fix: критични бъгове в storage, MVCC, auth, Raft, query executor

Поправени проблеми:
- MVCC: aborted транзакции вече не стават видими (добавено abortedTxns множество)
- LSM-Tree: поправена загуба на данни при immutable memtable overwrite
- LSM-Tree: поправена SSTable search order (сортиране по id вместо minKey)
- WAL: sync()/close() вече извикват posix.fsync()
- Auth: simpleHash (djb2) заменен с HMAC-SHA256
- Recovery: summary() вече не мутира базата данни
- Raft: поправена majority calculation за четен брой нодове
- Query: EXISTS subqueries вече изпълняват подзаявката
- Nimble: поправен build (-d:ssl) и bench task

Добавен BUG_AUDIT.md с пълен доклад и план за подобрения.

292 теста, 0 failure-а.

src/barabadb/core/mvcc.nim

@@ -43,6 +43,8 @@ type
     nextLsn: uint64
     activeTxns: Table[TxnId, Transaction]
     committedTxns: seq[TxnId]
+    committedTxnsSet: HashSet[TxnId]
+    abortedTxns: HashSet[TxnId]
     globalVersions*: Table[string, seq[VersionedRecord]]
     txnTimeoutMs: int64
     deadlockDetector*: DeadlockDetector
@@ -59,6 +61,8 @@ proc newTxnManager*(txnTimeoutMs: int64 = 30000): TxnManager =
   result.nextLsn = 1
   result.activeTxns = initTable[TxnId, Transaction]()
   result.committedTxns = @[]
+  result.committedTxnsSet = initHashSet[TxnId]()
+  result.abortedTxns = initHashSet[TxnId]()
   result.globalVersions = initTable[string, seq[VersionedRecord]]()
   result.txnTimeoutMs = txnTimeoutMs
   result.deadlockDetector = newDeadlockDetector()
@@ -112,8 +116,13 @@ proc isVisible(tm: TxnManager, txn: Transaction, version: VersionedRecord): bool
     return false
   if creator in txn.snapshotTxns:
     return false
+  if creator in tm.abortedTxns:
+    return false
   if creator in tm.activeTxns and tm.activeTxns[creator].state != tsCommitted:
     return false
+  # If creator is not active and not committed, it must be aborted
+  if creator notin tm.activeTxns and creator notin tm.committedTxnsSet:
+    return false
 
   # Deleter must not be a visible committed txn
   if deleter != TxnId(0):
@@ -246,6 +255,7 @@ proc commit*(tm: TxnManager, txn: Transaction): bool =
 
   txn.state = tsCommitted
   tm.committedTxns.add(txn.id)
+  tm.committedTxnsSet.incl(txn.id)
   tm.activeTxns.del(txn.id)
   tm.deadlockDetector.removeTxn(uint64(txn.id))
   release(tm.lock)
@@ -258,6 +268,7 @@ proc abortTxn*(tm: TxnManager, txn: Transaction): bool =
     return false
   txn.state = tsAborted
   tm.activeTxns.del(txn.id)
+  tm.abortedTxns.incl(txn.id)
   tm.deadlockDetector.removeTxn(uint64(txn.id))
   release(tm.lock)
   return true

src/barabadb/core/raft.nim

@@ -290,7 +290,7 @@ proc handleAppendReply*(node: RaftNode, peerId: string, reply: RaftMessage) =
       matchIndices.add(idx)
     matchIndices.sort()
 
-    let medianIdx = matchIndices[matchIndices.len div 2]
+    let medianIdx = matchIndices[(matchIndices.len - 1) div 2]
     if medianIdx > node.commitIndex:
       if medianIdx <= node.lastLogIndex and
          node.log[medianIdx - 1].term == node.currentTerm: