store: Report if transient polling errors persist.

Signed-off-by: Zixuan James Li <zixuan@zulip.com>

Author: PIG208

lib/model/store.dart

@@ -764,6 +764,8 @@ class UpdateMachine {
 
   void poll() async {
     BackoffMachine? backoffMachine;
+    int accumlatedTransientFailureCount = 0;
+    const transientFailureCountNotifyThreshold = 5;
 
     while (true) {
       if (_debugLoopSignal != null) {
@@ -791,8 +793,11 @@ class UpdateMachine {
           case Server5xxException() || NetworkException():
             assert(debugLog('Transient error polling event queue for $store: $e\n'
                 'Backing off, then will retry…'));
-            // TODO tell user if transient polling errors persist
             // TODO reset to short backoff eventually
+            accumlatedTransientFailureCount++;
+            if (accumlatedTransientFailureCount > transientFailureCountNotifyThreshold) {
+              reportErrorToUserBriefly('Failed to reach server. Will retry: $e');
+            }
             await (backoffMachine ??= BackoffMachine()).wait();
             assert(debugLog('… Backoff wait complete, retrying poll.'));
             continue;
@@ -825,6 +830,7 @@ class UpdateMachine {
       // and failures, the successes themselves should space out the requests.
       backoffMachine = null;
       store.isLoading = false;
+      accumlatedTransientFailureCount = 0;
 
       final events = result.events;
       for (final event in events) {

test/model/store_test.dart

@@ -430,6 +430,7 @@ void main() {
 
     void checkRetry(String description, void Function() prepareError, {
       String? errorMessage,
+      int failureCountNofityThreshold = 0,
     }) {
       test(description, () {
         awaitFakeAsync((async) async {
@@ -438,18 +439,24 @@ void main() {
           updateMachine.poll();
           check(async.pendingTimers).length.equals(0);
 
-          // Make the request, inducing an error in it.
-          prepareError();
-          updateMachine.debugAdvanceLoop();
-          check(debugLastReportedError).isNull();
-          async.elapse(Duration.zero);
-          if (errorMessage == null) {
-            check(takeLastReportedError()).isNull();
-          } else {
-            check(takeLastReportedError()).isNotNull().contains(errorMessage);
+          for (int i = 0; i < failureCountNofityThreshold + 1; i++) {
+            // Make the request, inducing an error in it.
+            prepareError();
+            if (i > 0) {
+              // End polling backoff from the previous iteration.
+              async.flushTimers();
+            }
+            updateMachine.debugAdvanceLoop();
+            check(debugLastReportedError).isNull();
+            async.elapse(Duration.zero);
+            if (i < failureCountNofityThreshold || errorMessage == null) {
+              check(takeLastReportedError()).isNull();
+            } else {
+              check(takeLastReportedError()).isNotNull().contains(errorMessage);
+            }
+            checkLastRequest(lastEventId: 1);
+            check(store).isLoading.isTrue();
           }
-          checkLastRequest(lastEventId: 1);
-          check(store).isLoading.isTrue();
 
           // Polling doesn't resume immediately; there's a timer.
           check(async.pendingTimers).length.equals(1);
@@ -471,11 +478,15 @@ void main() {
     }
 
     group('retries', () {
-      checkRetry('retries on Server5xxException',
-        () => connection.prepare(httpStatus: 500, body: 'splat'));
-
-      checkRetry('retries on NetworkException',
-        () => connection.prepare(exception: Exception("failed")));
+      checkRetry('too many retries on Server5xxException',
+        () => connection.prepare(httpStatus: 500, body: 'splat'),
+        errorMessage: 'Failed to reach server. Will retry',
+        failureCountNofityThreshold: 5);
+
+      checkRetry('too many retries on NetworkException',
+        () => connection.prepare(exception: Exception("failed")),
+        errorMessage: 'Failed to reach server. Will retry',
+        failureCountNofityThreshold: 5);
 
       checkRetry('retries on ZulipApiException',
         () => connection.prepare(httpStatus: 400, json: {