retry broadcast fetch without etag

something in the http implementation of http://liveskak.dk
is very broken. Requests containing an etag can start producing
responses with error code 400, seemingly randomly, even if the
exact same request has succeeded seconds before.

when it happens, we try again without etag.

modules/relay/src/main/HttpClient.scala

@@ -42,11 +42,21 @@ private final class HttpClient(
       CanProxy
   ): Fu[(Option[Body], Option[Etag])] =
     val req = etag.foldLeft(toRequest(url))((req, etag) => req.addHttpHeaders("If-None-Match" -> etag))
-    fetchResponse(req).map: res =>
-      val newEtag = extractEtagValue(res)
-      if res.status == 304
-      then none                         -> newEtag.orElse(etag)
-      else decodeResponseBody(res).some -> newEtag
+    fetchResponse(req)
+      .map: res =>
+        val newEtag = extractEtagValue(res)
+        if res.status == 304
+        then none                         -> newEtag.orElse(etag)
+        else decodeResponseBody(res).some -> newEtag
+      .recoverWith:
+        case Status(400, _) if etag.isDefined =>
+          val prevEtag = etag.get // terrible, I wish it could be extracted from the match above
+          fetchBodyAndEtag(url, none)
+            .addEffects: res =>
+              val success = if res.isSuccess then "succeeded" else "failed"
+              logger.info(s"Retrying $url without etag $prevEtag -> $success")
+            .map: (body, etag) =>
+              (body, etag.filter(_ != prevEtag))
 
   // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag#w
   private def extractEtagValue(res: StandaloneWSResponse): Option[Etag] =