Introduce tracing output to a file for t8n

Co-authored-by: Paweł Bylica <pawel@ethereum.org>

test/t8n/t8n.cpp

@@ -31,6 +31,7 @@ int main(int argc, const char* argv[])
     fs::path output_body_file;
     std::optional<uint64_t> block_reward;
     uint64_t chain_id = 0;
+    bool tracing_enabled = false;
 
     try
     {
@@ -63,6 +64,8 @@ int main(int argc, const char* argv[])
                 chain_id = intx::from_string<uint64_t>(argv[i]);
             else if (arg == "--output.body" && ++i < argc)
                 output_body_file = argv[i];
+            else if (arg == "--trace")
+                tracing_enabled = true;
         }
 
         state::BlockInfo block;
@@ -95,6 +98,9 @@ int main(int argc, const char* argv[])
 
             evmc::VM vm{evmc_create_evmone(), {{"O", "0"}}};
 
+            if (tracing_enabled)
+                vm.set_option("trace", "0");
+
             std::vector<state::Log> txs_logs;
 
             if (j_txs.is_array())
@@ -107,8 +113,6 @@ int main(int argc, const char* argv[])
                     auto tx = test::from_json<state::Transaction>(j_txs[i]);
                     tx.chain_id = chain_id;
 
-                    auto res = state::transition(state, block, tx, rev, vm);
-
                     const auto computed_tx_hash = keccak256(rlp::encode(tx));
                     const auto computed_tx_hash_str = hex0x(computed_tx_hash);
 
@@ -123,6 +127,22 @@ int main(int argc, const char* argv[])
                                                    hex0x(loaded_tx_hash_opt.value()));
                     }
 
+                    std::ofstream trace_file_output;
+                    const auto orig_clog_buf = std::clog.rdbuf();
+                    if (tracing_enabled)
+                    {
+                        const auto output_filename =
+                            output_dir /
+                            ("trace-" + std::to_string(i) + "-" + computed_tx_hash_str + ".jsonl");
+
+                        // `trace` flag enables trace logging to std::clog.
+                        // Redirect std::clog to the output file.
+                        trace_file_output.open(output_filename);
+                        std::clog.rdbuf(trace_file_output.rdbuf());
+                    }
+
+                    auto res = state::transition(state, block, tx, rev, vm);
+
                     if (holds_alternative<std::error_code>(res))
                     {
                         const auto ec = std::get<std::error_code>(res);
@@ -156,6 +176,10 @@ int main(int argc, const char* argv[])
                         transactions.emplace_back(std::move(tx));
                         receipts.emplace_back(std::move(receipt));
                     }
+                    // Set original std::clog buffer
+                    // Have to do it. Otherwise, it's crashing
+                    if (tracing_enabled)
+                        std::clog.rdbuf(orig_clog_buf);
                 }
             }