[bug]: lnd sends "internal error" to peer, when peer tries to update channel during shutdown

[DerEwige]

Background

When the shutdown of lnd is initiated.
LND starts to shudown all internal systems.
If a peer tries to add an HTLC to this channel while LND is shutting down, LND sends back an error to the peer.
On LND you will see this error in the log:
failing link: unable to update commitment: link shutting down with error: internal error
The peer will only get this error: internal error

If the peer follows the specs this will cause a force close.

Currently the only affected implementation seems to be eclair.
LND simply ignores these kind of errors.
CLN currently has a workaround in place to treat lnd "internal error" as a warning

Your environment

LND Version 0.16.1-beta.rc2
eclair version 0.8.0

Steps to reproduce

Shutdown LND
While lnd is shutting down: try to add as many HTLCs to the channel from another peer

2023-04-22 12:35:41.685 [INF] HSWC: ChannelLink(abc425691bacf13aad6749416c17302ebfe2eb8a45bffa05c07c0a78c41d543e:0): stopping
2023-04-22 12:35:41.866 [ERR] HSWC: ChannelLink(abc425691bacf13aad6749416c17302ebfe2eb8a45bffa05c07c0a78c41d543e:0): failing link: unable to update commitment: link shutting down with error: internal error
2023-04-22 12:35:41.866 [ERR] HSWC: ChannelLink(abc425691bacf13aad6749416c17302ebfe2eb8a45bffa05c07c0a78c41d543e:0): link failed, exiting htlcManager
2023-04-22 12:35:41.866 [INF] HSWC: ChannelLink(abc425691bacf13aad6749416c17302ebfe2eb8a45bffa05c07c0a78c41d543e:0): exited

lnd/htlcswitch/link.go

Line 2232 in c2851c2

"unable to update commitment: %v", err)

Expected behaviour

I expect one of two behavious:
Option 1.) LND sends a Warning instead of Error in this scenario
Option 2.) LND disconnects the peer and does not allow new connections, before shuting down the link

Actual behaviour

LND just starts to shutdown the channels while the peers are still connected and try to use them.

[Roasbeef]

Will be fixed by #6760

[Crypt-iQ]

Not fixed by #6760 since this isn't related to coop close. It's because we send an Error in the link and other impl's force close on Error

[DerEwige]

I think this might be similar/related to:
#6522