Python subprocess call hungs when running rpm2cpio

Question

I'm running the below command using python subprocess to extract files from rpm. But the command failes when the rpm size is more than 25 - 30 MB. Tried the command using Popen, call, with stdout as PIPE and os.system as well. This command is working fine when i run it in shell directly. The problem is only when i invoke this by some means from Python

Command:

rpm2cpio <rpm_name>.rpm| cpio -idmv

I did an strace on the process id and found that its always hung on some write system call

ps -ef | grep cpio
root      4699  4698  4 11:05 pts/0    00:00:00 rpm2cpio kernel-2.6.32-573.26.1.el6.x86_64.rpm
root      4700  4698  0 11:05 pts/0    00:00:00 cpio -idmv

strace -p 4699
Process 4699 attached
write(10, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0rc_pixelview_new"..., 8192

strace -p 4700
Process 4700 attached
write(2, "./lib/modules/2.6.32-573.26.1.el"..., 94

I have 2 questions:

Can someone figure out what is the problem here? Why is it failing when the rpm size is more than 25 MB.
Is there any other way i can extract the rpm contents from python?

Please post the python code you used. Did you redirect stdout to a PIPE and then forget to read the pipe as one of the below answers suggest? We can't debug this without the code needing to be debugged. — tdelaney, Jul 23 '16 at 16:37

score 1 · Answer 1 · answered Jul 23 '16 at 14:59

Your output pipe is full. The python docs note in many places not to do what you are doing:

Do not use stdout=PIPE or stderr=PIPE with this function as that can deadlock based on the child process output volume. Use Popen with the communicate() method when you need pipes.

score 0 · Answer 2 · edited May 23 '17 at 12:14

If all you want is the payload of a *.rpm package, then do the computations to find the beginning of the compressed cpio payload and do the operations directly in python.

See How do I extract the contents of an rpm? for a rpm2cpio.sh shell script that documents the necessary computations. The only subtlety is ensuring that the padding (needed for alignment) between the signature and metadata headers is correct.

Python subprocess call hungs when running rpm2cpio

2 Answers2