Re: [PATCH v3 2/2] btrfs-progs: device stats: add json output format

[David Sterba <dsterba@suse.cz>]

On Wed, Dec 16, 2020 at 06:23:50PM +0100, David Sterba wrote:
> On Fri, Dec 11, 2020 at 06:09:11PM +0000, Sidong Yang wrote:
> > On Fri, Dec 11, 2020 at 06:46:29PM +0100, David Sterba wrote:
> > > On Fri, Dec 11, 2020 at 06:30:25PM +0100, David Sterba wrote:
> > > > On Fri, Dec 11, 2020 at 04:48:12PM +0000, Sidong Yang wrote:
> > > > > Example json format:
> > > > > 
> > > > > {
> > > > >   "__header": {
> > > > >     "version": "1"
> > > > >   },
> > > > >   "device-stats": [
> > > > >     {
> > > > >       "device": "/dev/vdb",
> > > > >       "devid": "1",
> > > > >       "write_io_errs": "0",
> > > > >       "read_io_errs": "0",
> > > > >       "flush_io_errs": "0",
> > > > >       "corruption_errs": "0",
> > > > >       "generation_errs": "0"
> > > > >     }
> > > > >   ],
> > > >      ^
> > > > 
> > > > I've verified that the comma is really there, it's not a valid json so
> > > > there's a bug in the formatter. To verify that the output is valid you
> > > > can use eg. 'jq', simply pipe the output of the commadn there.
> > > > 
> > > >   $ ./btrfs --format json dev stats /mnt | jq
> > > >   parse error: Expected another key-value pair at line 16, column 1
> > > 
> > > I've pushed the updated plain text formatting to devel, so the only
> > > remaining bug is the above extra comma.
> > 
> > I've tested devel branch. but there is no extra comma and I also tested
> > with jq you said. jq results no error. I think that It's just mistype in
> > message.
> 
> That's strange because the formatter is buggy, eg. the simplest
> fmt_start()/fmt_end() leads to
> 
> $ ./json-formatter-test
> {
>   "__header": {
>     "version": "1"
>   },
> }
> ---
> 
> Which is of course invalid and jq confirms that.

The reason was a garbage value on the depth 0 of the stack that got
interpreted as "there were values printed, insert the comma". That also
explains why it worked for you.