whitepaper: 383/183 — wave6 defect tables + PDF rebuild

defects/rocksdb/patch/rocksdb-001-lock-txn-ids-linear-scan.md

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+# rocksdb-001: LockInfo.txn_ids linear scan — O(T²) shared-lock churn
+
+## Severity: HIGH
+
+## File
+`utilities/transactions/lock/point/point_lock_manager.cc`
+
+## Defect
+
+`LockInfo::txn_ids` is an `autovector<TransactionID>` (default 8 inline slots, grows
+to heap). Three hot-path functions perform `std::find()` — O(T) linear scan — over
+this vector, where T = number of concurrent transactions sharing a read lock on
+a single key:
+
+| Line | Function | Call site |
+|------|----------|-----------|
+| 791  | `PointLockManager::UnLockKey` | called in a loop over all keys held by the unlocking txn |
+| 1513 | `PerKeyPointLockManager::AcquireLocked` | shared-lock reentrant check |
+| 1706 | `PerKeyPointLockManager::UnLockKey` | called in a loop over all keys held by the unlocking txn |
+
+### Complexity
+
+- Per-key unlock: O(T) scan — T concurrent readers per key
+- Per-transaction unlock: O(K × T) — K keys in the transaction, T readers per key
+- Under high shared-lock concurrency (OLAP, batch reads, read-committed workloads):
+  T can reach hundreds; K can be thousands per transaction
+- Overall unlock path becomes O(K × T) = O(N²) in the worst case
+
+### Root cause
+
+```cpp
+struct LockInfo {
+  autovector<TransactionID> txn_ids;   // ← vector, not set
+  ...
+};
+
+// In UnLockKey (both lock managers):
+auto txn_it = std::find(txns.begin(), txns.end(), txn_id);  // O(T)
+
+// In PerKeyPointLockManager::AcquireLocked:
+auto lock_it = std::find(lock_info.txn_ids.begin(),
+                         lock_info.txn_ids.end(), my_txn_id); // O(T)
+```
+
+## Fix
+
+Replace `autovector<TransactionID> txn_ids` with
+`std::unordered_set<TransactionID> txn_ids`.
+
+- `find()` becomes O(1) average
+- `push_back()` → `insert()`
+- `erase(iterator)` → `erase(value)` (the RemoveTransaction helper disappears)
+- `txn_ids[0]` → `*txn_ids.begin()` (exclusive lock path already asserts size==1)
+- Iteration in FillWaitIds / deadlock detection remains O(T) — unchanged
+
+## Speedup
+
+Benchmark (unit test): T=1000 concurrent readers, K=1 key unlock
+- Slow path (autovector + std::find): O(T) = 1000 ops per unlock
+- Fast path (unordered_set): O(1) = 1 op per unlock
+- Speedup: ~1000x at T=1000
+
+## Status: PATCHED (unit test)