[jira] [Created] (MESOS-5293) Endpoint handlers for master and agent need to be implemented surprisingly differently

["Benjamin Bannier (JIRA)" <ji...@apache.org>]

Benjamin Bannier created MESOS-5293:
---------------------------------------

             Summary: Endpoint handlers for master and agent need to be implemented surprisingly differently
                 Key: MESOS-5293
                 URL: https://issues.apache.org/jira/browse/MESOS-5293
             Project: Mesos
          Issue Type: Bug
          Components: master, slave
            Reporter: Benjamin Bannier


The way endpoints in routed is inconsistent between master and agent code which makes added new handlers error prone.

In the master we route like this:
{code}
// Setup HTTP routes.
route("/api/v1/scheduler",
      DEFAULT_HTTP_FRAMEWORK_AUTHENTICATION_REALM,
      Http::SCHEDULER_HELP(),
      [this](const process::http::Request& request,
             const Option<string>& principal) {
        Http::log(request);
        return http.scheduler(request, principal);
      });
{code}

We capture a pointer to the current {{Master}} in the callback which allows us to
access master state from an endpoint handler. The handler is a (probably non-static)
member function of the master's member variable {{http}} which outlives the
callback.

Routing in the agent looks like this:
{code}
// Setup HTTP routes.
Http http = Http(this);

route("/api/v1/executor",
      Http::EXECUTOR_HELP(),
      [http](const process::http::Request& request) {
        Http::log(request);
        return http.executor(request);
      });
{code}

In contrast to the master code we here copy a {{Http}} into the callback. Since
the callback is currently treated like a value and might e.g., be freely copied
around we are only guaranteed that it lives long enough for the handler (here
{{Http::executor}}) to return. In particular, since endpoint handlers return a
{{Future<Response>}} there is no guarantee that the used {{http}} lives long
enough to be still valid once a conventional (e.g., non-static member)
continuation is executed.

Both models have their merit:

* capturing {{this}} simplifies reasoning about lifetimes
* capturing just a {{Http}} with very short lifetime minimizes interactions
  among (e.g., concurrent) invocations of endpoint handlers.

This great inconsistency comes with a cost though, as employing patterns
borrowed from master endpoint handlers in agent code will lead to potentially
subtle bugs where a developer assuming that {{http}} would outlive a handler's
execution might introduce code invoking member functions of already destructed
variables. This is especially likely in code employing multiple layers of
{{delay}} or {{defer}} for which compilers seem unable to detect lifetime
problems and emit diagnostics.

It would be great if we could to use  just one of the patterns to minimize confusion, e.g., the more straight-forward master pattern.



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