Return local channel alias in payment failures

[t-bast]

When using option_scid_alias, we must be careful not to expose the real scid to remote payers. We never generate a channel_update with the real scid so this isn't a risk, but we're incorrectly returning the remote alias instead of the returning the local one (which should match the invoice).

[pm47]

Good catch!

I think I would have addressed this differently:

• always generate our channel_update with our local alias or real scid depending on channel params (this is the fix to the present bug)
• re-sign the channel_update with remote alias in the onTransition within the if (sendToPeer) {...} so we make it clear that using remote alias is the exception not the rule

[t-bast]

I think I would have addressed this differently

That makes sense, it feels more consistent this way, I'll try it out.

[t-bast]

I've updated the PR to:

• always use either our local alias or the real scid in our channel updates
• when directly sending to our peer, override it with their remote alias when necessary
• when returning onion failures, use the incoming scid when applicable

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.70%. Comparing base (8af14ac) to head (8687bfa).
⚠️ Report is 632 commits behind head on master.
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2323      +/-   ##
==========================================
- Coverage   84.71%   84.70%   -0.02%     
==========================================
  Files         194      194              
  Lines       14633    14641       +8     
  Branches      612      589      -23     
==========================================
+ Hits        12397    12402       +5     
- Misses       2236     2239       +3     

Files with missing lines	Coverage Δ
...c/main/scala/fr/acinq/eclair/channel/Helpers.scala	95.04% <100.00%> (+<0.01%)	⬆️
...in/scala/fr/acinq/eclair/channel/fsm/Channel.scala	87.66% <100.00%> (-0.12%)	⬇️
...q/eclair/channel/fsm/ChannelOpenSingleFunder.scala	91.37% <100.00%> (ø)
.../scala/fr/acinq/eclair/payment/PaymentEvents.scala	97.61% <100.00%> (+0.05%)	⬆️
...a/fr/acinq/eclair/payment/relay/ChannelRelay.scala	98.09% <100.00%> (+0.03%)	⬆️
...fr/acinq/eclair/payment/relay/ChannelRelayer.scala	100.00% <100.00%> (ø)
.../scala/fr/acinq/eclair/payment/relay/Relayer.scala	90.90% <100.00%> (ø)
...n/scala/fr/acinq/eclair/router/Announcements.scala	100.00% <100.00%> (ø)

... and 7 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[pm47]

The router is left unchanged despite us using localAlias instead of remoteAlias for our own channel_update, because we resolve based on the channelId or the realScid:
https://github.com/ACINQ/eclair/blob/master/eclair-core/src/main/scala/fr/acinq/eclair/router/Validation.scala#L300