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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Markus Armbruster (armbru@redhat.com) wrote:
> = Introduction =
> 

<snip>

> = Structured option argument syntax =
> 
> == JSON ==
> 
> The obvious way to provide the expressiveness of JSON on the command
> line is JSON.  Easy enough[2].  However, besides not being compatible,
> it's rather heavy on syntax, at least for simple cases.  Compare:
> 
>     -machine q35,accel=kvm
>     -machine '{ "type": "q35", "accel": "kvm"}'
> 
> It compares a bit more favourably in cases that use our non-flat hacks.
> Here's a flat list as KEY=VALUE,... with repeated keys, and as JSON:
> 
>     -semihosting-config enable,arg=eins,arg=zwei,arg=drei
>     -semihosting-config '{ "enable": true, "arg": [ "eins", "zwei", "drei" ] }'
> 
> Arbitrary nesting with dotted key convention:
> 
>     -drive driver=qcow2,file.driver=gluster,
>            file.volume=testvol,file.path=/path/a.qcow2,file.debug=9,
>            file.server.0.type=tcp,
>            file.server.0.host=1.2.3.4,
>            file.server.0.port=24007,
>            file.server.1.type=unix,
>            file.server.1.socket=/var/run/glusterd.socket
>     -drive '{ "driver": "qcow2",
>               "file": {
>                   "driver": "gluster", "volume": "testvol",
>                   "path": "/path/a.qcow2", "debug": 9,
>                   "server": [ { "type": "tcp",
>                                 "host": "1.2.3.4", "port": "24007"},
>                               { "type": "unix",
>                                 "socket": "/var/run/glusterd.socket" } ] } }'

So while I generally hate JSON, the -drive dotted key syntax makes
me mad when it gets like this;  have a look
at the block replication and quorum setups especially, that can end up
with (from docs/COLO-FT.txt):

  -drive if=virtio,id=primary-disk0,driver=quorum,read-pattern=fifo,vote-threshold=1,\
         children.0.file.filename=1.raw,\
         children.0.driver=raw -S

   that's just way too many .'s to ever properly understand.
(I'm sure it used to be more complex).

> Lines broken and indented for legibility; you need to join them for
> actual use.

Why? What's a \n between friends for JSON?

> Once you do, both variants are basically illegible.  This
> is simply something that belongs into a config file rather than the
> command line.  In a config file, JSON would be a better choice.
> 
> 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.
> 
> 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.
> 
> Since structured option arguments are always dictionaries, a JSON option
> argument always starts with '{'.  If no QemuOpts argument can ever start
> with '{', accepting either QemuOpts or a JSON object is unambiguous.
> For a more detailed discussion of the following argument, see [3].
> 
> A QemuOpts argument normally starts with KEY.  We need to outlaw KEYs
> starting with '{'.  QAPI outlaws such names, see docs/qapi-code-gen.txt.
> QOM doesn't, but no such keys exist as far as I know.
> 
> QemuOpts permit abbreviating KEY=VALUE to just VALUE for one specific
> KEY (the "implied" key).  We need to limit this to KEYs whose VALUE
> can't start with '{'.  Most implied keys can't have such values.
> Troublemakers include qemu-img's use of implied "file" keys.  You'd have
> to say "file={my-tastelessly-named-file}" instead of just
> "{my-tastelessly-named-file}".

What worries me a bit is building shell scripts which include ['s and {'s
tends to be painful.

<snip>

> === 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.

I don't understand why [ could imply a string.

> 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.

How about a tighter rule that if you've got a structured value - i.e. you're
inside either of [ or {, then you must " all strings (except keys that we keep clean).

> So, if escaped ',' wasn't ugly and confusing enough for you...
> 
> === 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?

Dave

> 
> 
> 
> 
> [1] [PATCH v14 00/21] QAPI/QOM work for non-scalar object properties
> (actually v15)
> Message-Id: <1475246744-29302-1-git-send-email-berrange@redhat.com>
> http://lists.gnu.org/archive/html/qemu-devel/2016-09/msg08238.html
> 
> [2] [RFC PATCH] block: Crude initial implementation of -blockdev
> Message-Id: <1485968933-9162-1-git-send-email-armbru@redhat.com>
> http://lists.gnu.org/archive/html/qemu-devel/2017-02/msg00182.html
> 
> [3] Message-ID: <87h989ncse.fsf@dusky.pond.sub.org>
> http://lists.gnu.org/archive/html/qemu-devel/2016-10/msg04046.html
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK