[Qemu-devel] Ignoring errno makes QMP errors suck

[Kevin Wolf <kwolf@redhat.com>]

Hi,

I keep getting reports of problems, with nice error descriptions that
usually look very similar to what I produced here:

{"execute":"blockdev-snapshot-sync","arguments":{"device":"ide0-hd0","snapshot-file":"/tmp/backing.qcow2"}}
{"error": {"class": "OpenFileFailed", "desc": "Could not open
'/tmp/backing.qcow2'", "data": {"filename": "/tmp/backing.qcow2"}}}

Who can tell me what has happened here? Oh, yes, the command failed, I
would have guessed that from the "error" key. But the actual error
description is as useless as it gets. It doesn't tell me anything about
_why_ the snapshot couldn't be created. ("Permission denied" would have
been the helpful additional information in this case)

How should management tools ever be able to provide a helpful error
message to their users if all they get is this useless "something went
wrong" error?

Kevin