Implement immediate flag for waitanyinvoice.

[ZmnSCPxj]

An alternative to #3449. Closes: #3449

This adds the immediate flag, equivalent to timeout=0 on #3449

[cdecker]

Very nice 👍

One thing that comes to mind is that most APIs that have a waitsomething call will have a timeout parameter, and sometimes they'll have a blocking=False argument (which is what your immediate flag compares to). I kind of like the timeout variant since it allows to semi-block a thread and do other work if the timeout expires.

Just as a discussion topic, would a timeout parameter, that may be 0 and return immediately, be a more flexible way to implement this? The downside obviously is that it'd require a timer and a timeout callback similar to the success callback.

[ZmnSCPxj]

Just as a discussion topic, would a timeout parameter, that may be 0 and return immediately, be a more flexible way to implement this? The downside obviously is that it'd require a timer and a timeout callback similar to the success callback.

I considered this as well --- wallet/invoices.c can certainly handle having the callback context tal-deleted under it --- but then I thought what would be the point of using anything other than 0 or UINT32_MAX for that.

Or maybe it is just better to have a consistent interface, and a good number of our commands do have timeout.

[ZmnSCPxj]

Changed to use a timeout instead of threading an immediate parameter through to wallet/invoices.c The wallet/invoices.c sub-module is perfectly capable of having its callback cancelled, so this is perfectly fine and thus immediate need not be passed in to the invoices sub-module.