Re: [MirageOS-devel] contributing best practices?

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

On 26 Mar 2014, at 09:34, Dave Scott <Dave.Scott@xxxxxxxxxx> wrote:

> 
> 
>> On Mar 26, 2014, at 9:02 AM, "Richard Mortier" 
>> <Richard.Mortier@xxxxxxxxxxxxxxxx> wrote:
>> 
>> 
>>> On 25 Mar 2014, at 22:49, Anil Madhavapeddy <anil@xxxxxxxxxx> wrote:
>>> 
>>> One quick trick that often helps in these sorts of situations is to wrap 
>>> the offending code in a try/with and print the backtrace:
>>> 
>>> try
>>> the_bad_code
>>> with exn ->
>>> Printf.printf "Exn: %s\n%!" (Printexc.to_string exn);
>>> Printexc.print_backtrace stdout; 
>>> raise exn
>> 
>> didn't i add something like this to the pre-refactoring mirage at some 
>> point? did it get lost, or is it not possible to have now after all the 
>> refactoring?  seems like dropping this to console before bailing out would 
>> be a useful default behaviour...  should i add to the mirage cli tool maybe 
>> so that main.ml is generated appropriately?
>> 
> 
> Not sure about your change specifically, but my usual problem with a global 
> try/with is that Lwt threads get in the way-- you end up with a boring stack 
> with Lwt_main.run (ie the scheduler) on it and a frame for the current thunk 
> you're running, without much clue as to how you got there. I'm not sure what 
> the best fix for this would be -- possibly copying some ideas from core async?

The Async way is to maintain a monitor (very similar to an Erlang process 
monitor) that keeps an eye on a group of threads.  The Async scheduler is thus 
responsible for keeping track of backtraces.

We're pretty close to being able to use Async in Mirage, but not quite yet.  
The critical path is:

- Core was split into Core_kernel and Core_unix, so we need to add the (very 
small) set of C stubs to support Core_kernel
- Async_kernel now exists, so we need to link that in and test it under Xen
- Async_unix needs to be rewritten entirely for a Unix-free world, just as we 
did for Lwt.  This is probably a week of expert work to get a prototype 
running, more to upstream.

To learn more about monitors:
https://realworldocaml.org/v1/en/html/concurrent-programming-with-async.html#exception-handling

-anil
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