Lnd: Concurrency issue with `update_fee`

[pm47]

Background

I don't know the version of lnd so this issue may have already been fixed, just thought it would be interesting to keep a reference just in case.

Right after having sent an update_fee, lnd rejects a commit_sig that I sent, and by looking at the attached error message, I can see that lnd was incorrectly expecting that I use the new unacknowledged feerate.

  eclair              lnd
    |                  |
    |-------sig------->|
    |<------fee--------|
    |<------err--------|
    |                  |

[halseth]

So is eclair or lnd sending the fee here? Should the fee come before the sig in the drawing?

[pm47]

lnd sends the fee, and eclair sends the signature roughly at the same time. The exact order doesn't really matter, the point is that the fee hasn't been acknowledged by eclair, if that makes sense?

[halseth]

It looks like there's an issue with how we reset our expected fee after signing our own commitment, making lnd expect the next message coming from the remote to have the updated fee.

Was able to trigger in a test case, thanks for the report!