SystemRestart exception to restart the interpreter

Context: code "autoreloading"[1] on webapp dev-servers, such as Django.

Problem: This kind of servers uses a forking/respawning strategy, which is very
costly for us (on startup time and loosing HotSpot optimizations). 

It is interesting to see how this "autoreloaders" work. I know the django
one[2], but I'm quite sure that the others are quite similar:

 * There is one "main supervisor". It just forks and wait the child process to
exit. If it exits with a special status code (the value `3` is used on django),
then the child it is spawned again.

 * The "runner", which is the process spawned by the "supervisor". It runs the
main code on a thead, while constantly checking for changes on  __file__'s of
loaded modules. If a modification is detected, it exits with the special code
that will make the "supervisor" to spawn the "runner" again.

This gave me the idea that would be good to start supporting this strategy,
obviously without the exit()/spawnve() bits. So I've implemented it by making
the jython command line interpreter (jython.java) work as the "supervisor", by
recognizing a new special exception: SystemRestart. This exception must be
raised on the main thread. On other threads it have the same effect as
SystemExit (i.e, exits the thread)

There are a few problems with this approach, such as the possibility that not
all running threads are stopped. Also, this isn't complete auto-restarting,
because the "runner" component is not included on Jython. Anyway, it is quite
easy to adapt existing "runners" to work on Jython[3]. Thus, by now, this is
targeted for dev-servers scenarios and not meant for mission-critical problems.

[1] I use quotes because it is not "reloading" in the sense of reimporting the
changed code and continue the execution, but simply restarting the interpreter
when some loaded module is changed.
[2]
http://code.djangoproject.com/browser/django/trunk/django/utils/autoreload.py
[3] http://pylonshq.com/pasties/913

[Leo Soto M.]

Issue created.

[Nicholas Riley]

http://codereview.appspot.com/2810/diff/1/8
File Lib/test/test_urllib2_localnet.py (right):

http://codereview.appspot.com/2810/diff/1/8#newcode229
Line 229: PORT = 8765
Seems to be an unrelated change; if this is a problem perhaps file a bug with
CPython? (There's only one line difference in the Jython version of this test
file.)

http://codereview.appspot.com/2810/diff/1/7
File src/org/python/util/jython.java (right):

http://codereview.appspot.com/2810/diff/1/7#newcode62
Line 62: private static boolean restart;
perhaps "shouldRestart"?

[Leo Soto M.]

http://codereview.appspot.com/2810/diff/1/8
File Lib/test/test_urllib2_localnet.py (right):

http://codereview.appspot.com/2810/diff/1/8#newcode229
Line 229: PORT = 8765
On 2008/08/04 02:59:41, Nicholas Riley wrote:
> Seems to be an unrelated change; if this is a problem perhaps file a bug with
> CPython? (There's only one line difference in the Jython version of this test
> file.)

Yup, this has nothing to do with the patch. Please ignore this change ;-)

[fwierzbicki]

http://codereview.appspot.com/2810/diff/1/5
File src/org/python/core/Py.java (right):

http://codereview.appspot.com/2810/diff/1/5#newcode215
Line 215: 
We should comment this as being experimental.

http://codereview.appspot.com/2810/diff/1/7
File src/org/python/util/jython.java (right):

http://codereview.appspot.com/2810/diff/1/7#newcode281
Line 281: /**
I like that this has been broken out into its own method.