DEV Community

Morgan Ma
Morgan Ma

Posted on

Use a Free Server to Catch an AI-Written Race

#cpp #ai #testing #debugging

Some bugs exist only on another machine. Your laptop is silent. Production crashes. Half the time the culprit is a data race. AI-generated code makes this worse. It writes threads confidently. It forgets the locks. A free server plus a sanitizer can find these bugs. I will show you a short workflow using MonkeyCode's free tokens and free server.

Disclosure: This article was prepared as part of MonkeyCode's product outreach.

The Reproducer

Consider this tiny C++ program. Four threads push into one std::vector. That operation is not thread-safe. It is undefined behavior.

#include <vector>
#include <thread>
#include <iostream>

int main() {
    std::vector<int> results;
    std::vector<std::thread> threads;
    for (int i = 0; i < 4; ++i) {
        threads.emplace_back([&results, i]() {
            for (int j = 0; j < 10000; ++j) {
                results.push_back(i * 10000 + j);
            }
        });
    }
    for (auto& t : threads) t.join();
    std::cout << results.size() << '\n';
}
Enter fullscreen mode Exit fullscreen mode

Compile it locally. It may print 40000. It may crash. The behavior depends on timing, CPU, and allocator state. That is the nature of UB.

Why AI Code Introduces Races

Large language models are trained on patterns. Threading patterns often omit synchronization. They look correct. They compile clean. The race appears only in production. This is why you need a second environment. Sound familiar?

The Free Server and Free Model

MonkeyCode is an open-source AI coding assistant. It offers 10 million free tokens. It also includes a free remote server. Use the model to generate test tools. Use the server to run them. No cloud bill arrives.

Step 1 — Generate a Sanitizer Build

Ask the free model to provide a ThreadSanitizer command. You get something like:

g++ -pthread -fsanitize=thread -g -O1 race.cpp -o race
Enter fullscreen mode Exit fullscreen mode

ThreadSanitizer (TSan) is a runtime checker. It tracks every memory access. It reports conflicting threads and source lines.

Step 2 — Put the Code on the Free Server

Upload race.cpp to the free server. You can use SSH. You can also use a browser terminal. Both work. Then run:

g++ -pthread -fsanitize=thread -g -O1 race.cpp -o race && ./race
Enter fullscreen mode Exit fullscreen mode

Step 3 — Read the Warning

TSan prints a data race report. Example output:

WARNING: ThreadSanitizer: data race (pid=12345)
  Write of size 8 at 0x7f... by thread T2:
    #0 main::$_0::operator() race.cpp:18 (race+0x...)

  Previous write of size 8 at 0x7f... by thread T1:
    #0 main::$_0::operator() race.cpp:18 (race+0x...)
Enter fullscreen mode Exit fullscreen mode

The report shows two stacks. Each stack points to the same line in your source. That line is the unprotected push_back. Root cause confirmed.

Step 4 — Fix the Race

Add a mutex around the shared vector.

#include <mutex>
std::mutex m;

// inside the lambda
std::lock_guard<std::mutex> lock(m);
results.push_back(i * 10000 + j);
Enter fullscreen mode Exit fullscreen mode

Rebuild. Run TSan again. No warning. The program also produces 40000 every time. Note: std::atomic works for simple counters. It does not make std::vector thread-safe.

The Review Trap

Imagine a code review. An AI wrote a thread pool. It looks fine. The reviewer approves it. Then production crashes once. Nobody can reproduce it. This is a familiar story. The missing step is a runtime check on a different architecture. This workflow fills that gap.

Decision Table

Use this free-server workflow when:

  • Production crashes are intermittent.
  • Local runs always pass.
  • You suspect threading errors.
  • You cannot access the production box.

Skip it when:

  • You already have a useful core dump.
  • The crash consistently happens on the first run.
  • You need exact production data and traffic.

Limitations

  • A free server is not a mirror of production.
  • TSan significantly slows execution.
  • TSan may report false positives with custom atomics.
  • MonkeyCode's free tier may change. Check its official repo for current limits.

Wrap-Up

The environment is part of the bug. A second machine is often enough to expose it. You do not need to buy hardware. A free server and a sanitizer will do. Let the AI write the helper. Let TSan find the truth. Why buy a server for one race? Try this flow the next time you debug a mystery crash.

Top comments (0)