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Date: Mon, 26 Mar 2012 09:46:10 -0300
From: Luiz Capitulino <lcapitulino@redhat.com>
Message-ID: <20120326094610.2ee5abc8@doriath.home>
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Subject: Re: [Qemu-devel] Ignoring errno makes QMP errors suck
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To: Kevin Wolf <kwolf@redhat.com>
Cc: Qemu-devel@nongnu.org, Anthony Liguori <anthony@codemonkey.ws>

On Mon, 26 Mar 2012 10:39:50 +0200
Kevin Wolf <kwolf@redhat.com> wrote:

> 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)

There's a function called qemu_fopen_err() in the screendump conversion series
that return more specific errors. It will be trivial to add qemu_open_err()
as soon as qemu_fopen_err() is merged.

We're adding a bunch of more precise errors (some map directly to errno). That's
the easy part. The hard part is to convert everything to use them.

Note that while it's true that this shouldn't have leaked to QMP, good error
reporting is a general problem in QEMU.