make some 'beginTransaction' usages safer

premix/debug.nim

@@ -19,8 +19,9 @@ proc executeBlock(blockEnv: JsonNode, memoryDB: TrieDatabaseRef, blockNumber: Ui
     header = chainDB.getBlockHeader(blockNumber)
     body   = chainDB.getBlockBody(header.blockHash)
 
+  let transaction = memoryDB.beginTransaction()
+  defer: transaction.dispose()
   let
-    transaction = memoryDB.beginTransaction()
     vmState = newBaseVMState(parent, chainDB)
     validationResult = processBlock(chainDB, parent, header, body, vmState)
 
@@ -29,9 +30,6 @@ proc executeBlock(blockEnv: JsonNode, memoryDB: TrieDatabaseRef, blockNumber: Ui
   else:
     info "block validation success", validationResult, blockNumber
 
-  # success or not dispose it
-  transaction.dispose()
-  
   dumpDebuggingMetaData(chainDB, header, body, vmState, false)
   let
     fileName = "debug" & $blockNumber & ".json"

premix/dumper.nim

@@ -16,8 +16,9 @@ proc dumpDebug(chainDB: BaseChainDB, blockNumber: Uint256) =
     captureTrieDB = trieDB captureDB
     captureChainDB = newBaseChainDB(captureTrieDB, false)
 
+  let transaction = memoryDB.beginTransaction()
+  defer: transaction.dispose()
   let
-    transaction = memoryDB.beginTransaction()
     parentNumber = blockNumber - 1
     parent = captureChainDB.getBlockHeader(parentNumber)
     header = captureChainDB.getBlockHeader(blockNumber)
@@ -28,8 +29,6 @@ proc dumpDebug(chainDB: BaseChainDB, blockNumber: Uint256) =
   captureChainDB.setHead(parent, true)
   let validationResult = processBlock(captureChainDB, parent, header, body, vmState)
 
-  # success or not dispose it
-  transaction.dispose()
   dumpDebuggingMetaData(captureChainDB, header, body, vmState, false)
 
 proc main() =

premix/hunter.nim

@@ -68,13 +68,13 @@ proc huntProblematicBlock(blockNumber: Uint256): ValidationResult =
 
   chainDB.setHead(parentBlock.header, true)
 
+  let transaction = memoryDB.beginTransaction()
+  defer: transaction.dispose()
   let
-    transaction = memoryDB.beginTransaction()
     vmState = newBaseVMState(parentBlock.header, chainDB)
     validationResult = processBlock(chainDB, parentBlock.header, thisBlock.header, thisBlock.body, vmState)
 
   if validationResult != ValidationResult.OK:
-    transaction.dispose()
     dumpDebuggingMetaData(chainDB, thisBlock.header, thisBlock.body, vmState, false)
 
   result = validationResult