Make stream state available for message field callbacks also during message size calculation

[reavertm]

This is more of a question rather than puill request.

I have simple message with arbitrarily big payload that I want to handle procedurally.
I would also like to use message field callbacks.

Unless I'm mistaken, I need to set option callback_datatype just to get message callbacks prototype generated even though I won't be using those pointers.

message Test {
  required bytes payload = 1 [(nanopb).callback_datatype = "void*"];
}

After implementing Test_callback everything works as expected except my payload size is not known inside the callback so I need to pass it as global variable at the moment.
ostream->state is only set on 2nd callback invocation (in encode mode).
Is it intentional, in ex. to tell user that it's "size calculation phase"? (so that one could just manually update bytes_written or do dummy pb_write(... NULL, size) instead of doing more sophisticated encoding)
If so, is there any way to pass user data than global variable?
One could also of course just encode message manually using encoding primitives functions, but I would prefer to use message field callbacks as much as possible (manual set callbacks are ..meh)

[PetteriAimonen]

Sizing streams are identified by stream->callback == NULL, e.g. in here.

So I agree that there seems to be no reason not to make the state available.

It would make sense to do the same in pb_get_encoded_size().
Ideally this pull request would also add a check to tests/encode_unittests, but if that seems complex, I can do it myself later.

[reavertm]

It seems pb_{i,o}stream_from_buffer() functions reuse stream state themselves.
I assume you would intend to keep it that way and not have dedicated additional let's say void* arg in stream structures, untouched by nanopb itself?

[PetteriAimonen]

@reavertm I'm not sure. I can see that having extra arg there would make sense in this case, but I'm not sure if it gets a bit strange to mix message state with stream state. E.g. pb_get_encoded_size() doesn't have a way to pass a stream.