Re: [PATCH 09/14] error: Use error_setg_file_open() for simplicity and consistency

["Dr. David Alan Gilbert" <dave@xxxxxxxxxxx>]

* BALATON Zoltan (balaton@xxxxxxxxxx) wrote:
> On Sat, 22 Nov 2025, Markus Armbruster wrote:
> > "Dr. David Alan Gilbert" <dave@xxxxxxxxxxx> writes:
> > 
> > > * Markus Armbruster (armbru@xxxxxxxxxx) wrote:
> > > > "Dr. David Alan Gilbert" <dave@xxxxxxxxxxx> writes:
> > > > 
> > > > > * Markus Armbruster (armbru@xxxxxxxxxx) wrote:
> > > > > > Replace
> > > > > > 
> > > > > >     error_setg_errno(errp, errno, MSG, FNAME);
> > > > > > 
> > > > > > by
> > > > > > 
> > > > > >     error_setg_file_open(errp, errno, FNAME);
> > > > > > 
> > > > > > where MSG is "Could not open '%s'" or similar.
> > > > > > 
> > > > > > Also replace equivalent uses of error_setg().
> > > > > > 
> > > > > > A few messages lose prefixes ("net dump: ", "SEV: ", __func__ ": ").
> > > > > > We could put them back with error_prepend().  Not worth the bother.
> > > > > 
> > > > > Yeh, I guess you could just do it with another macro using
> > > > > the same internal function just with string concatenation.
> > > > 
> > > > I'm no fan of such prefixes.  A sign of developers not caring enough to
> > > > craft a good error message for *users*.  *Especially* in the case of
> > > > __func__.
> > > > 
> > > > The error messages changes in question are:
> > > > 
> > > >     net dump: can't open DUMP-FILE: REASON
> > > >     Could not open 'DUMP-FILE': REASON
> > > > 
> > > >     SEV: Failed to open SEV-DEVICE: REASON
> > > >     Could not open 'SEV-DEVICE': REASON
> > > > 
> > > >     sev_common_kvm_init: Failed to open SEV_DEVICE 'REASON'
> > > >     Could not open 'SEV-DEVICE': REASON
> > > > 
> > > > I think these are all improvements, and the loss of the prefix is fine.
> > > 
> > > Yeh, although I find the error messages aren't just for users;
> > > they're often for the first dev to see it to guess which other
> > > dev to pass the problem to, so a hint about where it's coming
> > > from can be useful.
> > 
> > I agree!  But I think an error message must be make sense to users
> > *first* and help developers second, and once they make sense to users,
> > they're often good enough for developers.
> > 
> > The common failures I see happen when developers remain caught in the
> > developer's perspective, and write something that makes sense to *them*.
> > Strawman form:
> > 
> >    prefix: failed op[: reason]
> > 
> > where "prefix" is a subsystem tag, or even __func__, and "reason" is
> > strerror() or similar.
> > 
> > To users, this tends to read as
> > 
> >    gobbledygook: techbabble[: reason]
> > 
> > When we care to replace "failed op" (developer's perspective) by
> > something that actually makes sense to users, "prefix" often becomes
> > redundant.
> > 
> > The error messages shown above aren't bad to begin with.  "failed to
> > open FILE", where FILE is something the user specified, should make
> > sense to the user.  It should also be good enough for developers even
> > without a prefix: connecting trouble with the DUMP-FILE to dump /
> > trouble with the SEV-DEVICE to SEV should be straightforward.
> > 
> > [...]
> 
> I think that
> 
> net dump: can't open random-filename: because of some error
> 
> shows better where to look for the problem than just
> 
> Could not open 'random-filename': because of some error
> 
> as the latter does not tell where the file name comes from or what is it. It
> could be added by a management application or added by the users randomly
> without really knowing what they are doing so repeating the option or part
> in the message that the error comes from can help to find out where to
> correct it. Otherwise it might be difficult to guess what random-filename is
> related to if it's not named something you'd expect.

Yeh agreed.  It very much depends if you think of a 'user' as the person
who typed a qemu command line, or pressed a button on a GUI that triggered
15 levels of abstraction that eventually ran a qemu.

Or for the support person who has a customer saying 'help I've got this error',
and now needs to route it to the network person rather than something else.

Dave

> Regards,
> BALATON Zoltan
-- 
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