Re: [PATCH 9/9] monitor/hmp: Prefer to use hmp_handle_error for error reporting in block hmp commands

[Dr. David Alan Gilbert]

* Maxim Levitsky (address@hidden) wrote:
> On Mon, 2020-01-27 at 14:44 +0100, Markus Armbruster wrote:
> > Maxim Levitsky <address@hidden> writes:
> > 
> > > On Wed, 2019-11-27 at 09:38 +0100, Markus Armbruster wrote:
> > > > Title is too long.  blockdev-hmp-cmds.c will become
> > > > block/monitor/block-hmp-cmds.c in v2.  With this in mind, suggest
> > > > 
> > > >     block/monitor: Prefer to use hmp_handle_error() to report HMP errors
> > > > 
> > > > Maxim Levitsky <address@hidden> writes:
> > > > 
> > > > > This way they all will be prefixed with 'Error:' which some parsers
> > > > > (e.g libvirt need)
> > > > 
> > > > Sadly, "all" is far from true.  Consider
> > > > 
> > > >     void hmp_drive_add(Monitor *mon, const QDict *qdict)
> > > >     {
> > > >         Error *err = NULL;
> > > >         DriveInfo *dinfo = NULL;
> > > >         QemuOpts *opts;
> > > >         MachineClass *mc;
> > > >         const char *optstr = qdict_get_str(qdict, "opts");
> > > >         bool node = qdict_get_try_bool(qdict, "node", false);
> > > > 
> > > >         if (node) {
> > > >             hmp_drive_add_node(mon, optstr);
> > > >             return;
> > > >         }
> > > > 
> > > >         opts = drive_def(optstr);
> > > >         if (!opts)
> > > >             return;
> > > > 
> > > > 
> > > > hmp_drive_add_node() uses error_report() and error_report_err().  Easy
> > > > enough to fix if you move the function here, as I suggested in my review
> > > > of PATCH 8.
> > > 
> > > To be honest that involves exporting the monitor_bdrv_states variable and
> > > bds_tree_init, which were both static before, but I created a patch that 
> > > does that,
> > > If that is all right, I'll squash it with some of my patches.
> > > 
> > > 
> > > > 
> > > > drive_def() is a wrapper around qemu_opts_parse_noisily(), which uses
> > > > error_report_err().  You can't change qemu_opts_parse_noisily() to use
> > > > hmp_handle_error().  You'd have to convert drive_def() to Error, which
> > > > involves switching it to qemu_opts_parse() + qemu_opts_print_help().
> > > > 
> > > > These are just the first two error paths in this file.  There's much
> > > > more.  Truly routing all HMP errors through hmp_handle_error() takes a
> > > > *massive* Error conversion effort, with a high risk of missing Error
> > > > conversions, followed by a never-ending risk of non-Error stuff creeping
> > > > in.
> > > 
> > > Oops. Active can of worms is detected. Take cover!
> > 
> > :)
> > 
> > > > There must be an easier way.
> > > > 
> > > > Consider vreport():
> > > > 
> > > >     switch (type) {
> > > >     case REPORT_TYPE_ERROR:
> > > >         break;
> > > >     case REPORT_TYPE_WARNING:
> > > >         error_printf("warning: ");
> > > >         break;
> > > >     case REPORT_TYPE_INFO:
> > > >         error_printf("info: ");
> > > >         break;
> > > >     }
> > > > 
> > > > Adding the prefix here (either unconditionally, or if cur_mon) covers
> > > > all HMP errors reported with error_report() & friends in one blow.
> > > 
> > > This is a very good idea.
> > > If feels like this should be done unconditionally, although that will
> > > break probably some scripts that depend on exact value of the error 
> > > message (but to be honest,
> > > scripts shouldn't be doing that in first place).
> > > 
> > > Doing that with cur_mon (took me some time to figure out what that is) 
> > > will
> > > limit the damage but its a bit of a hack.
> > > 
> > > 
> > > I think that this is a very good change anyway though so if everyone 
> > > agrees,
> > > I will be more that happy to do this change.
> > > Thoughts?
> > 
> > I think adding an "error: " tag has been proposed before.
> > 
> > I dislike overly decorated error messages, because decoration tends to
> > obscure information.
> > 
> > However, when there's significant non-error output, or even uncertainty
> > of what's an error and what's something else, decoration can help.
> Yes, also this way it is consistent

Yes I also like it; I wouldn't worry too much about things parsing error
messages for the exact error message; if anything is doing that then the
corresponding case needs to have big red flags around it.

Dave

> > 
> > Perhaps you can give some examples where the proposed decoration helps.
> It helps to tag most monitor messages with error prefix which was the root 
> cause of
> me starting to work on this refactoring.
> You suggested this, and I kind of like that idea.
> 
> > 
> > > > That leaves the ones that are still reported with monitor_printf().
> > > > Converting those to error_report() looks far more tractable to me.
> > > 
> > > Yep, in fact I grepped the tree for monitor_printf and there are not
> > > that much instances of this used for error reporting, so it might
> > > be possible to have 'error' prefix on all monitor errors that way
> > > and not only for the block layer.
> > 
> > I figure "all" would be more useful than "just for the block layer".
> Yep, the cover letter is outdated, now this patch series touch way
> more that the block layer.
> 
> Best regards,
>       Maxim Levitsky
> 
> 
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK