Re: [rdiff-backup-users] rdiff-backup crash

[Martin Pool]

On 16 May 2004, Ben Escoto <address@hidden> wrote:
> >>>>> Dave Hawkes <address@hidden>
> >>>>> wrote the following on Mon, 19 Apr 2004 08:56:43 -0400
> 
> >>> I've been running rdiff-backup (ver. 13.4) reliably for quite some
> >>> time but now consistently get the following crash when trying to
> >>> backup up a particular machine (see below). Has anyone any ideas?
> ...
> >>>  File "/usr/lib/python2.2/site-packages/rdiff_backup/log.py", line 134, 
> >>> in log_to_term
> >>>   termfp.write(self.format(message, self.term_verbosity))
> >>> IOError: [Errno 11] Resource temporarily unavailable
> 
> > I managed to fix the problem by lowering the terminal-verbosity setting 
> > to 4 from 5. It would appear that there was some contention for stdout, 
> > though I'm not sure how that could happen. The problem can likely be 
> > fixed with an exception handler around the code that writes to stdout 
> > and then deals with this event in an intelligent way.
> 
> You're right---it seems stdout is temporarily unavailable.  Does
> anyone know how this can happen?  The whole server is built around
> writing to stdout, so checking for this error could add a lot of
> overhead.
> 
> Perhaps some un*x gurus can tell me 1) when this error occurs and 2)
> is it really necessary to check for EAGAIN every time data is written
> to a file descriptor?  The examples I see in, say, books covering C on
> unix generally don't check.

I haven't reproduced the bug or anything so this is kind of handwavey:

You should only see EAGAIN on a file descriptor in nonblocking mode.
This can sometimes happen on programs invoked over SSH.  In this case
the fd is a pipe or unix domain socket connecting the rdiff-backup
server to the sshd child.  So EAGAIN (aka EWOULDBLOCK) will happen
when rdiff-backup produces output faster than sshd can consume it.  As
you might expect lowering the terminal verbosity fixes that.

So there are basically two things you can do:

1 - Check for EAGAIN; when you get it then select() on the socket and
retry.  Presumably a good way in Python is to just use a little
decorator object around the real file.

2 - Make the socket nonblocking.  But I don't know if that would upset
sshd.

(Congratulations on your dissertation submission and job.)

-- 
Martin

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