[Ur] callbacks from C FFI (again)

[Sergey]

On 05.01.2014 22:10, Adam Chlipala wrote:
>> I think it is better to direct efforts into improvement of the FFI
>> interface and urweb API because we will need them anyway. If I only
>> had methods for
>>
>> 1) clone the context to use it with a new thread inside an FFI
>> 2) call the url handler (I already know that `uw_begin' or
>> `uw_get_app(ctx)->handle(ctx, uri)' may help me here)
>
> I think I'm basically on board with this proposal, but there is it 
> least one potential complication that seems worth bringing up.
>
> Any Ur/Web code may encounter a benign exception requiring transaction 
> restart.  The canonical example is a serialization failure from 
> optimistic SQL concurrency.  Your suggestions above don't make it 
> clear how transactions should be handled in general, including where 
> to jump back to when a transaction needs restarting.

I assumed the strategy to be an FFI-specific. As soon as FFI API 
provides the basic combinators (uw_begin, uw_commit, uw_rollback, ...), 
the programmer is able to implement any algorithm they want. The 
`periodic_loop()' from request.c demonstrates one possible algorithm, 
probably, the most popular one.
>
> For simplicity's sake, what do you think of implementing the following 
> two functions instead?
> 1) clone context but don't start a transaction for it (note that this 
> also opens a database connection, checks the schema, and creates 
> prepared statements!)
> 2) given a context and a URI, run the associated Ur/Web code in that 
> context in a new transaction associated only with this operation
Fine! I'll draft a patch and publish it.
> My main recommendation, though, is to do like this for a record that 
> needs to cross the Ur-C boundary.  This would be code in an FFI .urs 
> file:
>     type myrecord
>     val myrecord : int -> string -> myrecord
>     val getInt : myrecord -> int
>     val getString : myrecord -> string
>
OK. I already use this method, the only thing that bothers me is the 
amount of boilerplate code. But It is not that important at the moment.

Regards,
Sergey