wip; inspect the cause

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/model/store.dart

@@ -777,6 +777,15 @@ class UpdateMachine {
     int accumulatedTransientFailureCount = 0;
     const transientFailureCountNotifyThreshold = 5;
 
+    /// This only reports transient errors after reaching
+    /// a pre-defined threshold of retries.
+    void maybeReportTransientError(String? message, {String? details}) {
+      accumulatedTransientFailureCount++;
+      if (accumulatedTransientFailureCount > transientFailureCountNotifyThreshold) {
+        reportErrorToUserBriefly(message, details: details);
+      }
+    }
+
     while (true) {
       if (_debugLoopSignal != null) {
         await _debugLoopSignal!.future;
@@ -802,12 +811,24 @@ class UpdateMachine {
             debugLog('… Event queue replaced.');
             return;
 
-          case Server5xxException() || NetworkException():
+          case Server5xxException():
+            assert(debugLog('Transient error polling event queue for $store: $e\n'
+                'Backing off, then will retry…'));
+            maybeReportTransientError(
+              localizations.errorConnectingToServerShort,
+              details: localizations.errorConnectingToServerDetails(
+                serverUrl, e.toString()));
+            await (backoffMachine ??= BackoffMachine()).wait();
+            assert(debugLog('… Backoff wait complete, retrying poll.'));
+            continue;
+
+          case NetworkException():
             assert(debugLog('Transient error polling event queue for $store: $e\n'
                 'Backing off, then will retry…'));
-            accumulatedTransientFailureCount++;
-            if (accumulatedTransientFailureCount > transientFailureCountNotifyThreshold) {
-              reportErrorToUserBriefly(
+            // Heuristic check to skip reporting boring errors.
+            // This error occurs when the client has no network connection.
+            if (!e.message.startsWith('Failed host lookup')) {
+              maybeReportTransientError(
                 localizations.errorConnectingToServerShort,
                 details: localizations.errorConnectingToServerDetails(
                   serverUrl, e.toString()));

test/model/store_test.dart

@@ -471,8 +471,17 @@ void main() {
     }));
 
     group('retries on errors', () {
+      /// Check if [UpdateMachine.poll] retries as expected when there are
+      /// errors.
+      ///
+      /// If `expectedFailureCountNotifyThreshold` is null,
+      /// no user-facing error message should be shown .
+      ///
+      /// If `expectedFailureCountNotifyThreshold` is non-null,
+      /// the first user-facing error message should be shown when
+      /// the `expectedFailureCountNotifyThreshold`'th polling failure occurs.
       void checkRetry(void Function() prepareError, {
-        int expectedFailureCountNotifyThreshold = 0,
+        int? expectedFailureCountNotifyThreshold = 0,
       }) {
         reportErrorToUserBriefly = logAndReportErrorToUserBriefly;
         addTearDown(() => reportErrorToUserBriefly = defaultReportErrorToUserBriefly);
@@ -489,7 +498,8 @@ void main() {
 
           // Need to add 1 to the upperbound for that one additional request
           // to trigger error reporting.
-          for (int i = 0; i < expectedFailureCountNotifyThreshold + 1; i++) {
+          final numRetries = expectedFailureCountNotifyThreshold ?? 0 + 1;
+          for (int i = 0; i < numRetries; i++) {
             // Make the request, inducing an error in it.
             prepareError();
             if (i > 0) {
@@ -499,15 +509,19 @@ void main() {
             updateMachine.debugAdvanceLoop();
             check(lastReportedError).isNull();
             async.elapse(Duration.zero);
-            if (i < expectedFailureCountNotifyThreshold) {
+
+            checkLastRequest(lastEventId: 1);
+            check(store).isLoading.isTrue();
+            if (expectedFailureCountNotifyThreshold == null
+                || i < expectedFailureCountNotifyThreshold) {
               // The error message should not appear until the `updateMachine`
               // has retried the given number of times.
               check(takeLastReportedError()).isNull();
-            } else {
+              continue;
+            }
+            if (i == expectedFailureCountNotifyThreshold) {
               check(takeLastReportedError()).isNotNull().contains(expectedErrorMessage);
             }
-            checkLastRequest(lastEventId: 1);
-            check(store).isLoading.isTrue();
           }
 
           // Polling doesn't resume immediately; there's a timer.