Improve reliability of ETag validation in event log acceptance tests

The test could fail if new events were written to the log between the initial request and the conditional poll. The validation now polls until the log stabilizes or a 304 Not Modified is received to avoid false positives caused by background activity.

Author: johnsimons

src/ServiceControl.AcceptanceTests/EventLogs/When_the_event_log_is_polled_with_an_etag.cs

@@ -52,12 +52,9 @@ await Define<ScenarioContext>()
                         return false;
                     }
 
-                    var current = await Poll(currentEtag);
+                    var current = await PollUntilTheLogGoesQuiet(currentEtag);
 
-                    // The endpoint keeps writing startup events, so a 200 here means either the
-                    // validator went stale between the two requests, or the server ignored
-                    // If-None-Match. Only the second is a failure, and the ETag tells them apart.
-                    if (current.StatusCode == HttpStatusCode.OK && ReadEtag(current) != currentEtag)
+                    if (current == null)
                     {
                         return false;
                     }
@@ -96,6 +93,35 @@ await Define<ScenarioContext>()
         static string ReadEtag(HttpResponseMessage response) =>
             response.Headers.TryGetValues("ETag", out var values) ? values.FirstOrDefault() : null;
 
+        // The endpoint keeps writing startup events, so a 200 carrying a *different* ETag only
+        // means the log moved on between the two requests, and says nothing about conditional
+        // GET support. Poll again with the ETag that response handed back, until either the log
+        // stops changing (304, or a 200 repeating the validator we sent, which is the real
+        // failure) or the attempts run out and the caller retries from a fresh read.
+        async Task<HttpResponseMessage> PollUntilTheLogGoesQuiet(string etag)
+        {
+            for (var attempt = 0; attempt < MaxPollAttempts; attempt++)
+            {
+                var response = await Poll(etag);
+
+                if (response.StatusCode != HttpStatusCode.OK)
+                {
+                    return response;
+                }
+
+                var newEtag = ReadEtag(response);
+
+                if (string.IsNullOrEmpty(newEtag) || newEtag == etag)
+                {
+                    return response;
+                }
+
+                etag = newEtag;
+            }
+
+            return null;
+        }
+
         Task<HttpResponseMessage> Poll(string ifNoneMatch)
         {
             var request = new HttpRequestMessage(HttpMethod.Get, "/api/eventlogitems/");
@@ -104,6 +130,8 @@ Task<HttpResponseMessage> Poll(string ifNoneMatch)
             return HttpClient.SendAsync(request);
         }
 
+        const int MaxPollAttempts = 5;
+
         public class StartingEndpoint : EndpointConfigurationBuilder
         {
             public StartingEndpoint() =>