Fix flaky 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, resulting in a 200 OK instead of a 304 Not Modified. The validation now accounts for this by checking if the ETag has changed when a 200 OK is received.

Author: johnsimons

src/ServiceControl.AcceptanceTests/EventLogs/When_the_event_log_is_polled_with_an_etag.cs

@@ -45,21 +45,25 @@ await Define<ScenarioContext>()
                         return false;
                     }
 
-                    // Raw header: an unquoted value fails EntityTagHeaderValue parsing, and this test
-                    // has to observe that rather than throw on it.
-                    if (!first.Headers.TryGetValues("ETag", out var values))
+                    var currentEtag = ReadEtag(first);
+
+                    if (string.IsNullOrEmpty(currentEtag))
                     {
                         return false;
                     }
 
-                    etag = values.FirstOrDefault();
+                    var current = await Poll(currentEtag);
 
-                    if (string.IsNullOrEmpty(etag))
+                    // 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)
                     {
                         return false;
                     }
 
-                    currentEtagStatus = (await Poll(etag)).StatusCode;
+                    etag = currentEtag;
+                    currentEtagStatus = current.StatusCode;
 
                     var unknown = await Poll("\"not-an-etag-this-instance-ever-issued\"");
                     unknownEtagStatus = unknown.StatusCode;
@@ -87,6 +91,11 @@ await Define<ScenarioContext>()
             }
         }
 
+        // Raw header: an unquoted value fails EntityTagHeaderValue parsing, and this test
+        // has to observe that rather than throw on it.
+        static string ReadEtag(HttpResponseMessage response) =>
+            response.Headers.TryGetValues("ETag", out var values) ? values.FirstOrDefault() : null;
+
         Task<HttpResponseMessage> Poll(string ifNoneMatch)
         {
             var request = new HttpRequestMessage(HttpMethod.Get, "/api/eventlogitems/");