Re: [PATCH] telemetry: fix autotest failures on Alpine

[Bruce Richardson <bruce.richardson@intel.com>]

On Fri, Mar 10, 2023 at 11:08:32AM -0800, Stephen Hemminger wrote:
> On Fri, 10 Mar 2023 18:18:36 +0000
> Bruce Richardson <bruce.richardson@intel.com> wrote:
> 
> > On Alpine linux, the telemetry_data_autotest was failing for the
> > test where we had dictionaries embedded in other dictionaries up
> > to three levels deep. Indications are that this issue is due to
> > excess data being stored on the stack, so replace stack-allocated
> > buffer data with dynamically allocated data in the case where we
> > are doing recursive processing of telemetry data structures into
> > json.
> > 
> > Bugzilla ID: 1177
> > Fixes: c933bb5177ca ("telemetry: support array values in data object")
> > Fixes: d2671e642a8e ("telemetry: support dict of dicts")
> > Cc: stable@dpdk.org
> 
> Looking at the telemetry code:
> 
> - why so many temporary buffers, could this be streamed or redesigned
>   so that an allocated buffer is returned.
> 

This is largely what the fix patch does. The temporary buffers are used to
ensure we never end up with a partially written buffer from any truncated
snprintf, since that could cause us to emit invalid json. I think as a
scheme it works well enough for what it was designed for, but this
particular test case has more levels of recursion than was expected, so the
limit of the design are showing. The downside of using memory allocation
using malloc as here, is just more failure cases.

> - why is rte_tel_json_XXX all inline?  These should just be internal
>   functions and not in a .h file.
>

Sure. Since these are all just internal functions used by the library, I'm
not sure it really matters. 

> FYI - if this library reused existing json writer it would have
> been much simpler.  
> https://github.com/shemminger/iproute2/blob/main/lib/json_writer.c

Yep. Looked at that previously, but it's at a lower level than what we
have. IMHO, the complicated bit of producing json output is not the
formatting characters for each data-type but ensuring correct termination
of the json string even in case of errors, so each function in our
telemetry json code always includes correct terminators at all points, i.e.
we have no separate functions to be called for terminating objects, arrays
etc. - they are always included in the output of the items. This is why so
many temporary buffers are used - the input string to a function is
well-formed json, and we only actually append to that buffer by copying
from the temporary buffer once we are sure that the output will also be
similarly well-formed.

That said, if you want to replace the current json implementation with a
better one, I'm not opposed to it. [though I'd definitely rather no
external dependencies which would mean we could no longer rely on it always
being available in default builds]

/Bruce