subprocess-support.rst

Subprocess support

Normally coverage writes the data via a pretty standard atexit handler. However, if the subprocess doesn't exit on its own then the atexit handler might not run. Why that happens is best left to the adventurous to discover by waddling through the Python bug tracker.

pytest-cov supports subprocesses and multiprocessing, and works around these atexit limitations. However, there are a few pitfalls that need to be explained.

If you use multiprocessing.Pool

pytest-cov automatically registers a multiprocessing finalizer. The finalizer will only run reliably if the pool is closed. Closing the pool basically signals the workers that there will be no more work, and they will eventually exit. Thus one also needs to call join on the pool.

If you use multiprocessing.Pool.terminate or the context manager API (__exit__ will just call terminate) then the workers can get SIGTERM and then the finalizers won't run or complete in time. Thus you need to make sure your multiprocessing.Pool gets a nice and clean exit:

from multiprocessing import Pool

def f(x):
    return x*x

if __name__ == '__main__':
    p = Pool(5)
    try:
        print(p.map(f, [1, 2, 3]))
    finally:
        p.close()  # Marks the pool as closed.
        p.join()   # Waits for workers to exit.

If you must use the context manager API (e.g.: the pool is managed in third party code you can't change) then you can register a cleaning SIGTERM handler like so:

from multiprocessing import Pool

def f(x):
    return x*x

if __name__ == '__main__':
    try:
        from pytest_cov.embed import cleanup_on_sigterm
    except ImportError:
        pass
    else:
        cleanup_on_sigterm()

    with Pool(5) as p:
        print(p.map(f, [1, 2, 3]))

If you use multiprocessing.Process

There's similar issue when using the Process objects. Don't forget to use .join():

from multiprocessing import Process

def f(name):
    print('hello', name)

if __name__ == '__main__':
    try:
        from pytest_cov.embed import cleanup_on_sigterm
    except ImportError:
        pass
    else:
        cleanup_on_sigterm()

    p = Process(target=f, args=('bob',))
    try:
        p.start()
    finally:
        p.join()  # necessary so that the Process exists before the test suite exits (thus coverage is collected)

If you got custom signal handling

pytest-cov 2.6 has a rudimentary pytest_cov.embed.cleanup_on_sigterm you can use to register a SIGTERM handler that flushes the coverage data.

pytest-cov 2.7 adds a pytest_cov.embed.cleanup_on_signal function and changes the implementation to be more robust: the handler will call the previous handler (if you had previously registered any), and is re-entrant (will defer extra signals if delivered while the handler runs).

For example, if you reload on SIGHUP you should have something like this:

import os
import signal

def restart_service(frame, signum):
    os.exec( ... )  # or whatever your custom signal would do
signal.signal(signal.SIGHUP, restart_service)

try:
    from pytest_cov.embed import cleanup_on_signal
except ImportError:
    pass
else:
    cleanup_on_signal(signal.SIGHUP)

Note that both cleanup_on_signal and cleanup_on_sigterm will run the previous signal handler.

Alternatively you can do this:

import os
import signal

try:
    from pytest_cov.embed import cleanup
except ImportError:
    cleanup = None

def restart_service(frame, signum):
    if cleanup is not None:
        cleanup()

    os.exec( ... )  # or whatever your custom signal would do
signal.signal(signal.SIGHUP, restart_service)

If you use Windows

On Windows you can register a handler for SIGTERM but it doesn't actually work. It will work if you os.kill(os.getpid(), signal.SIGTERM) (send SIGTERM to the current process) but for most intents and purposes that's completely useless.

Consequently this means that if you use multiprocessing you got no choice but to use the close/join pattern as described above. Using the context manager API or terminate won't work as it relies on SIGTERM.

However you can have a working handler for SIGBREAK (with some caveats):

import os
import signal

def shutdown(frame, signum):
    # your app's shutdown or whatever
signal.signal(signal.SIGBREAK, shutdown)

try:
    from pytest_cov.embed import cleanup_on_signal
except ImportError:
    pass
else:
    cleanup_on_signal(signal.SIGBREAK)

The caveats being roughly:

• you need to deliver signal.CTRL_BREAK_EVENT
• it gets delivered to the whole process group, and that can have unforeseen consequences