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

[Eric Blake]

On 6/28/19 2:17 PM, Richard W.M. Jones wrote:

>>
>> 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.

Yeah, avoiding the segfault is still worthwhile - but our lack of
PyFOO_BAR() error checking is more pervasive than just in
raise_exception(), so a patch to audit all of our generated code will
pick that up along with the rest.

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

Maybe we want two fields, both .errno (string name, or None if Python
errno.errorcode couldn't map it to a name), and .errnum (raw numeric
value, accessible no matter what).

Maybe as simple as this (or with one further tweak to __str__ to at
least output .errnum when .errno is None):

diff --git i/generator/generator w/generator/generator
index 7c2fb59..9192988 100755
--- i/generator/generator
+++ w/generator/generator
@@ -3944,11 +3944,14 @@ from libnbdmod import Error
 Error.__doc__ = '''
 Exception thrown when the underlying libnbd call fails.

-This exception has two properties to query the error.  Use
+This exception has three 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).
+the error message.  Use the .errnum property for the associated
+numeric error value (which may be 0 if the error did not
+correspond to a system call failure), or the .errno property to
+return a string containing the Python errno name if one is known
+(which may be None if the numeric value does not correspond to
+a known errno name).
 '''

 Error.string = property (lambda self: self.args[0])
@@ -3961,6 +3964,8 @@ def _errno (self):
         return None
 Error.errno = property (_errno)

+Error.errnum = property (lambda self: self.args[1])
+
 def _str (self):
     if self.errno:
         return (\"%%s (%%s)\" %% (self.string, self.errno))




-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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