fix(learn): the instance asks LanguageTool for its languages again a minute after it didn't answer

Switching the sidecar on learn.halkyon.eu's server recreated the server
beside LanguageTool, whose start takes a minute: the server's first look
at the languages failed, and it looked again only an hour later, as
after one that worked. Until then instance.info() named no languages,
and an app that asked meanwhile keeps that answer, so its editors check
nothing until it starts anew. A look LanguageTool doesn't answer is now
tried again a minute later, by a timer, until it answers, the languages
known before kept meanwhile; one that worked still counts for an hour.

The server's tests take fake_async (^1.3.3, as Learn's app, already in
the lock) for the timer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_server/lib/src/grammar/grammar_checking.dart

@@ -42,11 +42,17 @@ class GrammarChecking {
   List<String>? _codes;
   DateTime? _lookedAt;
   Future<void>? _looking;
+  Timer? _again;
   final _asked = HashMap<UuidValue, Queue<DateTime>>();
 
   /// How long a look at the languages counts.
   static const _fresh = Duration(hours: 1);
 
+  /// How soon a look LanguageTool didn't answer is tried again: it takes
+  /// a minute to start, and a server started beside it looks at once, and
+  /// apps keep the languages they were told.
+  static const _retry = Duration(minutes: 1);
+
   /// The languages checked here (`de`, `ja`); null without a LanguageTool,
   /// or while it hasn't answered. Looks again in the background once the
   /// last look is an hour old, and never waits.
@@ -62,12 +68,14 @@ class GrammarChecking {
   }
 
   /// Asks LanguageTool which languages it checks; one look at a time. A
-  /// failed look forgets none it knew.
+  /// failed look forgets none it knew, and looks again after [_retry],
+  /// until LanguageTool answers.
   Future<void> look() => _looking ??= () async {
+    _again?.cancel();
     try {
       _codes = await languageTool?.languages() ?? _codes;
     } on LanguageToolFailed {
-      // Unknown until the next look.
+      _again = Timer(_retry, () => unawaited(look()));
     } finally {
       _lookedAt = clock.now();
       _looking = null;

apps/learn/halkyon_learn_server/pubspec.yaml

@@ -24,6 +24,8 @@ dependencies:
   serverpod_auth_idp_server: 4.0.4
   serverpod_offline_sync_server: 0.0.10
 dev_dependencies:
+  # Time in tests that wait on timers (looking again for LanguageTool's languages).
+  fake_async: ^1.3.3
   serverpod_test: 4.0.4
   test: ^1.25.5
 

apps/learn/halkyon_learn_server/test/unit/grammar/language_tool_test.dart

@@ -3,6 +3,7 @@ import 'dart:convert';
 import 'dart:io';
 
 import 'package:clock/clock.dart';
+import 'package:fake_async/fake_async.dart';
 import 'package:halkyon_learn_server/src/generated/protocol.dart';
 import 'package:halkyon_learn_server/src/grammar/grammar_checking.dart';
 import 'package:halkyon_learn_server/src/grammar/language_tool.dart';
@@ -285,6 +286,51 @@ void main() {
       });
     });
 
+    test('looks again a minute after LanguageTool failed to answer, as it '
+        'does while starting beside the server, until it answers', () {
+      fakeAsync((async) {
+        final fake = _FakeLanguageTool()..fails = true;
+        final checking = GrammarChecking(languageTool: fake);
+        unawaited(checking.look());
+        async.flushMicrotasks();
+        expect(checking.languages, isNull);
+        expect(fake.looks, 1);
+
+        async.elapse(const Duration(seconds: 61));
+        expect(fake.looks, 2, reason: 'still failing: again a minute on');
+
+        fake.fails = false;
+        async.elapse(const Duration(seconds: 61));
+        expect(fake.looks, 3);
+        expect(checking.languages, ['de', 'en', 'ja']);
+
+        async.elapse(const Duration(minutes: 30));
+        expect(fake.looks, 3, reason: 'answered: nothing until an hour on');
+      });
+    });
+
+    test('keeps the languages it knows when a later look fails, and looks '
+        'again a minute after', () {
+      fakeAsync((async) {
+        final fake = _FakeLanguageTool();
+        final checking = GrammarChecking(languageTool: fake);
+        unawaited(checking.look());
+        async.elapse(const Duration(minutes: 61));
+        fake
+          ..fails = true
+          ..codes = ['de-DE', 'en-US', 'ja-JP', 'el-GR'];
+        expect(checking.languages, ['de', 'en', 'ja']);
+        async.flushMicrotasks();
+        expect(fake.looks, 2);
+        expect(checking.languages, ['de', 'en', 'ja']);
+
+        fake.fails = false;
+        async.elapse(const Duration(seconds: 61));
+        expect(fake.looks, 3);
+        expect(checking.languages, ['de', 'el', 'en', 'ja']);
+      });
+    });
+
     test('checks in the code the language becomes, nothing for an empty '
         'text', () async {
       final fake = _FakeLanguageTool();
@@ -374,11 +420,15 @@ class _FakeLanguageTool implements LanguageTool {
   bool fails = false;
   final checked = <(String, String)>[];
 
+  /// How often it was asked for its languages.
+  int looks = 0;
+
   @override
   Uri get address => Uri.parse('http://languagetool:8081/');
 
   @override
   Future<List<String>> languages() async {
+    looks++;
     if (fails) {
       throw const LanguageToolFailed('down');
     }