Re: [Qemu-devel] Non-flat command line option argument syntax

["Daniel P. Berrange" <berrange@redhat.com>]

On Thu, Feb 02, 2017 at 08:42:33PM +0100, Markus Armbruster wrote:
> === Dotted keys ===
> 
> One sufficiently powerful syntax extension already exists: the dotted
> key convention.  It's syntactically unambiguous only when none of the
> KEYs involved contains '.'  To adopt it across the board, we'd have to
> outlaw '.' in KEYs.  QAPI outlaws '.' already, but we have a bunch of
> QOM properties names with '.'.  We'd have to rename at least the ones
> that need to be accessible in -object.

We don't neccessarily need to rename - there's a couple of alternatives

 - Allow registration of property name aliases, so there's two
   names to access the same property. The old one with the '.'
   in it for back compat, and a new one without the '.'.

 - Magically remap '.' to '_'. eg if user specifies foo_bar and
   there's no foo_bar present, then also look for foo.bar

> Dotted keys can't express member names that look like integers.  We'd
> have to outlaw them at least for the objects that are accessible on the
> command line.  Once again, QAPI outlaws such names already.  QOM is
> anarchy when it comes to names, however.
> 
> The way dotted keys do arrays is inconsistent with how QOM's automatic
> arrayification (commit 3396590) do them: foo.0 vs. foo[0].  Backward
> compatibility makes changing the dotted key convention awkward.  Perhaps
> we can still change QOM.

It is a shame - foo[0] is clearly better as it avoids need to outlaw
member names that are pure integers.

IIRC, we outlaw members names that begin with a "_", so if we must
support member names that are integers, we could allow them to be
given unambigously by using  foo._0  to dinstiguish from foo.0
array access.
 
> === Structured values ===
> 
> The dotted key convention messes with KEY syntax to permit structured
> values.  Works, but the more conventional way to support structured
> values is a syntax for structured values.  
> 
> An obvious one is to use { KEY=VALUE, ...} for objects, and [ VALUE,
> ... ] for arrays.  Looks like this:
> 
>     -drive 'driver=quorum,
>             child=[{ driver=file, filename=disk1.img },
>                    { driver=host_device, filename=/dev/sdb },
>                    { driver=nbd, host=localhost } ]'
> 
> Again, lines broken and indented for legibility; you need to join them
> for actual use.
> 
> There's a syntactic catch, though: a value of the form [ ... ] can
> either be an array or a string.  Which one it is depends on the type of
> the key.  To parse this syntax, you need to know the types, unlike JSON
> or traditional QemuOpts.  Unless we outlaw strings starting with '{' or
> '[', which feels impractical.
> 
> But wait, there's another syntactic catch: in traditional QemuOpts, a
> value ends at the next unescaped ',' or '\0'.  Inside an object, it now
> also ends at the next unescaped '}', and inside an array, at the next
> unescaped ']'.  Or perhaps at the next space (the example above assumes
> it does).  That means we either have to provide a way to escape '}', ']'
> and space, or find another way to delimit string values, say require '"'
> around strings whenever the string contains "funny" characters.
> 
> So, if escaped ',' wasn't ugly and confusing enough for you...

I'm not a fan of this because it essentially introduces yet another
syntax, different from everything that exists today.

> === Comparison ===
> 
> In my opinion, dotted keys are weird and ugly, but at least they don't
> add to the quoting mess.  Structured values look better, except when
> they do add to the quoting mess.
> 
> I'm having a hard time deciding which one I like less :)
> 
> Opinions?  Other ideas?

I think the JSON approach is fine, *iff*, QEMU is being managed by
a machine (ie libvirt). If QEMU is being managed by a human, then
typing & quoting json in the CLI is pretty horrible - when I write
json I pretty much always get the syntax wrong several times, with
the trailing comma rules being particularly tedious. IMHO this is
why humans tend to still use the HMP (or the HMP-lookalike QMP-shell
to talk to QMP), rather ever using QMP directly.

The block layer allows both dotted keys and JSON already IIUC.

As such I'm inclined to say we should support dotted syntax as the
primary human facing syntax and JSON as the machine targetted
syntax. Essentially just do what the block layer already does
and avoid inventing something new. It might not be perfectly
to our taste if we had a green-field, but given historical
baggage, I think its basically fine.

Regards,
Daniel
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