[Python-bugs-list] [ python-Bugs-689895 ] Imports can deadlock

[SourceForge.net]

Bugs item #689895, was opened at 2003-02-20 09:39
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https://sourceforge.net/tracker/?func=detail&atid=105470&aid=689895&group_id=5470

Category: Python Interpreter Core
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: M.-A. Lemburg (lemburg)
Assigned to: Nobody/Anonymous (nobody)
Summary: Imports can deadlock

Initial Comment:
[Just copying some discussion details from python-dev
into this
bug report so that we can reopen it to Python 2.4]

[MAL]

>> I wonder whether a general lock such as the one used
in import
>> is such a good idea. Perhaps it should only lock the
importing
>> of a specific module, keeping the locks in a
dictionary indexed by
>> module name instead of a static C variable ?!


I've been thinking along the same lines.  We could make
the import
locking much finer-grained, and limit the blocking only
to threads
that are importing a module that is in the middle of
being loaded by
some other thread.

But this is hard work, and I suggest that we put this
off until Python
2.4 so we can do it right.


>> Then again it's hard to know the real name of the
module being
>> searched before finding it...


There could be a short-lived lock for that problem.


>> I see a more general problem here: the lock prevent
starting
>> up threaded applications which use client-server
logic between
>> the threads. If the application's main thread starts
a client
>> thread as a result of an import which then tries to
import
>> other Python modules, you have a deadlock. (At least
that's how
>> I understand the current implementation.)


Correct.


>> Don't know about others, but I frequently use the
idiom of
>> placing the server's main code in a separate module
and then
>> have small startup script importing this module.
This kind
>> of setup is also advertised for CGI programs, so it
may not
>> be uncommon out there.


We had this problem with Zope2 -- I don't know why the
import lock
didn't bite us before, but we decided to change the
Zope startup code
so that you have to import Zope first and then,
separately, make a
call to start it.  You could do the same for your
application, and I
recommend that Mark does the same for his.

--Guido van Rossum (home page:
http://www.python.org/~guido/)



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