fix: memiavl-nil-deref-during-state-sync

[blindchaser]

Summary

During state-sync the mempool reactor runs CheckTx (→ GetConsensusParams → RouterCommitKVStore.Has → Router.Read) before memiavl's LoadVersion opens the underlying *DB. With cs.db == nil this nil-derefs and crashes the node. Affects any node still backed by memiavl (default MemiavlOnly included), not just EVM-migration nodes.

This makes the memiavl read path safe during that pre-load window:

• memiavl/db.go: TreeByName returns nil on a nil receiver instead of locking db.mtx.
• memiavl/store.go: GetChildStoreByName returns a nil interface when unloaded; adds IsLoaded().
• migration/router_builder.go: memiavl closures gate on IsLoaded(). Reads return key-not-found (no error), so Has/Get no longer panic. Writes/iterators/proofs return an explicit error rather than corrupting state or faking a proof.

Scope / known limits:

• Only the read path is fully panic-free. Iterator/GetProof/Set still surface as a panic at RouterCommitKVStore (now with a clear "not loaded yet" message instead of a nil deref).
• cs.db is still accessed without synchronization vs. LoadVersion; this fixes the deterministic nil-deref, not a potential data race.
• Defensive fix at the storage layer; does not change whether the reactor dispatches CheckTx during state-sync.

Test plan

• memiavl/unloaded_store_test.go: IsLoaded transitions; nil-safe GetChildStoreByName/TreeByName before load.
• migration/router_builder_unloaded_test.go: reader returns not-found without error; writer/iterator/proof error before load; TestModuleRouter_Read_BeforeLoad_DoesNotPanic reproduces the original panic stack.

[cursor]

PR Summary

Medium Risk
Touches consensus-adjacent read paths during state-sync (params/CheckTx); behavior change is intentional (not-found vs panic) but must not mask real misconfiguration after load.

Overview
Prevents crashes when CheckTx or other reads hit memIAVL before LoadVersion finishes during state-sync (mempool can run while the snapshot is still applying).

memiavl now treats an unopened store as safe nil: DB.TreeByName and CommitStore.GetChildStoreByName no longer dereference a nil *DB, and CommitStore.IsLoaded() exposes whether the DB is open. The migration router uses that to return key-not-found on reads in the pre-load window, while writes, iteration, and proofs fail with explicit errors instead of silent corruption.

New tests lock in nil-receiver behavior and the router contract on an unloaded CommitStore.

^{Reviewed by Cursor Bugbot for commit 1c80d55. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	May 28, 2026, 9:06 PM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.65%. Comparing base (8567422) to head (1c80d55).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3518      +/-   ##
==========================================
- Coverage   59.03%   58.65%   -0.38%     
==========================================
  Files        2199     2159      -40     
  Lines      182207   178092    -4115     
==========================================
- Hits       107569   104468    -3101     
+ Misses      64975    64315     -660     
+ Partials     9663     9309     -354     

Flag	Coverage Δ
sei-db	70.41% <ø> (ø)
sei-db-state-db	?
sei-db-state-db-pr	77.79% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sei-db/state_db/sc/memiavl/db.go	66.76% <100.00%> (+0.09%)	⬆️
sei-db/state_db/sc/memiavl/store.go	92.80% <100.00%> (+0.21%)	⬆️
sei-db/state_db/sc/migration/router_builder.go	57.82% <100.00%> (+1.10%)	⬆️

... and 41 files with indirect coverage changes

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