Re: [Qemu-devel] [PATCH 07/22] qemu-error: Introduce get_errno_string()

[Luiz Capitulino <lcapitulino@redhat.com>]

On Wed, 21 Apr 2010 19:13:16 +0200
Markus Armbruster <armbru@redhat.com> wrote:

> "Daniel P. Berrange" <berrange@redhat.com> writes:
> 
> > On Wed, Apr 21, 2010 at 12:12:14PM -0300, Luiz Capitulino wrote:
> >> On Wed, 21 Apr 2010 18:42:38 +0400 (MSD)
> >> malc <av1474@comtv.ru> wrote:
> >> 
> >> > On Wed, 21 Apr 2010, Kevin Wolf wrote:
> >> > 
> >> > > Am 21.04.2010 10:28, schrieb Daniel P. Berrange:
> >> > > > On Tue, Apr 20, 2010 at 06:09:37PM -0300, Luiz Capitulino wrote:
> >> > > >> There are error handling functions in QEMU which print errno codes
> >> > > >> to the user. While it's debatable if this is good from a user
> >> > > >> perspective, sometimes it's the best you can do because it's what
> >> > > >> system calls return and this is also useful for debugging.
> >> > > >>
> >> > > >> So, we need a way to expose those codes in QMP. We can't use the
> >> > > >> codes themselfs because they may vary between systems.
> >> > > >>
> >> > > >> The best solution I can think of is returning the string
> >> > > >> representation of the name. For example, EIO becomes "EIO".
> >> > > >>
> >> > > >> This is what get_errno_string() does.
> >> > > >>
> >> > > >> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> >> > > >> ---
> >> > > >>  qemu-error.c |   25 +++++++++++++++++++++++++
> >> > > >>  qemu-error.h |    1 +
> >> > > >>  2 files changed, 26 insertions(+), 0 deletions(-)
> >> > > >>
> >> > > >> diff --git a/qemu-error.c b/qemu-error.c
> >> > > >> index 5a35e7c..55ce133 100644
> >> > > >> --- a/qemu-error.c
> >> > > >> +++ b/qemu-error.c
> >> > > >> @@ -207,3 +207,28 @@ void error_report(const char *fmt, ...)
> >> > > >>      va_end(ap);
> >> > > >>      error_printf("\n");
> >> > > >>  }
> >> > > >> +
> >> > > >> +/*
> >> > > >> + * This is probably only useful for QMP
> >> > > >> + */
> >> > > >> +const char *get_errno_string(int err)
> >> > > >> +{
> >> > > >> +    assert(err < 0);
> >> > > >> +
> >> > > >> +    switch (err) {
> >> > > >> +    case -EINVAL:
> >> > > >> +        return "EINVAL";
> >> > > >> +    case -EIO:
> >> > > >> +        return "EIO";
> >> > > >> +    case -ENOENT:
> >> > > >> +        return "ENOENT";
> >> > > >> +    case -ENOMEDIUM:
> >> > > >> +        return "ENOMEDIUM";
> >> > > >> +    case -ENOTSUP:
> >> > > >> +        return "ENOTSUP";
> >> > > >> +    default:
> >> > > >> +        return "unknown";
> >> > > >> +    }
> >> > > >> +
> >> > > >> +    abort();
> >> > > >> +}
> >> > > > 
> >> > > > Wouldn't it be nicer to return strerror_r()  output instead of errno
> >> > > > names ?
> >> > > 
> >> > > I agree. And it would be more complete, too.
> >> > 
> >> > OTOH it has a problem of returning translated messages (subject to
> >> > LC_MESSAGES value).
> >> 
> >>  Exactly, and I'm not sure if there's anything that ensure they're
> >> exactly the same among different systems.
> >
> > I thought QMP already declared that the printable error strings are subject
> > to arbitrary change at any time, which includes translation? Apps needing 
> > something reliable should be hooking onto the error code.
> 
> Yes, but the value of get_errno_string() is put into the error's data
> object, where the "client should not attempt to parse this" clause does
> not apply.
> 
> We need to decide whether clients need to know the errno or not.
> 
> If they do, we need to encode errno in a way that doesn't depend on
> QEMU's host system.  The encoding proposed by Luiz is as good as any, I
> think.
> 
> If they don't, then substituting text obtained from strerror_r() into
> desc is the way to go.  There's currently no way to do that without
> putting something it the error's data object, so that would need fixing.
> Simple: don't send members whose names start with '_' across the wire.

 That's even better indeed, actually we could do both: put the errno
in the data object _and_ the strerror_r() text in desc.