[jira] [Assigned] (MESOS-5293) Endpoint handlers for master and agent are implemented surprisingly differently.

["Abhishek Dasgupta (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/MESOS-5293?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Abhishek Dasgupta reassigned MESOS-5293:
----------------------------------------

    Assignee: Abhishek Dasgupta

> Endpoint handlers for master and agent are 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
>            Assignee: Abhishek Dasgupta
>              Labels: tech-debt
>
> The way endpoints are routed is inconsistent between master and agent code which makes adding or extending 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}} by value 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 {{http}} 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 a pointer to the master 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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