[Python-Dev] Deadlock by a second import in a thread

[Adam Olsen]

On 10/19/07, Facundo Batista <facundobatista at gmail.com> wrote:
> Hi!
>
> I was looking to this bug:   http://bugs.python.org/issue1255
>
> It basically creates a deadlock in Python by doing the following:
>
> - aa.py imports bb.py
> - bb.py imports time and generates a thread
> - the thread uses time.strptime
>
> The deadlock is because the strptime function imports another module,
> line 517 of timemodule.c:
>
>       PyObject *strptime_module = PyImport_ImportModule("_strptime");
>
> This situation is well known, found a lot of references to this
> import-thread-import problem in discussions and previous bugs (i.e.:
> http://bugs.python.org/issue683658).
>
> What I did *not* find, and why I'm asking here, is how to solve it.
>
> Exists a known solution to this?

When python encounters a recursive import within a single thread it
allows you to get access to partially-imported modules, making the
assumption that you won't do any significant work until the entire
import process completes.

Only one thread is allowed to do an import at a time though, as
they'll do significant work with it immediately, so being
partially-imported would be a race condition.

Writing a python file as a script means you do significant work in the
body, rather than in a function called after importing completes.
Importing this python file then violates the assumption for
single-threaded recursive imports, and creating threads then violates
their safety assumptions.

The solution then is, if your python file will ever be imported, you
must write a main function and do all the work there instead.  Do not
write it in the style of a script (with significant work in the global
scope.)

-- 
Adam Olsen, aka Rhamphoryncus