Improve "mempool min fee not met" error to include information on what the min fee is #11955

<!-- Describe the issue -->

When sending a transaction with a low fee, you can sometimes get an error saying "mempool min fee not met".

<!--- What behavior did you expect? -->

I would expect an error like this to include information about what the minimum fee is.

<!-- What version of Bitcoin Core are you using, where did you get it (website, self-compiled, etc)? -->

I'm using Electrum 3.0.2 in which you can see the raw error from this when you send a low fee transaction when its relayed to an electrum server.

@fresheneesz Could you explain as to where you saw this error? Some more info like logs would be really useful. Thanks!

There's not much more to explain than is in this issue and the electrum issue. I tried to increase the fee with a replace-by-fee transaction and it popped up a message box with an error that said "mempool min fee not met" with some additional information (none of which looked like it included the min fee). I can't repro right now and even if I could, electrum displayed the error in a way where the whole thing couldn't be seen fully or copied.

I get the same message.

Using Electrum 3.0.3 / Windows 10 64bit

The amount of system memory allocated for unconfirmed transactions is defined with the option maxmempool. maxmempool defaults to 300MB of RAM. Once the maxmempool-memory is fully used, the lowest-fee transactions are dropped from the nodes mempool and mempoolminfee is increased to reject lower-fee transactions.

The mempoolminfee thus is dependent on the current memory usage and the configured maxmempool. Connecting to another Electrum Server connected to a node with custom (higher) maxmempool could work. However the underlying node might accept the transaction but then failes to broadcast it to a peer/peers with a default maxmempool of 300MB. The transaction does not meet their min-fee expecations.

A core node could return it's current mempoolminfee in the error output, but that would not inform about the mempoolminfee of it's peers.

bitcoin-cli getmempoolinfo shows the current mempoolminfee.

Q: Why is the mempoolminfee not specifyed as fee-rate (sat/vbyte)?

@spot88 That's the one that got me...

A core node could return it's current mempoolminfee in the error output, but that would not inform about the mempoolminfee of it's peers.

I'd be perfectly happy with that. Nothing's perfect. But if your aim is to submit something at the minimum and you know that risks that the transaction might be evicted, right now I have pretty much no recourse. I have no feedback on what min fee might be accepted. At least if your peer tells you their min fee, you can submit back to them, or it should give you some idea of the min fee for other peers (who will then reject you with their own min fee message that you can then use to resubmit with that higher fee).

bitcoin-cli getmempoolinfo shows the current mempoolminfee.

So something like electrum could use this to determine a remote node's minimum fee?

@fresheneesz: Yes, electrum could use the call to get the minimum mempool fee or we could edit the error returned. But since that's already there in the logs, I think it would be better if you request the folks over at electrum. @0xB10C: You could open an issue to change it from btc/KB now :)

@Varunram Thanks for the info guys! I'll throw that back into the electrum issue.

Now that #12356 is merged, the error message includes the attempted fee and the minimum fee (both in satoshis), e.g.: mempool min fee not met, 166 < 411 (code 66). I think we can close this. /cc @laanwj

166 < 411 ? Units, people!

I agree that specifying units would be nice, but think enough information has been added to close the issue (all the internal low-level stuff uses satoshis.)