[Python-Dev] Can someone look at dummy_thread (#622537)? (original) (raw)

Brett Cannon brett@python.org
Fri, 29 Nov 2002 20:17:35 -0800 (PST)

There is a slight chance some of you remember the discussion that Zack Weinberg brought up here back in September about his experiences in rewriting tempfile. One of the things he brought up was having to write his own fake lock so that the code would work when thread was not available. Guido suggested writing a module called dummy_thread that was API compatible with thread but was available on all platforms; he said he would check it in if it was written.

Well, I wrote it. But Guido has told me that he is too busy to check it in and so I am emailing you guys to see if someone can look at it and check it in. I would like to get it in before 2.3a so as to get the extra testing the alpha will get.

The patch is #622537 ( http://www.python.org/sf/622537 ). It is complete to the best of my abilities. I have docs and the module and the tests and such. Should be pretty straight-forward to apply assuming I didn't muck up the docs (first attempt at doing any docs from scratch).

The only thing beyond quality of code and documentation that might have to be dealt with is how far to integrate dummy_thread into the stdlib. I have a patch to have Queue, threading, and tempfile all use dummy_thread when thread is not available. Now I tested all of them with and without thread available and they all pass. But of course I wonder if this will break any code.

The problem is that since dummy_thread basically just executes thread calls serially there is a problem when sommething like blocking I/O is used that blocks waiting for another thread (test_asynchat and test_socket do that, but they require thread so there is no issue there outright). So there is a chance that some code, when trying to run using threading will lock up.

But I don't think this is an issue. I say in the docs that if you want to guarantee true threading to run import thread; del thread before importing threading. But the main reason I don't think it will be an issue is that all other code that runs fine (but slowly) using dummy_thread now can be run where threads are not available. And those programs where deadlock would occur do not lose or gain anything since they couldn't run on a platform without thread anyway. The only issue is that if they don't check for thread someone trying to run the program will have a deadlocked program (although they shouldn't have been trying to run it in the first place).

This is the only real questionable thing. Obviously the module can be checked in with no problem without touching the other modules (although Queue and tempfile should be patched regardless) if this seems to dicey to do.

-Brett