Unexpected -maxtxfee behavior

[Celean]

The documentation for -maxtxfee seems to imply that it will abort a transaction if the transaction's fee is larger than the specified amount. The actual behavior is to just use a fee equal to maxtxfee when the calculated fee is larger.

-maxtxfee=
Maximum total fees (in BTC) to use in a single wallet transaction or raw
transaction; setting this too low may abort large transactions
(default: 0.10)

In a specific case I observed, a large transaction created with sendtransaction and paytxfee 0.0012 ended up being broadcast with a fee of 15 sat/byte rather than aborted, which seems unintuitive.

However, looking at https://github.com/bitcoin/bitcoin/blob/master/src/wallet/wallet.cpp#L2747 in CWallet::GetMinimumFee this seems intended, and as far as I can tell, the only situation where a transaction will be aborted is if if the fee is below minrelaytxfee, rather than paytxfee.

Is this really the intended behavior? I feel like it would make far more sense that, if paytxfee is set explicitly, the transaction should be aborted if maxtxfee causes the calculated fee to drop below that value.

If not, I would argue that at the very least, the documentation for this setting should refer to the minrelaytxfee threshold specifically.

[jnewbery]

I believe we won't broadcast any transaction with fees > maxtxfee, because they won't be admitted to our mempool (all calls to AcceptToMemoryPool() in the wallet use maxTxFee, which gets set from the -maxtxfee argument here:

bitcoin/src/wallet/wallet.cpp

Line 3918 in 8c28670

maxTxFee = nMaxFee;

).

I can't comment on the specific case you observed, since you haven't included details about the total fee attached to the transaction. Note that -mxtxfee is limiting the total fee, not the fee rate. paytxfee, on the other hand, sets the feerate in BTC per kB.

[maflcko]

Indeed, there is a belt and suspender check in ATMP that rejects any transactions with fees larger than nAbsurdFee (==maxTxFee for the wallet). Additionally, it is wanted that the wallet does not create transactions that would be rejected by the mempool, so capping the total fee to maxTxFee makes sense. I'd suggest to update the documentation for -maxtxfee.

…

On Fri, Apr 7, 2017 at 2:51 PM, John Newbery ***@***.***> wrote: I believe we *won't* broadcast any transaction with fees > maxtxfee, because they won't be admitted to our mempool (all calls to AcceptToMemoryPool() in the wallet use maxTxFee, which gets set from the -maxtxfee argument here: https://github.com/bitcoin/bitcoin/blob/ 8c28670/src/wallet/wallet.cpp#L3918). I can't comment on the specific case you observed, since you haven't included details about the total fee attached to the transaction. Note that -mxtxfee is limiting the *total* fee, not the fee rate. paytxfee, on the other hand, sets the feerate in BTC per kB. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#10122 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AGGmvwW6Cp_JDp8ZpnJr5Y4dT8UCi8y3ks5rtjE6gaJpZM4Mufxp> .

[Celean]

Capping transactions to maxtxfee is all well and good, but the unexpected part was the transaction being broadcast with a very low (capped) fee despite the "setting this too low may abort large transactions" wording. I believe the transaction in question was in the ~60 KB range and hit the 0.01 limit configured for that wallet, leaving the fee at ~15 sat/byte.

My suggestion is that it would make sense if a transaction is aborted (i.e. not broadcast) if maxtxfee ends up capping the fee to less than paytxfee, as that could indicate a problem. I assume a setting like this is intended as a safeguard against fat-fingered or erroneously calculated fees, but right now with the default setting, it would just limit the fee to ~$120 at today's rate.

[Leviathn]

Is this still the case in .15 @Celean? I think the transaction fee logic and coin-selection upgrades should have resolved this..

[maflcko]

Yes, this is still the case. I'd assume you can read the FeeReason and abort if it is set to MAXTXFEE?

bitcoin/src/wallet/fees.cpp

Line 28 in cead84b

if (feeCalc) feeCalc->reason = FeeReason::MAXTXFEE;

[maflcko]

Some recent txs on mainnet that might be affected by this bug:

• https://blockstream.info/tx/e55ffe698b977690086adcbb19470b95d82a226d8797d7c5db4274869642686b
• https://blockstream.info/tx/69614d6ab9f44113b8451d3bb15e89890682f61b160103a787e0b6e0c318ec76

[Sjors]

Fixed in #16257

[maflcko]

This will be fixed in 0.18.1