Dumping the state of a deadlocked process

[andre . naess]

Hi all

I'm currently having some issues with a process getting deadlocked. The
problem is that the only way I can seem to find information about where
it deadlocks is by making a wild guess, insert a pdb.set_trace() before
this point, and then step until it locks up, hoping that I've guessed
right.

The frustrating part is that most of the time my guesses are wrong.

It would be really nice if I could send the python process some signal
which would cause it to print the current stacktrace and exit
immediately. That way I would quickly be able to pinpoint where in the
code the deadlock happens. Java has a somewhat similar feature where
you can send a running VM process a SIGQUIT, to which it will respond
by dumping all current threads and lots of other information on stdout.

Is this possible somehow?

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Re: Dumping the state of a deadlocked process

[andre . naess]

MrJean1 wrote:
> Did you try using the signal module?  If not, a basic example is here
>  which may need to be
> extended.

I looks useful. I gave it a try, and the only weakness it has is that
when my process locks, it locks so badly that it doesn't respond to
CTRL-C, or any other signal. But by sending it a SIGQUIT which causes
it to dump the current state, and then kill it, I get the dump I need.

This is actually not a multi-threaded app. It's an application which
uses a SQL DB. The problem I was having was that I had a cursor which
started a transaction, and then never finished. Then some other cursor
came along and tried to perform a delete table, and they both locked
up. The cursor isn't ending it's transaction, and the transaction
prevents the delete table from being executed. Luckily Postgresql
allows me to list current activity, otherwise I would have been
scratching my head still.

Using logging or print statements to debug this sort of things is
highly unsatisfactory. I think the way Java uses SIGQUIT is pretty
neat, are there any reasons why Python can't adopt something similar?

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