Python child process in while True loop blocks parent

[Barry Scott]

> On 29 Nov 2021, at 22:31, Jen Kris <jenkris at tutanota.com> wrote:
> 
> Thanks to you and Cameron for your replies.  The C side has an epoll_ctl set, but no event loop to handle it yet.  I'm putting that in now with a pipe write in Python-- as Cameron pointed out that is the likely source of blocking on C.  The pipes are opened as rdwr in Python because that's nonblocking by default.  The child will become more complex, but not in a way that affects polling.  And thanks for the tip about the c-string termination. 
> 

flags is a bit mask. You say its BLOCKing by not setting os.O_NONBLOCK.
You should not use O_RDWR when you only need O_RDONLY access or only O_WRONLY access.

You may find

	man 2 open

useful to understand in detail what is behind os.open().

Barry



> 
> 
> Nov 29, 2021, 14:12 by barry at barrys-emacs.org:
> 
> On 29 Nov 2021, at 20:36, Jen Kris via Python-list <python-list at python.org> wrote:
> 
> I have a C program that forks to create a child process and uses execv to call a Python program. The Python program communicates with the parent process (in C) through a FIFO pipe monitored with epoll(). 
> 
> The Python child process is in a while True loop, which is intended to keep it running while the parent process proceeds, and perform functions for the C program only at intervals when the parent sends data to the child -- similar to a daemon process. 
> 
> The C process writes to its end of the pipe and the child process reads it, but then the child process continues to loop, thereby blocking the parent. 
> 
> This is the Python code:
> 
> #!/usr/bin/python3
> import os
> import select
> 
> #Open the named pipes
> pr = os.open('/tmp/Pipe_01', os.O_RDWR)
> Why open rdwr if you are only going to read the pipe?
> pw = os.open('/tmp/Pipe_02', os.O_RDWR)
> Only need to open for write.
> 
> ep = select.epoll(-1)
> ep.register(pr, select.EPOLLIN)
> 
> Is the only thing that the child does this:
> 1. Read message from pr
> 2. Process message
> 3. Write result to pw.
> 4. Loop from 1
> 
> If so as Cameron said you do not need to worry about the poll.
> Do you plan for the child to become more complex?
> 
> while True:
> 
> events = ep.poll(timeout=2.5, maxevents=-1)
> #events = ep.poll(timeout=None, maxevents=-1)
> 
> print("child is looping")
> 
> for fileno, event in events:
> print("Python fileno")
> print(fileno)
> print("Python event")
> print(event)
> v = os.read(pr,64)
> print("Pipe value")
> print(v)
> 
> The child process correctly receives the signal from ep.poll and correctly reads the data in the pipe, but then it continues looping. For example, when I put in a timeout:
> 
> child is looping
> Python fileno
> 4
> Python event
> 1
> Pipe value
> b'10\x00'
> The C code does not need to write a 0 bytes at the end.
> I assume the 0 is from the end of a C string.
> UDS messages have a length.
> In the C just write 2 byes in the case.
> 
> Barry
> child is looping
> child is looping
> 
> That suggests that a while True loop is not the right thing to do in this case. My question is, what type of process loop is best for this situation? The multiprocessing, asyncio and subprocess libraries are very extensive, and it would help if someone could suggest the best alternative for what I am doing here. 
> 
> Thanks very much for any ideas. 
> 
> 
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>