closing incorrectly using `fee_range`

[t-bast]

Our node received some warnings that seems to indicate a c-lightning node that wants to use lightning/bolts#847 improperly: Feerange 177sat-182sat below minimum acceptable 183sat

I observe the following flow of messages:

  c-lightning                      eclair
       |                              |
       |          shutdown            |
       |----------------------------->|
       |          shutdown            |
       |<-----------------------------|
       |   closing_signed (183 sat)   |
       |----------------------------->|
       |   closing_signed (176 sat)   |
       |<-----------------------------|
       |           warning            |
       |----------------------------->|

And then the c-lightning node disconnects.
The closing_signed messages do not contain a tlv stream with a fee_range, so it should use the legacy closing negotiation flow instead of disconnecting and refusing to make progress.

I don't know what version of c-lightning our peer is using, I can share their node_id if that's useful.

[cdecker]

Could this be related to this comment by @rustyrussell in #4599:

On the way I found that our fee estimation for closing txs was not actually using the weight of the closing tx, but the final commitment tx. This is v. different for anchor_outputs!

[rustyrussell]

I saw this too, closing with a c-lightning node. It's complaining because it considers 183 the minimum valid fee. It probably started at (say) 183-1000, then it sent 183, and when it gets 176 the possible range is now 175 - 182, which is below its minimum.

I think this is OK...