[webkit-dev] Protect promise capabilities from being vended again

[Xabier Rodríguez Calvar]

	Hello,

When you create a promise capability, there's nothing preventing you
from calling again @resolve and @reject on the capability to vend the
promise twice. In the case of the Streams implementation to create
capabilities are are already vended, we do it as { @promise: @Promise.@
resolve } or @reject and those capability objects don't have the
@resolve and @reject slots so calling them would obviously fail.

Vending a promise twice means probably an issue in the algorithm that
you are implementing and if it were properly designed that shouldn't
happen. Software and specs can have bugs though and we could end up
hitting those unexpected cases.

For that I think we could change the internal promise capabilities to
use the newly added @assert when they are vended twice and also add two
functions to create those vended capabilities in a single call.

Any comments?

-- 
Xabier Rodríguez Calvar
Software Engineer
IGALIA http://www.igalia.com

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