wally_psbt_to_bytes() difficult to use

[rustyrussell]

Without any way of knowing the length to allocate, how do we call wally_psbt_to_bytes?

For the moment, we allocate 1k and loop, doubling each time.
But worse, it returns WALLY_EINVAL for reasons OTHER than "you didn't give enough room".

There are two ways of fixing this:

1. Expose psbt_get_length(). This is inefficient.
2. Change the semantics of bytes_written to "bytes_needed", and set it to the number of bytes you want. Then return WALLY_ENOMEM to distinguish from unrelated errors.

The second allows a more optimal implementation in future.

[achow101]

Isn't psbt_get_length() already exposed?

Change the semantics of bytes_written to "bytes_needed", and set it to the number of bytes you want. Then return WALLY_ENOMEM to distinguish from unrelated errors.

I don't quite understand this. Do you mean that wally_psbt_to_bytes should compute how many bytes are needed and return that if the buffer is big enough (i.e. output the result of psbt_get_length)?