Connector errors after ColdFusion 2018 Update 5: Bad gateway, Tomcat is down or refused connection

An update.

One part of our application runs on CFCs and CFM pages. The other is a REST API.

With the assistance of the ColdFusion Team at Adobe, we have discovered the following:

The 502 Bad Gateway occurs only in the REST API. We narrowed the root-cause down to the onRESTRequest event-handler in Application.cfc. The fact that OnRESTRequest is not single-threaded is the root-cause.

It turns out that, in our application, multiple REST requests sharing the same URI do occasionally have access to onRESTRequest. Some of the code in onRESTRequest involves flushing a servlet response outputstream to the client, as follows:

var out = response.getOutputStream();     
out.write(toBinary(toBase64(content)));       
out.flush();       
out.close();

As onRESTRequest is not single-threaded, two such responses sometimes create a race-condition. One of them is flushed to the client, as expected. But parts of the second response (for example, headers) may not be flushed or may be flushed in the wrong order. The result is a 502 Bad Gateway error.

Workaround:

Use a ColdFusion flush after the servlet flush.

var out = response.getOutputStream();     
out.write(toBinary(toBase64(content)));       
out.flush();       
out.close();

// Ensure ColdFusion flushes every response
cfflush();

BKBK, you say this is since applying update 5. Did you first try to revert to whatever update you were on before, to prove is this is indeed "the cause"? And if you may not want to do that, have you tried using whatever isapi_redirect.dll you had from BEFORE the update 5? Sadly, the CF wsconfig "update" feature does not save a backup (I have a feature request for that).

And in doing the manual update to the dll, did you restart IIS after that? 

Was it failing the same way after update 5 but before the new dll? Had you tried to upate the connector AS PER THE UPDATED CONNECTOR THAT CAME with update 5? Or did you perhaps skip that step? What update were you on before?

Finally, it's possible that your problem is NOT about the update at all. I assume the site was working fine before the update, right? Did you do ANYTHING else after applying the CF update? Did you update the connector (with the wsconfig, or not)? Did you change anything in IIS? Did you perhaps "remove" and re-add a connector, instead of using the "update" feature of the wsconfig tool?

Answers to any of the above could provide a clue as to what is amiss. I appreciate folks often just wish to "get the solution", but as you are finding this is a more challenging problems for you, and it may take a bit more digging to resolve. (I will assume you are not interested in a consulting session to try to address this, if the above does not help. But I will say to other readers who may find this that I can often help solve such things, and perhaps in just minutes when I can actually see what's amiss on your server.)

Hope something there helps.