[New-bugs-announce] [issue7736] ctypes freezes/deadlocks process

[Nikolaus Rath]

New submission from Nikolaus Rath <Nikolaus at rath.org>:

The following is a very subtle bug and it took me a couple of days to reduce it to a manageable test case. So please bear with me even if it's tedious.

The problem is that in some cases the use of ctypes for a callback function freezes the entire process. Even 'gdb -p' freezes when trying to attach to the affected PID. The process can not be killed with SIGTERM.

Please consider the attached example. It needs the fuse headers installed to work (package libfuse-dev under Debian).

Step 1: Compile hello_ll.c with   

gcc -Wall -g3 -O0 -fPIC `pkg-config fuse --cflags` -c hello_ll.c
gcc -Wall -g3 -O0 -shared `pkg-config fuse --libs` -o hello_ll.so hello_ll.o

into a shared library. The do_mount() function will be called with ctypes from call_hello.py.

Step 2: In call_hello.py you need to adjust the import of libfuse to match your installed FUSE version (2.6 or 2.8). The libfuse2{6,8}.py files have been autogenerated with ctypeslib. I can provide versions for other FUSE versions as well if necessary. Note that the problem is most likely not with the definitions in this file, I checked all of them carefully by hand.


Step 3: Create an empty directory 'mnt' in the working directory

Step 4: Run call_hello.py. It will mount a simple example FUSE file system in mnt, read the directory, umount the fs and exit.

Step 5: Uncomment line 36 of call_hello.py. This will substitute the C function hello_ll_opendir() (defined in hello_ll.c) by the Python function fuse_opendir(). Note that both functions do exactly the same thing, they just call the fuse function fuse_reply_err.

Step 6: Run call_hello.py again. The program will hang completely. You can salvage the mount point with "fusermount -u -z mnt".

Step 7: Comment out line 47 in call_hello.py, so that the process itself does not try to access the mountpoint anymore.

Step 8: Run call_hello.py again. Observe that the file system mounts and umounts fine. Accessing the mountpoint with a different process does not trigger the freeze either.


In summary, the process hangs only if:

1) The opendir function is implemented as a ctypes callback

and

2) the mounting process itself tries to access the mountpoint


I suspect that the ctypes callback may somehow deadlock with the thread that tries to access the mountpoint.

If someone can tell me how to get around the hanging gdb process, i'll be happy to provide a backtrace. I have already installed debugging symbols for both Python and libfuse.


Thank you for taking the time to read up to this point :-)

----------
assignee: theller
components: ctypes
messages: 98038
nosy: Nikratio, theller
severity: normal
status: open
title: ctypes freezes/deadlocks process
type: crash
versions: Python 2.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue7736>
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