[ python-Bugs-924301 ] A leak case with cmd.py & readline & exception

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Bugs item #924301, was opened at 2004-03-27 00:28
Message generated for change (Comment added) made by mwh
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Category: Python Library
Group: Python 2.3
Status: Open
Resolution: None
Priority: 5
Submitted By: Sverker Nilsson (svenil)
Assigned to: Michael Hudson (mwh)
Summary: A leak case with cmd.py & readline & exception

Initial Comment:
A leak to do with readline & cmd, in Python 2.3.

I found out what hold on to my interactive objects
too long ('for ever') in certain circumstances.  The
circumstance had to do with an exception being
raised in Cmd.cmdloop and handled (or not handled)
outside of Cmd.cmdloop.

In cmd.py, class Cmd, in cmdloop(), if an
exception is raised and propagated out from the
interior of cmdloop, the function postloop() is
not called. The default function of this, (in 2.3)
when the readline library is present, is to
restore the completer, via:

readline.set_completer(self.old_completer)

If this is not done, the newly (by preloop)
inserted completer will remain. Even if the loop
is called again and run without exception, the new
completer will remain, because then in postloop
the old completer will be set to our new
completer. When we exit, the completer will remain
the one we set. This will hold on to our object,
aka 'leak'. - In cmd.py in 2.2 no attempt was made
to restore the completer, so this was also a kind
of leak, but it was replaced the next time a Cmd
instance was initialized.  Now, however, the next
time we will not replace the old completer, but
both of them will remain in memory. The old one
will be stored as self.old_completer.  If we
terminate with an exception, bad luck... the
stored completer retains both of the instances. If
we terminate normally, the old one will be
retained.  In no case do we restore the space of
the first instance. The only way that would
happen, would be if the second instance first
exited the loop with an exception, and then
entered the loop again an exited normally. But
then, the second instance is retained instead! If
each instance happens to terminate with an
exception, it seems well possible that an ever
increasing chain of leaking instances will be
accumulated.

My fix is to always call the postloop, given the
preloop succeeded. This is done via a try:finally
clause.

    def cmdloop(self, intro=None):
	...

        self.preloop()
	try:
		...
	finally: # Make sure postloop called
	    self.postloop()

I am attaching my patched version of cmd.py.  It
was originally from the tarball of Python 2.3.3
downloaded from Python.org some month or so ago in
which cmd.py had this size & date:

14504 Feb 19 2003 cmd.py


Best regards,

Sverker Nilsson


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>Comment By: Michael Hudson (mwh)
Date: 2004-05-19 10:38

Message:
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This is where I go "I wish I'd reviewed that patch more
carefully".

In particular, the documentation of {pre,post}loop is now
out of date.

I wonder setting/getting the completer in these functions
was a good idea.  Hmm.  This bug report confuses me :-) but
I can certainly see the intent of the patch...

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Comment By: Raymond Hettinger (rhettinger)
Date: 2004-05-19 02:52

Message:
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user_id=80475

Michael, this touches some of your code.  Do you want to
handle this one?

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https://sourceforge.net/tracker/?func=detail&atid=105470&aid=924301&group_id=5470