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

From: Kevin Wolf
Subject: Re: [Qemu-block] Non-flat command line option argument syntax
Date: Mon, 6 Feb 2017 14:23:21 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

Am 02.02.2017 um 20:42 hat Markus Armbruster geschrieben:
> = Brief recap of dotted key convention =
> We'll discuss use of dotted key convention later, so let me explain it
> briefly for the readers who don't know it already.
> The dotted key convention interprets the KEY part as a sequence of names
> separated by dots.  If a name looks like an integer *handwave*, it's an
> array index, else it's an object member name.

I think that's not really what we have, but just what it looks like to
you from your perspective of someone who wants to write a parser that
generically parses these things into nested QObjects.

In reality, pretty much all users of the dotted syntax know from context
whether this is an array or an object. In the former case they call
something like qdict_array_entries()/split(), and in the latter case
qemu_opts_absorb_qdict(). Both of them are happy with numeric keys.

The one exception is qdict_crumple(), which already is a parser like
you're intending to write. The difference is that it's used only in very
few specific cases where we know that the assumption holds true.

In other words: To do things properly, you'd have to look at the schema.

> = Structured option argument syntax =
> == JSON ==
> [...]
> There's also the -drive file=json:... syntax.  It's a bad fit for
> QemuOpts, because QemuOpts and JSON fight for the comma.  I'd show you
> if I could get it to work.

Just double all the commas in the JSON object. It's not really that hard
to come up with something working, but it makes it even uglier than
plain JSON on the command line.

Bonus: You get to guess which options are parsed by QemuOpts and which
aren't. qemu-img usually takes parameters directs from argv, so doubling
the comma there like you have to in -drive will result in an error.

> We obviously can't replace QemuOpts with JSON.  But accepting JSON in
> addition to QemuOpts is a debatable feature: it lets management
> applications reuse the code to build QMP arguments for option arguments.

Management applications already have working code for the existing
syntax, they might not want to switch just because (and probe whether
qemu is new enough to even support JSON for QemuOpts).

When I talked to Peter, his concern wasn't really about what the exact
syntax was, but just that the content is a 1:1 mapping of QMP arguments.

I take this as a sign that we should find something that works well
for human users, and management tools can easily cope with whatever we

> === 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.
> 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.

Dotted key syntax is a bit longwinded, but it's the simplest thinkable
extension of key=value and seems to be relatively easy to implement; the
necessary renaming should be possible to do.

This is not perfect for human users because of its repetitive nature,
but it could be the first step for 2.9.

> === 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.

This looks more like what you really want to use. However, being able to
write a={b=x,c=y} for a.b=x,a.c=y is really just syntactic sugar and
could be a second step after we got the basics working.

Note that treating it simply as syntactic sugar for the expanded dotted
form would also allow mixing (and I think that's a good thing):

    -drive 'driver=qcow2,
            file={driver=file, filename=overlay.qcow2, aio=native}'

Or even add to a previously defined thing, which should make Max happy
when he forgot a nested option at first:

    -drive 'driver=qcow2,

> 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.

We would have to look at the schema and only treat it as a nested object
or an array if the expected type has one there.

Your other mail says that even this doesn't work because of "any" types,
but I don't think this is a real problem: In that case, you simply use
the type that we always used, i.e. string. That's the fully backwards
compatible way.

Want to make use of the shiny new QemuOpts and get things parsed into
a nested object? Well, provide a real schema instead of "any" then.

> 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...

This is actually the part that troubles me the most about adding such a
syntax with new special characters. Otherwise it's pretty close to
optimal, in my opinion.


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