MacOS 10.9.2: threading error using python.org 2.7.6 distribution

[Matthew Pounsett]

On Sunday, 27 April 2014 10:33:38 UTC-4, Chris Angelico  wrote:
> In most contexts, "thread unsafe" simply means that you can't use the 
> same facilities simultaneously from two threads (eg a lot of database
> connection libraries are thread unsafe with regard to a single
> connection, as they'll simply write to a pipe or socket and then read
> a response from it). But processes and threads are, on many systems,
> linked. Just the act of spinning off a new thread and then forking can
> potentially cause problems. Those are the exact sorts of issues that
> you'll see when you switch OSes, as it's the underlying thread/process
> model that's significant. (Particularly of note is that Windows is
> *very* different from Unix-based systems, in that subprocess
> management is not done by forking. But not applicable here.)
> 

Thanks, I'll keep all that in mind.  I have to wonder how much of a problem it is here though, since I was able to demonstrate a functioning fork inside a new thread further up in the discussion.

I have a new development that I find interesting, and I'm wondering if you still think it's the same problem.

I have taken that threading object and turned it into a normal function definition.  It's still forking the external tool, but it's doing so in the main thread, and it is finished execution before any other threads are created.   And I'm still getting the same error.

Turns out it's not coming from the threading module, but from the subprocess module instead.  Specifically, like 709 of /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py
which is this:

        try:
            self._execute_child(args, executable, preexec_fn, close_fds,
                                cwd, env, universal_newlines,
                                startupinfo, creationflags, shell, to_close,
                                p2cread, p2cwrite,
                                c2pread, c2pwrite,
                                errread, errwrite)
        except Exception:

I get the "Warning: No stack to get attribute from" twice when that self._execute_child() call is made.  I've tried stepping into it to narrow it down further, but I'm getting weird behaviour from the debugger that I've never seen before once I do that.  It's making it hard to track down exactly where the error is occurring.

Interestingly, it's not actually raising an exception there.  The except block is not being run.