[Python-Dev] test_pty.py hangs in verbose mode on Mac OS X? (original) (raw)

Jean-Paul Calderone exarkun at divmod.com
Fri Apr 13 16:57:31 CEST 2007

On Fri, 13 Apr 2007 10:32:28 -0400, Barry Warsaw <barry at python.org> wrote:

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1

I've been getting some test failures in Python 2.5 svn head on Mac OS X 10.4.9 which I'm not getting on Linux (Ubuntu feisty beta). testsqlite and testzipimport both fail, however, when run in verbose mode (e.g. ./python.exe Lib/test/testsqlite.py) both pass. But that's not exactly why I'm writing this email . In the course of trying to debug this, I ran the following on my Mac: make TESTOPTS=-v test This runs the entire test suite in verbose mode, and you do get a lot of output. However the test suite hangs on testpty.py. In fact, if you run that test alone: ./python.exe Lib/test/testpty.py it too hangs for me. The reason is that in verbose mode, debug() actually prints stuff to stdout and on the Mac, when the child of the pty.fork() writes to its stdout, it blocks and so the parent's waitpid () never returns. This doesn't happen on Linux though; the child's stdout prints don't block, it exits, and the parent continues after the waitpid(). Here's a very simple program that reproduces the problem: - -----snip snip----- import os, pty, sys pid, fd = pty.fork() print >> sys.stderr, pid, fd if pid: os.waitpid(pid, 0) else: os.exit(0) - -----snip snip----- stderr, stdout, doesn't matter. This hangs on the Mac but completes successfully on Linux. Of course, in neither case do you see the child's output. I don't know if this is caused by a bug in the Mac's pty implementation or something we're doing wrong on that platform. I played around with several modifications to pty.fork() on the Mac, including letting it drop down to the openpty()/os.fork() code, even adding an explicit ioctl(slavefd, TIOCSCTTY) call which Stevens chapter 19 recommends for 4.3+BSD. I can't get it to not block.

What about reading from the child in the parent before calling waitpid?

Barring a fix to pty.fork() (or possibly os.forkpty()) for the Mac, then I would like to at least make testpty.py not block when run in verbose mode. A very simple hack would add something like this to the "if pid == pty.CHILD" stanza: "def debug(msg): pass", possibly protected by a "if verbose:". A less icky hack would be to read the output from the masterfd in the parent, though you have to be careful with that on Linux else the read can throw an input/output error. Disabling debug output is band-aid yes, and any application on the Mac like the above snippet will still fail. If anybody has any suggestions, I'm all ears, but I've reached the limit of my pty-fu.

I don't think this is an OS X PTY bug. Writing to a blocking file descriptor can block. Programs that do this need to account for the possibility.

Jean-Paul

More information about the Python-Dev mailing list