stream_id);
+        return;
+    }
+
+    const url = "/json/users/me/" + stream_id + "/topics";
+    void channel.get({
+        url,
+        data: {allow_empty_topic_name: true},
+        success(raw_data) {
+            const data = stream_topic_history_response_schema.parse(raw_data);
+            const server_history = data.topics;
+            stream_topic_history.add_history(stream_id, server_history);
+            stream_topic_history.remove_request_pending_for(stream_id);
+            for (const callback of pending_on_success_callbacks.get(stream_id)!) {
+                callback();
+            }
+            pending_on_success_callbacks.delete(stream_id);
+        },
+        error(xhr) {
+            const retry_delay_secs = util.get_retry_backoff_seconds(xhr, attempt);
+            setTimeout(() => {
+                fetch_channel_history_with_retry(stream_id, attempt + 1);
+            }, retry_delay_secs * 1000);
+        },
+    });
+}
+
 export function get_server_history(stream_id: number, on_success: () => void): void {
     if (stream_topic_history.has_history_for(stream_id)) {
         on_success();
@@ -30,27 +63,7 @@ export function get_server_history(stream_id: number, on_success: () => void): v
     stream_topic_history.add_request_pending_for(stream_id);
     pending_on_success_callbacks.set(stream_id, [on_success]);
 
-    const url = "/json/users/me/" + stream_id + "/topics";
-    void channel.get({
-        url,
-        data: {allow_empty_topic_name: true},
-        success(raw_data) {
-            const data = stream_topic_history_response_schema.parse(raw_data);
-            const server_history = data.topics;
-            stream_topic_history.add_history(stream_id, server_history);
-            stream_topic_history.remove_request_pending_for(stream_id);
-            for (const callback of pending_on_success_callbacks.get(stream_id)!) {
-                callback();
-            }
-            pending_on_success_callbacks.delete(stream_id);
-        },
-        error() {
-            // TODO: Implement some sort of retry logic,
-            // before giving up on the first failure.
-            pending_on_success_callbacks.delete(stream_id);
-            stream_topic_history.remove_request_pending_for(stream_id);
-        },
-    });
+    fetch_channel_history_with_retry(stream_id);
 }
 
 export function update_topic_last_message_id(

web/tests/stream_topic_history.test.cjs

@@ -2,7 +2,7 @@
 
 const assert = require("node:assert/strict");
 
-const {mock_esm, zrequire} = require("./lib/namespace.cjs");
+const {mock_esm, set_global, zrequire} = require("./lib/namespace.cjs");
 const {run_test, noop} = require("./lib/test.cjs");
 
 const channel = mock_esm("../src/channel");
@@ -291,7 +291,6 @@ test("server_history_end_to_end", () => {
     ];
 
     let get_success_callback;
-    let get_error_callback;
     let on_success_called;
 
     channel.get = (opts) => {
@@ -299,7 +298,6 @@ test("server_history_end_to_end", () => {
         assert.deepEqual(opts.data, {allow_empty_topic_name: true});
         assert.ok(stream_topic_history.is_request_pending_for(stream_id));
         get_success_callback = opts.success;
-        get_error_callback = opts.error;
     };
 
     stream_topic_history_util.get_server_history(stream_id, noop);
@@ -309,9 +307,6 @@ test("server_history_end_to_end", () => {
     stream_topic_history_util.get_server_history(stream_id, noop);
     assert.ok(stream_topic_history.is_request_pending_for(stream_id));
 
-    get_error_callback();
-    assert.ok(!stream_topic_history.is_request_pending_for(stream_id));
-
     stream_topic_history_util.get_server_history(stream_id, () => {
         on_success_called = true;
     });
@@ -345,6 +340,29 @@ test("server_history_end_to_end", () => {
     assert.ok(on_success_called);
 });
 
+test("server_history_error", () => {
+    set_global("setTimeout", (f) => {
+        f();
+    });
+    stream_topic_history.reset();
+
+    const channel_id = 99;
+
+    let total_attempts = 0;
+    channel.get = (opts) => {
+        assert.equal(opts.url, "/json/users/me/99/topics");
+        assert.ok(stream_topic_history.is_request_pending_for(channel_id));
+        // This mocks error on each GET request.
+        opts.error();
+        total_attempts += 1;
+    };
+
+    stream_topic_history_util.get_server_history(channel_id, noop);
+    // Verify that we stop after MAX_RETRIES attempt.
+    assert.deepEqual(total_attempts, stream_topic_history_util.MAX_RETRIES);
+    assert.ok(!stream_topic_history.is_request_pending_for(channel_id));
+});
+
 test("ask_server_for_latest_topic_data", () => {
     stream_topic_history.set_update_topic_last_message_id((stream_id, topic_name) => {
         stream_topic_history_util.update_topic_last_message_id(stream_id, topic_name, noop);