eth: extract check for tracing transaction in block file (#19107)

Simplifies the transaction presense check to use a function to
determine if the transaction is present in the block provided
to trace, which originally had a redundant parenthesis and used
a `exist` flag to dictate control flow.

eth/api_tracer.go

@@ -526,13 +526,7 @@ func (api *PrivateDebugAPI) traceBlock(ctx context.Context, block *types.Block,
 func (api *PrivateDebugAPI) standardTraceBlockToFile(ctx context.Context, block *types.Block, config *StdTraceConfig) ([]string, error) {
 	// If we're tracing a single transaction, make sure it's present
 	if config != nil && config.TxHash != (common.Hash{}) {
-		var exists bool
+		if !containsTx(block, config.TxHash) {
-		for _, tx := range block.Transactions() {
-			if exists = (tx.Hash() == config.TxHash); exists {
-				break
-			}
-		}
-		if !exists {
 			return nil, fmt.Errorf("transaction %#x not found in block", config.TxHash)
 		}
 	}
@@ -625,6 +619,17 @@ func (api *PrivateDebugAPI) standardTraceBlockToFile(ctx context.Context, block
 	return dumps, nil
 }
 
+// containsTx reports whether the transaction with a certain hash
+// is contained within the specified block.
+func containsTx(block *types.Block, hash common.Hash) bool {
+	for _, tx := range block.Transactions() {
+		if tx.Hash() == hash {
+			return true
+		}
+	}
+	return false
+}
+
 // computeStateDB retrieves the state database associated with a certain block.
 // If no state is locally available for the given block, a number of blocks are
 // attempted to be reexecuted to generate the desired state.