Description

Repro

git am the patch below and run the new test with bazel test py:common-chrome
https://gist.github.com/victorvianna/d3930c0962b8fe322d6341b3f29f1fe5

Expected: works fine

Actual: "local variable 'ws_url' referenced before assignment"

We have concrete cases in Chromium benchmarks where this is a problem.

Root cause

devtools is a global and thus outlives individual WebDrivers. When start_devtools() is called the second time, the if not devtools block containing ws_url initialization is skipped

Discussion

Having the devtools auto-generated module be a global seems conceptually wrong. For one thing, this value depends on the browser version (the protocol changes over time). The least invasive change is probably to turn it into a WebDriver member like _websocket_connection.

On a separate note, I wonder:

1. Why was a global used in the first place? Would it be costly to include the module again on every call? Because bidi_connection() is doing exactly that ATM (notice how there's no global devtools line in the function)

2. _websocket_connection a single central websocket, shared with other methods. But why limit to a single socket, why not return a new one on every call? Does the browser not support it? One advantage I could imagine is guaranteeing that WebDriver calls close() when its done, but that's not done right now AFAICT

Reproducible Code

See above

Debugging Logs

See above