1Why Hand Off via Subprocess?

The existing codebase was built on a multiprocess architecture. Each task spawned a subprocess for processing, while the main process acted purely as an orchestrator. Adding Selenium-based tasks into this pattern is where things went wrong.

Chrome is heavy, so the plan was to launch it in a subprocess using--remote-debugging-port, then connect from the main process. The subprocess would handle login, and the main process would receive the authenticated session. But the requirement to use UC (undetected-chromedriver) for anti-bot bypass is where the real problems began.

# In subprocess:
# UC starts Chrome → login → open debug port

# In main process:
# Connect via regular selenium to debug port
options.add_experimental_option("debuggerAddress", "localhost:9222")
driver = webdriver.Chrome(options=options)

2Problem 1: UC Ignores --remote-debugging-port

options = uc.ChromeOptions()
options.add_argument("--remote-debugging-port=9222")  # ← ignored

driver = uc.Chrome(options=options)
# Actual port: random (not 9222)

Digging into UC’s source code revealed it manages port assignment internally and overwrites any user-specified port. You can technically read UC’s internal port at runtime, but safely passing that value between subprocesses introduces yet another layer of complexity.

3Problem 2: No detach Support

# Regular selenium (works)
options.add_experimental_option("detach", True)
driver = webdriver.Chrome(options=options)
# Chrome survives after process exit

# UC (does NOT work)
options = uc.ChromeOptions()
options.add_experimental_option("detach", True)  # ← ignored or errors
driver = uc.Chrome(options=options)
# Chrome dies when driver exits

UC tightly couples the lifecycle of chromedriver and Chrome. This design makes detach fundamentally incompatible. The entire concept of spawning Chrome in a subprocess and handing it off conflicts with UC’s architecture.

4Problem 3: stdout Pipe Inheritance Hell

proc = subprocess.Popen(
    ["python", "worker.py"],  # worker.py runs UC inside
    stdout=subprocess.PIPE,
    stderr=subprocess.PIPE
)
# This line never returns
stdout, stderr = proc.communicate()  # ← infinite wait

proc = subprocess.Popen(
    ["python", "worker.py"],
    stdout=subprocess.PIPE,
    stderr=subprocess.PIPE,
    close_fds=True  # ← UC still inherits some pipes
)

UC’s chromedriver launch mechanism makes it difficult to prevent pipe inheritance. Even if you solve this issue, the previous problems still remain — by this point the limitations were clear.

5Problem 4: Invisible Tabs with Regular chromedriver

# In main process
options.add_experimental_option("debuggerAddress", f"localhost:{port}")
driver = webdriver.Chrome(options=options)

# Visible tabs
print(driver.window_handles)  # ['CDP_unknown_handle']

# The tab opened by subprocess is under a different handle
# Not properly listed in window_handles

There’s a fundamental mismatch between Selenium’s managed tab context and the actual browser tabs when connecting via debug port. And with regular chromedriver, bot detection triggers anyway — defeating the whole purpose.

6Conclusion: UC Must Run in a Single Process

We abandoned the multiprocess design. UC now runs directly in the main process, and any Selenium work is handled there. The code structure diverged from the original intent, but it runs reliably.