graph: fix graph-cache issue

[ziggie1984]

The Problem (Before the Fix)

Initial Population:

• In graph_cache.go, the old AddChannel function would return early if both policies were disabled (before this fix)
• This meant the channel structure was never added to the graph cache
• However, the channel still existed in the database

When a Policy Update Arrives Later:

1. Builder.updateEdge() in builder.go:1137 is called
2. It calls HasChannelEdge() which checks the database (not the graph cache) - see kv_store.go:1265
3. Since the channel exists in the DB, exists=true is returned
4. The policy update proceeds and updates the database successfully
5. Then ChannelGraph.UpdateEdgePolicy() is called (graph.go:581)
6. It calls graphCache.UpdatePolicy() (graph.go:590)
7. But - UpdatePolicy() in graph_cache.go:193-206 checks if the channel exists in the graph cache:
   channel, ok := c.nodeChannels[nodeKey][policy.ChannelID]
   if !ok {
   log.Warnf("Channel=%v not found in graph cache", policy.ChannelID)
   return // ← Channel never gets added!
   }
8. The channel is not found, so it just logs a warning and returns
9. The policy update never makes it into the graph cache

Why AddEdge Won't Help:

You're also correct that Builder.addEdge() won't re-add it because:

• Lines 1048-1061 in builder.go check HasChannelEdge() which checks the DB
• If the channel exists in the DB, it returns early with ErrIgnored
• So AddChannelEdge() is never called, and the graph cache never gets the channel structure

The Fix (After the Commit)

The fix correctly:

1. Always adds the channel structure to the graph cache (lines 128-140)
2. Then checks if both policies are disabled and skips adding the policies (lines 142-153)
3. But the channel structure is already in the cache by this point
4. When a policy update arrives later, UpdatePolicy() finds the channel and successfully updates it

Fixes: #10342

[ziggie1984]

/gemini review

[gemini-code-assist]

Code Review

This pull request provides a well-reasoned fix for a subtle bug in the graph cache. The issue, where channels with initially disabled policies were not cached correctly, is resolved by ensuring the channel structure is always added. The changes in graph/db/graph_cache.go are clear and effective. My main feedback is to add a regression test to prevent this issue from resurfacing in the future. I've also included a minor formatting suggestion for the new release notes file.

[bhandras]

Looks good! Could you please cover this case with a small test? So we don't accidentally regress.

[bitromortac]

LGTM, nice catch ⚡

[bhandras]

LGTM, thank you! 🎉

[yyforyongyu]

Pending linter fix, otherwise can be merged!