Don't generate a commitment if we cannot afford a holding cell feerate

[tankyleo]

While an update_fee is in the holding cell, it is possible for HTLCs to get added to the commitment transaction such that when we release the holding cell, we can no longer afford this new feerate.

In that case, we previously would drop the fee update, but still send a commitment (at the old feerate), which is a break of the specification.

We now stop generating this lonely commitment when the fee update gets dropped upon release from the holding cell.

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[codecov]

Codecov Report

Attention: Patch coverage is 97.84173% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.36%. Comparing base (6133a6c) to head (cc03f48).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/update_fee_tests.rs	97.77%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3828      +/-   ##
==========================================
+ Coverage   89.90%   90.36%   +0.46%     
==========================================
  Files         160      160              
  Lines      129272   133275    +4003     
  Branches   129272   133275    +4003     
==========================================
+ Hits       116218   120432    +4214     
+ Misses      10360    10142     -218     
- Partials     2694     2701       +7     

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[TheBlueMatt]

I'm confused, why does this "violate the specification"? Our peer has no idea we had this update_fee queued, we never sent it to them AFAIU. It makes sense to me that we should want to send the commitment even if we can't update the fee, we definitely shouldn't sit on HTLCs!

[tankyleo]

I'm confused, why does this "violate the specification"? Our peer has no idea we had this update_fee queued, we never sent it to them AFAIU. It makes sense to me that we should want to send the commitment even if we can't update the fee, we definitely shouldn't sit on HTLCs!

The problem is that currently the code makes it possible to send a commitment with zero updates, in the case we cannot afford the new feerate and there are no pending HTLC updates.