Shield HTTP request handlers from async cancellations.

docs/pageserver-thread-mgmt.md

@@ -52,9 +52,7 @@ completion, or shield the rest of the code from surprise cancellations
 by spawning a separate task. The code that handles incoming HTTP
 requests, for example, spawns a separate task for each request,
 because Hyper will drop the request-handling Future if the HTTP
-connection is lost.  (FIXME: our HTTP handlers do not do that
-currently, but we should fix that. See [issue
-3478](https://github.com/neondatabase/neon/issues/3478)).
+connection is lost.
 
 
 #### How to cancel, then?

libs/utils/src/http/endpoint.rs

@@ -40,6 +40,12 @@ struct RequestId(String);
 ///
 /// This also handles errors, logging them and converting them to an HTTP error response.
 ///
+/// NB: If the client disconnects, Hyper will drop the Future, without polling it to
+/// completion. In other words, the handler must be async cancellation safe! request_span
+/// prints a warning to the log when that happens, so that you have some trace of it in
+/// the log.
+///
+///
 /// There could be other ways to implement similar functionality:
 ///
 /// * procmacros placed on top of all handler methods