[Libguestfs] [PATCH libnbd v3] python: Raise a custom exception containing error string and errno.

[Richard W.M. Jones]

On Fri, Jun 28, 2019 at 01:53:11PM -0500, Eric Blake wrote:
> On 6/28/19 1:27 PM, Richard W.M. Jones wrote:
> > Previously errors caused a RuntimeException to be raised.  This commit
> > defines a custom exception (nbd.Error) which has two parameters, the
> > required error string, and the optional errno (which may be 0 if
> > unavailable).
> > 
> > For example:
> > 
> > $ ./run nbdsh -c 'h.pread(0, 0)'
> > Traceback (most recent call last):
> >   File "/usr/lib64/python3.7/runpy.py", line 193, in _run_module_as_main
> >     "__main__", mod_spec)
> >   File "/usr/lib64/python3.7/runpy.py", line 85, in _run_code
> >     exec(code, run_globals)
> >   File "/home/rjones/d/libnbd/python/nbd.py", line 1163, in <module>
> >     nbdsh.shell()
> >   File "/home/rjones/d/libnbd/python/nbdsh.py", line 62, in shell
> >     exec (c)
> >   File "<string>", line 1, in <module>
> >   File "/home/rjones/d/libnbd/python/nbd.py", line 483, in pread
> >     return libnbdmod.pread (self._o, count, offset, flags)
> > nbd.Error: nbd_pread: invalid state: START: the handle must be connected and finished handshaking with the server: Transport endpoint is not connected (ENOTCONN)
> 
> Cool - in the time I spent writing my reply to v2 1/1, your reaction to
> my reply on 0/1 figured out the way to get what we want:
> 
> 
> > @@ -3917,6 +3938,36 @@ Read the libnbd(3) man page to find out how to use the API.
> >  
> >  import libnbdmod
> >  
> > +# Re-export Error exception as nbd.Error, adding some methods.
> > +from libnbdmod import Error
> 
> Implement all the cool stuff in pure Python on top of the bare-bones
> minimum :)  Lots less hassle than writing it in C code.  I like it!
> 
> > +
> > +Error.__doc__ = '''
> > +Exception thrown when the underlying libnbd call fails.
> > +
> > +This exception has two properties to query the error.  Use
> > +the .string property to return a printable string containing
> > +the error message.  Use the .errno property to return a
> > +Python errno (which may be None in some cases if the error
> > +did not correspond to a system call failure).
> > +'''
> > +
> > +Error.string = property (lambda self: self.args[0])
> > +
> > +def _errno (self):
> > +    import errno
> > +    try:
> > +        return errno.errorcode[self.args[1]]
> > +    except KeyError:
> > +        return None
> > +Error.errno = property (_errno)
> > +
> > +def _str (self):
> > +    if self.errno:
> > +        return (\"%%s (%%s)\" %% (self.string, self.errno))
> > +    else:
> > +        return (\"%%s\" %% self.string)
> > +Error.__str__ = _str
> 
> Looks good to me now!  Thanks for figuring this out while I was
> struggling with reading lots of documentation on C bindings.
> 
> ACK

I pushed it, but there may be a few issues still:

- Still no error checking in raise_exception().  We're on an error
  path already here so it's hard to do anything useful, although
  perhaps we should not segfault.

- The .errno attribute returns a (Python module) errno value, not a
  number, so the number is effectively lost, should that really be an
  issue.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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