Eclair rejected its own closing fee

[TheBlueMatt]

I had a peer force-close after it rejected us sending the shutdown fee it explicitly said it would accept while running Eclair 0.7.0-a804905.

The messages on the wire were as follows:

2023-04-28 03:26:16.027 TRACE [lightning::ln::peer_handler:1381] Received message Shutdown(Shutdown { channel_id: [72, 81, 46, 104, 1, 1, 180, 227, 215, 81, 24, 133, 221, 50, 47, 63, 114, 246, 4, 195, 165, 83, 227, 240, 135, 149, 142, 55, 111, 38, 201, 244], scriptpubkey: Script(OP_0 OP_PUSHBYTES_20 c19fd4b5862382c607cdb7bf79b1637738b5010b) }) from 02ad0d9e1fb59a7f491b4785395b4a456075a436fa392fd501d9c9a67509b788c6
2023-04-28 03:26:16.028 TRACE [lightning::ln::peer_handler:1071] Enqueueing message Shutdown { channel_id: [72, 81, 46, 104, 1, 1, 180, 227, 215, 81, 24, 133, 221, 50, 47, 63, 114, 246, 4, 195, 165, 83, 227, 240, 135, 149, 142, 55, 111, 38, 201, 244], scriptpubkey: Script(OP_0 OP_PUSHBYTES_20 a20f92037eb14943f092631165c3d5f2deea152a) } to 02ad0d9e1fb59a7f491b4785395b4a456075a436fa392fd501d9c9a67509b788c6
2023-04-28 03:26:16.499 TRACE [lightning::ln::peer_handler:1381] Received message ClosingSigned(ClosingSigned { channel_id: [72, 81, 46, 104, 1, 1, 180, 227, 215, 81, 24, 133, 221, 50, 47, 63, 114, 246, 4, 195, 165, 83, 227, 240, 135, 149, 142, 55, 111, 38, 201, 244], fee_satoshis: 4556, signature: 30450221009cfe8e44525f2e1d3498c6820eadc96f47697fb7e5634dbdbaba376a982a7a9b022074106615ca12fa5bc3f597d98a0ba84e62cece1b4e1777234eac939c664a449d, fee_range: Some(ClosingSignedFeeRange { min_fee_satoshis: 373, max_fee_satoshis: 9112 }) }) from 02ad0d9e1fb59a7f491b4785395b4a456075a436fa392fd501d9c9a67509b788c6
2023-04-28 03:26:16.500 TRACE [lightning::ln::peer_handler:1071] Enqueueing message ClosingSigned { channel_id: [72, 81, 46, 104, 1, 1, 180, 227, 215, 81, 24, 133, 221, 50, 47, 63, 114, 246, 4, 195, 165, 83, 227, 240, 135, 149, 142, 55, 111, 38, 201, 244], fee_satoshis: 9112, signature: 3044022002ea4728c66dc08be0168fdb9dea2ae68974d4bc5ff217375341dd0f9fa6ed1002202d12180bf9d04f3fc4dac37c9b6566969c7544d0372fe6977a0004cf235f8d13, fee_range: Some(ClosingSignedFeeRange { min_fee_satoshis: 380, max_fee_satoshis: 2000000 }) } to 02ad0d9e1fb59a7f491b4785395b4a456075a436fa392fd501d9c9a67509b788c6
2023-04-28 03:26:16.979 TRACE [lightning::ln::peer_handler:1381] Received message Error(ErrorMessage { channel_id: [72, 81, 46, 104, 1, 1, 180, 227, 215, 81, 24, 133, 221, 50, 47, 63, 114, 246, 4, 195, 165, 83, 227, 240, 135, 149, 142, 55, 111, 38, 201, 244], data: "invalid close fee: fee_satoshis=9112 sat" }) from 02ad0d9e1fb59a7f491b4785395b4a456075a436fa392fd501d9c9a67509b788c6

I received a closing signed with a max fee of 9112, then responded with a closing signed with a fee of 9112, then eclair said that that was an invalid fee!

[pm47]

This is the cause:

eclair/eclair-core/src/main/scala/fr/acinq/eclair/channel/Helpers.scala

Lines 671 to 672 in 0d3de8f

	log.error(s"remote proposed a commit fee higher than the last commitment fee: remote closing fee=${remoteClosingFee.toLong} last commit fees=$lastCommitFeeSatoshi")
	Left(InvalidCloseFee(commitment.channelId, remoteClosingFee))

After some digging, this was due to an ancient BOLT2 requirement which stated that:

The sending node:

• MUST set fee_satoshis less than or equal to the
  base fee of the final commitment transaction, as calculated in BOLT #3.

But this requirement was dropped in lightning/bolts#847 authored by none other than... @t-bast 😁 Oops!