routing: repopulate missing edges in graph for local channels

[JssDWt]

Change Description

Some channels missed corresponding edges in the graph database. This had a few implications:

• lncli getchaninfo would return "edge not found"
• forwarding htlcs over a channel with a missing edge would use the default channel policy
• lncli updatechanpolicy would fail to the edge missing

This commit adds a flag to LND startup: repopulate-missing-edges. When LND is started with this flag, any missing edges will be prefilled in the database with the default routing policy. This allows the user to later update the channel policy of these channels.

Note that 784dc8d partly addresses the issue for new channels, but old channels were still affected by this issue.

Fixes #7261

Steps to Test

I'm not entirely sure how to test this properly yet. Have a database where the channel edge is missing. lncli getchaninfo should fail. Then run LND with the repopulate-missing-edges flag. It should fix the issue.

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[JssDWt]

I'm not entirely sure how to test. Ideally I'd like to run an integration test where 2 nodes have a channel open. Then stop one of the nodes, delete the edge from the database and run the node again. getchaninfo should fail. Then run the node again with the new flag. getchaninfo should then succeed.

Not sure how to access the node's database in the test though, with all the different backends available.

[yyforyongyu]

Do we know the root cause of this issue, like why it's missing in the first place?

[yyforyongyu]

This looks more like a db migration for the affected nodes than a config to me.

[JssDWt]

If the problem was permanently fixed in #7613 that could be a better solution. What do you think @ellemouton? Was the 'edge not found' issue reliably fixed by that PR?

[JssDWt]

Do we know the root cause of this issue, like why it's missing in the first place?

It happens mostly on zero conf private channels. Also at least one public channel on our node is affected. I don't know what causes this. I assume it has to do with a peer disconnect during the initial channel setup process.

[JssDWt]

Another option is to add the missing edge in the default case here if the edge was not updated, but the channel exists, what do you think @yyforyongyu?

[JssDWt]

There's a few ways to go with this PR

1. Make it a database migration
   a. The question is whether a single database migration is enough. I'm not sure whether (actually I suspect that it isn't) the underlying issue is fixed in funding: use default forwarding policy if persisted values not found #7613. If it turns out to be not enough, I don't think this is the way to go.
2. Make a different PR that inserts a missing edge if it's not found when trying to updatechanpolicy to set a fee. That way you'll be able to fix missing edges manually by calling updatechanpolicy. This seems like a fine solution to me.
3. Leave it as-is. Seems like a fine solution to me too.

Please provide some guidance on what I should do.

[yyforyongyu]

Thanks for digging into this! Left some comments in the original issue, I still want to understand why it happened, to make sure we are not hiding the underlying problem by re-populating the policy. Meanwhile,

2. Make a different PR that inserts a missing edge if it's not found when trying to updatechanpolicy to set a fee. That way you'll be able to fix missing edges manually by calling updatechanpolicy. This seems like a fine solution to me.

I'd prefer this option, with a new param in the updatechanpolicy request, sth like RecreateEdgePolicy. This way we can mitigate this issue while staying aware of it.

[JssDWt]

@yyforyongyu I had made a PR for inserting the edge in updatexhanpolicy here #8805. I'll add the flag there. Can you do a review round on that, especially around the way the edge will eventually be persisted?

[lightninglabs-deploy]

@JssDWt, remember to re-request review from reviewers when ready

[yyforyongyu]

Think it's replaced by #8805, feel free to re-open if otherwise.