Prior to coin selection we need to indicate that the issuances will take
extra space, otherwise we may fail to select enough coins to cover our
fees, triggering the new "fee needed exceeds fees available" assertion.
The Elements 22 blinding logic has an edge case where when we drop change,
leaving only a single blinded output, we recompute a bunch of blinding
data to handle the potential for us to have 0 inputs and 1 output to blind.
(BlindTransaction will fail in this case because it cannot make the
transaction balance with only one output to mess with.)
In this recomputation, we dropped more data than we meant to, causing us
to incorrectly blind an output.
First, this reverts commit ca2d72ae8b to reinstate
an assertion that was added in Bitcoin #22686. It did not compile because our
`change_and_fee` variable is a map rather than number; I changed it to use
`map_change_and_fee.at(policyAsset)` to match the equivalent change 2 lines down
from a5d97b363b (merge of Bitcoin #22008).
Then fix the following bugs:
1. Change the new test in rpc_fundrawtransaction.py to bump the -maxtxfee value,
which we'd otherwise exceed, failing the test and masking actual failures.
(This was just caused by the extreme fee settings of the test combined with
Elements' large transactions.)
2. Change the fee-output size estimation for `tx_noinputs_size` to be 46 rather
than 44 bytes; we forgot that even null surjection/rangeproofs need a 0 byte
when output witnesses are present. This mistake triggered the new assertion.
3. Correct the logic in which change outputs are sometimes dropped even when
they are the only blinded output in a transaction with blinded inputs. This
would cause the new test to fail with `bad-txn-inputs-ne-outputs`; I'm very
surprised that no existing tests hit this.
(I have an existing comment block in this code where I "promise" that I had
a good reason for doing something mysterious related to blinding. I was not
able to reverse-engineer my intention here, though I think it is related to
this, but since I couldn't understand it I just left this block intact and
worked around it.)
4. This then triggered the assertion again since the coin selection code
assumes that sufficiently-small change will always be dropped. If we prevent
this drop we will have under-funded the transaction.
To fix this we add Yet Another Flag `may_need_blinded_dummy` in which we add
extra weight to `tx_noinputs_size` in the case that we're doing a blinded tx
but have no blind destinations. We turn this off after coin selection if it
turns out that we don't have any blinded inputs, though ofc at that point
much of the damage/inefficiency has already been done..
5. Fix some constants in other functional tests which assumed precise fee
calculations; these precise values changed because of fixes (2) and (4).
There is one new FIXME, which is that the "dummy change" value will now be a
zero-valued OP_RETURN but we still put a full-size rangeproof and surjection
proof on it. There is some plausible privacy benefit to this but not much,
and wasting 5000+ bytes rather than the ~65 needed for an exact-value proof
is not worth it. We will fix this in the future when we overhaul the wallet
blinding logic.
When the fee is not subtracted from the outputs, the amount that has
been reserved for the fee (change_and_fee - change_amount) must be
enough to cover the fee that is needed. It would be a bug to not do so,
so use an assert to make this obvious if such a situation were to occur.
Github-Pull: bitcoin/bitcoin#22686
Rebased-From: d9262324e8
Modifies the getnewblockhex json rpc call to accept an array of
commitments instead of a single commitment.
Backwards compatibility is maintained by first attempting to parse as a
string for a singular commitment.
