Bug in os.spawnv with P_NOWAIT

[nushin]

If you do not null out the output of your parent process that launches
a child process by os.spawnv( ) in P_NOWAIT mode, you will see that
your child process is launched *synchronously* instead of
asynchronously. This is a bug!

Here's a code sample:

In your parent.py file, call child.py as

os.spawnv(os.P_NOWAIT,'/usr/bin/python',('python', 'child.py'))

then, make sure your child.py prints a *hello child* to screen about
2000000 times, and your parent.py prints a *hello parent* message,
then execute your file in a shell as:

python parent.py

then, you would see that whenever your child.py is finished, then
parent.py finishes, i.e., a synchronous call.

Now, null out the output of your parent process as:

python parent.py > /dev/null

then, your parent.py finishes first while child.py is still running,
i.e., an asynchronous call. Please correct me if i am wrong.

Regards,
Nushin

 

[nushin]

(e-mail address removed) (nushin) wrote in message

If you do not null out the output of your parent process that launches
a child process by os.spawnv( ) in P_NOWAIT mode, you will see that
your child process is launched *synchronously* instead of
asynchronously. This is a bug!

Here's a code sample:

In your parent.py file, call child.py as

os.spawnv(os.P_NOWAIT,'/usr/bin/python',('python', 'child.py'))

then, make sure your child.py prints a *hello child* to screen about
2000000 times, and your parent.py prints a *hello parent* message,
then execute your file in a shell as:

python parent.py

then, you would see that whenever your child.py is finished, then
parent.py finishes, i.e., a synchronous call.

Now, null out the output of your parent process as:

python parent.py > /dev/null

then, your parent.py finishes first while child.py is still running,
i.e., an asynchronous call. Please correct me if i am wrong.

Regards,
Nushin

Jeff Epler has shed some light on this issue. Thanks to him.
===========================================================

I don't see any problem with P_NOWAIT. Take the following for example:

$ cat nushin.py
import os

p = os.spawnvp(os.P_NOWAIT, 'sh',
['sh', '-c', 'sleep 1; echo from spawnv'])
print "from program"
print "waitpid returns:", os.waitpid(p, 0)
$ python -u nushin.py
from program
waitpid returns:from spawnv
(2826, 0)

Now, if the program completed very quickly, it's a coin-flip whether its
output would appear before "from program". In this case, I made sure
the program would take a really long time (1 second) to complete.

When running without -u but not on a terminal, you might see
$ python nushin.py | cat
from spawnv
from program
waitpid returns: (2832, 0)
.. this is because the Python process has printed "from program", but
stdio buffering has kept it from actually being written to the output
yet.

Here's what you see on a terminal without -u:
$ python nushin.py
from program
from spawnv
waitpid returns: (2835, 0)
This is the same thing you'd see if the program said
print "from program"; sys.stdout.flush()

Jeff

Thanks Jeff. Yes, i think it's the stdio buffering that causes
P_NOWAIT act asif it's a P_WAIT. I wish an additional parameter could
be added to spawnv( ) to toggle its stdout on/off.

Regards,
Nushin