Re: Blocks runtime in the kernel

On 3/17/23 11:08 AM, Brooks Davis wrote:
> On Thu, Mar 16, 2023 at 10:06:11AM -0400, Justin Hibbits wrote:
>> Most probably know I've been working on the IfAPI conversion of all
>> network drivers in order to hide the contents of `struct ifnet`.  I'm
>> pretty much done with the development, and it's all in review.
>> However, there's one bit that I've thought is very clunky since I added
>> it, the if_foreach() iterator function, which iterates over all
>> interfaces in the current VNET, and calls a callback to operate on each
>> interface.  I've noticed that oftentimes I end up with a 2 line
>> callback, which just calls if_foreach_addr_type(), so I end up with
>> just trivial callback functions, which seems like a waste.
>>
>> All that backstory to say, would it be beneficial to anyone else to
>> add a (very basic) blocks runtime to the kernel for doing things like
>> this?  The rough change to the IfAPI becomes:
>>
>> int if_foreach_b(int (^)(if_t));
>>
>> __block int foo = 0;
>>
>> if_foreach_b(^(if_t ifp) {
>>    if (if_getlinkstate(ifp) == LINK_STATE_UP)
>>      foo++;
>> });
>>
>> The same could be done for other *_foreach KPIs as well, if this proves
>> out.  I think I could have something working in the next several days.
>>
>> The only technical snag I see with this would be other compilers.  I'm
>> not sure if GCC still supports blocks, it did at one point.
> 
> I think two things make this a non-starter.  First, GCC doesn't support
> this feature and I don't think we want to lose that for the reasons
> Warner outlines elsewhere.  Second, it seems moderately likely that C2Y
> will include lambdas in some form which fills the same niche.  This
> will further reduce the likelihood of blocks support being widespread
> (already extremely unlikely).  At this point I think we just need to
> live with the clunky syntax. :(

Alternatively one could use C++.  I think that would be an easier sell
than Blocks TBH.  It's not hard to write the little bits you would need
to let you use a ranged-for for this (which I find more readable than the
lambda approach anyway).

If you don't need to perform cleanup actions when terminating an iteration
you could also provide helper functions that let you implement a
IF_FOREACH() wrapper macro that would function similar to TAILQ_FOREACH().
You would just need 'if_first()' and 'if_next()' helpers.

-- 
John Baldwin