Re: GSoC Intro - TUI interface for QMP

[Markus Armbruster <armbru@redhat.com>]

John Snow <jsnow@redhat.com> writes:

> On 6/9/21 7:56 AM, Markus Armbruster wrote:
>>> The client could cache the information. (Against what kind of an
>>> identifier? Can QEMU report some kind of token that uniquely
>>> identifies its binary or uniquely identifies the set of QAPI commands
>>> it supports?)
>
>> I proposed something like it to permit QMP clients cache
>> query-qmp-schema output.  Libvirt didn't want it, so it never got beyond
>> the idea stage.
>> 
>
> What ideas did you have for a cache key? We don't need to uniquely
> identify every instance or even every binary.
>
> I suppose we could use an md5/sha1 checksum of the QMP introspection output?

commit 39a181581650f4d50f4445bc6276d9716cece050
Author: Markus Armbruster <armbru@redhat.com>
Date:   Wed Sep 16 13:06:28 2015 +0200

    qapi: New QMP command query-qmp-schema for QMP introspection

[...]

    New QMP command query-qmp-schema takes its return value from that
    variable.  Its reply is some 85KiBytes for me right now.

Has since grown to ~160KiB.
    
    If this turns out to be too much, we have a couple of options:
    
    * We can use shorter names in the JSON.  Not the QMP style.
    
    * Optionally return the sub-schema for commands and events given as
      arguments.
    
      Right now qmp_query_schema() sends the string literal computed by
      qmp-introspect.py.  To compute sub-schema at run time, we'd have to
      duplicate parts of qapi-introspect.py in C.  Unattractive.
    
    * Let clients cache the output of query-qmp-schema.
    
      It changes only on QEMU upgrades, i.e. rarely.  Provide a command
      query-qmp-schema-hash.  Clients can have a cache indexed by hash,
      and re-query the schema only when they don't have it cached.  Even
      simpler: put the hash in the QMP greeting.
    
    Signed-off-by: Markus Armbruster <armbru@redhat.com>
    Reviewed-by: Eric Blake <eblake@redhat.com>

Note this glosses over what exactly is hashed.  Back then, we generated
query-qmp-schema's output as a string, so we would have hashed that.
Today, we generate a QLitObject.  Less trivial to hash.  Quite feasible
all the same.

NB: Commit messages are love letters to your future self :)

[...]