Re: [PATCH v3 6/6] tests/qapi-schema: Test cases for aliases

[Markus Armbruster]

Kevin Wolf <kwolf@redhat.com> writes:

> Am 05.10.2021 um 15:49 hat Markus Armbruster geschrieben:
>> Kevin Wolf <kwolf@redhat.com> writes:
>> 
>> > Am 02.10.2021 um 15:33 hat Markus Armbruster geschrieben:
>> >> I apologize for this wall of text.  It's a desparate attempt to cut
>> >> through the complexity and my confusion, and make sense of the actual
>> >> problems we're trying to solve.
>> >> 
>> >> So, what problems exactly are we trying to solve?
>> >
>> > I'll start with replying to your final question because I think it's
>> > more helpful to start with the big picture than with details.
>> >
>> > So tools like libvirt want to have a single consistent interface to
>> > configure things on startup and at runtime. We also intend to support
>> > configuration files that should this time support all of the options and
>> > not just a few chosen ones.
>> 
>> Yes.
>> 
>> > The hypothesis is that QAPIfying the command line is the correct
>> > solution for both of these problems, i.e. all available command line
>> > options must be present in the QAPI schema and will be processed by a
>> > single parser shared with QMP to make sure they are consistent.
>> 
>> Yes.
>> 
>> This leads us to JSON option arguments and configuration files.
>> Well-suited for management applications that already use QMP.
>> 
>> > Adding QAPIfied versions of individual command line options are steps
>> > towards this goal. As soon as they exist for every option, the final
>> > conversion from an open coded getopt() loop (or in fact a hand crafted
>> > parser in the case of vl.c) to something generated from the QAPI schema
>> > should be reasonably easy.
>> 
>> Yes.
>> 
>> > You're right that adding a second JSON-based command line interface for
>> > every option can already achieve the goal of providing a unified
>> > external interface, at the cost of (interface and code) duplication. Is
>> > this duplication desirable? Certainly no. Is it acceptable? You might
>> > get different opinions on this one.
>> 
>> We'd certainly prefer CLI options to match corresponding QMP commands
>> exactly.
>> 
>> Unfortunately, existing CLI options deviate from corresponding QMP
>> commands, and existing CLI options without corresponding QMP commands
>> may violate QMP design rules.
>> 
>> Note: these issues pertain to human-friendly option syntax.  The
>> machine-friendly option syntax is still limited to just a few options,
>> and it does match QMP there.
>
> On the other hand, we only have a handful of existing options that are
> very complex. Most of them aren't even structured and just take a single
> scalar value. So I'm relatively sure that we know the big ones, and
> we're working on them.
>
> Another point here is that I would argue that there is a difference
> between existing options and human-friendly options. Not all existing
> options are human-friendly just because they aren't machine friendly
> either, and there is probably potential for human-friendly syntax that
> doesn't exist yet.
>
> So I would treat support for existing options (i.e. compatibility) and
> support for human-friendly options (i.e. usability) as two separate
> problems.

That's fair.

Instead of "human-friendly option syntax", I could've written "our
traditional option syntax (which is better suited to humans than to
machines, although it still fails to be truly friendly to anyone)", but
that's a tad long, isn't it?

>> > In my opinion, we should try to get rid of hand crafted parsers where
>> > it's reasonably possible, and take advantage of the single unified
>> > option structure that QAPI provides. -chardev specifically has a hand
>> > crafted parser that essentially duplicates the automatically generated
>> > QAPI visitor code, except for the nesting and some differences in option
>> > names.
>> 
>> We should definitely parse JSON option arguments with the QAPI
>> machinery, and nothing more.
>
> Yes, no doubt. (And we don't even consistently do that yet, like
> device-add going through QemuOpts after going through QAPI and throwing
> away all type information and silently ignoring anything it doesn't know
> to handle.)

At least device-add is the last remaining user of 'gen': false.

The 'any' type also leaves type checking to handwritten code, but at
least it doesn't throw away type information first.  Used by qom-get and
qom-set.

>> Parsing human-friendly arguments with it is desirable, but the need for
>> backward compatibility can make it difficult.  Even where compatibility
>> is of no concern, simply swapping concrete JSON syntax for dotted keys
>> may result in human interfaces that are less than friendly.
>> 
>> Are we in agreement that this is the problem at hand?
>
> As far as I am concerned, compatibility is the problem at hand,
> usability isn't.
>
> This doesn't mean that I'm opposed to having actually human friendly
> options, quite the opposite. But getting machine friendly options is
> already a big project. Making human interfaces friendlier would be
> adding another project of similar size, and I don't feel like tackling
> a second project at the same time when the first one is already hard.

Fair enough.

>> > Aliases are one tool that can help bridge these differences in a generic
>> > way with minimal effort in the individual case. They are not _necessary_
>> > to solve the problem; we could instead just use manually written code to
>> > manipulate input QDicts so that QAPI visitors accept them. Even with
>> > aliases, there are a few things left in the chardev code that are
>> > converted this way. Aliases just greatly reduce the amount of this code
>> > and make the conversion declarative instead.
>> 
>> Understood.
>> 
>> > Now a key point in the previous paragraph is that aliases add a generic
>> > way to do this. So even if they are immediately motivated by -chardev,
>> > it might be worth looking at other cases they could enable if you think
>> > that -chardev alone isn't sufficient justification to have this tool.
>> > I guess this is the point where things become a bit less clear because
>> > people are just hand waving with vague ideas for additional uses.
>> >
>> > Do we need to invest more thought on these other cases? We probably do
>> > if it makes a difference for the answer to the question whether we want
>> > to add aliases to our toolbox. Does it?
>> 
>> I hope we can make a case for aliases without looking beyond CLI
>> QAPIfication.  That's a wide field already, with enough opportunity to
>> get lost in details.
>> 
>> If we later put aliases to other uses, we might have to adapt them some.
>> That's okay.  Designing for one problem we have and understand has a
>> much better chance of success than trying to design for all problems we
>> might have.
>> 
>> There are many CLI options to be QAPIfied.  -chardev is one of the more
>> thornier ones, which makes it a useful example.
>
> Good, I agree.
>
>> >> But what about the dotted keys argument?
>> >> 
>> >> One point of view is the difference between the JSON and the dotted keys
>> >> argument should be concrete syntax only.  Fine print: there may be
>> >> arguments dotted keys can't express, but let's ignore that here.
>> >> 
>> >> Another point of view is that dotted keys arguments are to JSON
>> >> arguments what HMP is to QMP: a (hopefully) human-friendly layer on top,
>> >> where we have a certain degree of freedom to improve on the
>> >> machine-friendly interface for human use.
>> >
>> > This doesn't feel unreasonable because with HMP, there is precedence.
>> > But this is not all what we have used dotted key syntax for so far. I'm
>> > not against making a change there if we feel it makes sense, but we
>> > should be clear that it is a change.
>> 
>> Our first uses of dotted keys were green fields.  Sticking to QAPI/QMP's
>> abstract syntax was simplest.
>> 
>> > I think we all agree that -blockdev isn't human-friendly. Dotted key
>> > syntax makes it humanly possible to use it on the command line, but
>> > friendly is something else entirely. It is still a 1:1 mapping of QMP to
>> > the command line, and this is how we advertised it, too.
>> 
>> Yes.
>> 
>> >                                                          This has some
>> > important advantages, like we don't have a separate parser for a
>> > human-friendly syntax, but the generic keyval parser covers it.
>> 
>> There are two generic parts in play: the keyval parser, and the QAPI
>> input visitor.
>
> Fair point.
>
>> Using identical abstract syntax both for JSON and dotted keys arguments
>> makes use of both parts simple: pass the argument to
>> qobject_input_visitor_new_str() to create a visitor, then visit the QAPI
>> type with it.
>> 
>> When abstract syntax differs, using the keyval parser is still simple,
>> but to use the QAPI input visitor, we need separate QAPI types.  Massive
>> code duplication in the QAPI schema.
>> 
>> To avoid the duplication, we can instead translate the parse tree
>> produced for the dotted keys argument.  Plenty of boring code.
>> 
>> In short: yes, using the same abstract syntax for both has advantages.
>
> Even with duplication in the schema, you still have to translate unless
> you want to duplicate all of the logic, too. The difference is just that
> instead of translating between QObjects, you would be translating
> between C structs.

True.

> So yes, using different abstract syntax means translation, which in turn
> can mean plenty of boring code (hopefully - might also be buggy rather
> than boring) if we don't support ways to automate the conversion.

We did it manually for SocketAddress / SocketAddressLegacy.  Worked out
okay.  We don't want to do it manually for chardev-add's argument type,
because the amount of translation / duplication is off-putting.

>> > HMP in contrast means separate code for every command, or translated to
>> > the CLI for each command line option. HMP is not defined in QAPI, it's
>> > a separate thing that just happens to call into QAPI-based QMP code in
>> > the common case.
>> >
>> > Is this what you have in mind for non-JSON command line options?
>> 
>> We should separate the idea "HMP wraps around QMP" from its
>> implementation as handwritten, boilerplate-heavy code.
>> 
>> All but the latest, QAPI-based CLI options work pretty much like HMP: a
>> bit of code generation (hxtool), a mix of common and ad hoc parsers,
>> boring handwritten code to translate from parse tree to internal C
>> interfaces (which are often QMP command handlers), and to stitch it all
>> together.
>> 
>> Reducing the amount of boring handwritten code is equally desirable for
>> HMP and CLI.
>> 
>> So is specifying the interface in QAPI instead of older, less powerful
>> schema languages like hxtool and QemuOpts.
>> 
>> There are at least three problems:
>> 
>> 1. Specifying CLI in QAPI hasn't progressed beyond the proof-of-concept
>> stage.
>> 
>> 2. Backward compatibility issues and doubts have defeated attempts to
>> replace gnarly stuff, in particular QemuOpts.
>> 
>> 3. How to best bridge abstract syntax differences has been unclear.
>> Whether the differences are historical accidents or intentional for ease
>> of human use doesn't matter.
>> 
>> Aliases feel like a contribution towards solving 3.
>> 
>> I don't think 1. is particularly difficult.  It's just a big chunk of
>> work that isn't really useful without solutions for 2. and 3.
>> 
>> To me, 2. feels intractable head on.  Perhaps we better bypass the
>> problem by weakening the compatibility promise like we did for HMP.
>
> Can we define 2 in a bit more specific terms? I feel the biggest part
> of 2 is actually 3.
>
> You mention QemuOpts, but how commonly are the potentially problematic
> features of it even used? Short booleans are deprecated and could be
> dropped in 6.2. merge_lists may or may not have to be replicated.
> I know building lists from repeated keys is said to be a thing, where
> does it happen? I've worked on some of the big ones (blockdev, chardev,
> object, device) and haven't come across it yet.

Alright, I'll look for you :) Check out commit 2d6dcbf93fb, 396f935a9af,
54cb65d8588, f1672e6f2b6.  These are fairly recent, which is kind of
depressing.  There may be more.

When I wrote 2., I was specifically thinking of -object, where we
decided not to mess the human-friendly syntax in part because we didn't
fully understand what a conversion from QemuOpts to keyval could break.

> Can we have an intermediate state where CLI parsing involves both QAPI
> generated options and manually written ones? So that we can actually put
> the QAPIfication work already done to use instead of just having a
> promise that it will make things possible eventually, in a few years?

We kind of sort of have that already: a few option arguments are
QAPI-based.

Perhaps it's time to crack 1., but in a way that accommodates
handwritten options.  Could be like "this option takes a string
argument, and the function to process the option is not to be generated,
only called (just like 'gen': false for commands).  We may want to make
introspection describe the argument as "unknown", to distinguish it from
genuine string arguments.

>> > What I had in mind was using the schema to generate the necessary code,
>> > using the generic keyval parser everywhere, and just providing a hook
>> > where the QDict could be modified after the keyval parser and before the
>> > visitor. Most command line options would not have any explicit code for
>> > parsing input, only the declarative schema and the final option handler
>> > getting a QAPI type (which could actually be the corresponding QMP
>> > command handler in the common case).
>> >
>> > I think these are very different ideas. If we want HMP-like, then all
>> > the keyval based code paths we've built so far don't make much sense.
>> 
>> I'm not sure the differences are "very" :)
>> 
>> I think you start at "human-friendly and machine-friendly should use the
>> abstract syntax defined in the QAPI schema, except where human-friendly
>> has to differ, say for backward compatibility".
>> 
>> This approach considers differences a blemish.  The "standard" abstract
>> syntax (the one defined in the QAPI schema) should be accepted in
>> addition to the "alternate" one whenever possible, so "modern" use can
>> avoid the blemishes, and blemishes can be removed once they have fallen
>> out of use.
>> 
>> "Alternate" should not be limited to human-friendly.  Not limiting keeps
>> things consistent, which is good, because differences are bad.
>> 
>> Is that a fair description?
>
> Hm, yes and no.
>
>
> I do think that making the overlap between "standard" and "alternate"
> abstract syntax as big as possible is a good thing because it means less
> translation has to go on between them, and ultimately the interfaces are
> more consistent with each other which actually improves the human
> friendliness for people who get in touch with both syntaxes.
>
> The -chardev conversion mostly deals with differences that I do consider
> a blemish: There is no real reason why options need to have different
> names on both interfaces, and there is also a lot of nesting in QMP for
> which there is no real reason.
>
> The reason why we need to accept both is compatibility, not usability.

Compatibility requires us to accept both, but each in its place.  It
doesn't actually require us to accept both in either place.

> There are also some differences that are justified by friendliness.
> Having "addr" as a nested struct in QMP just makes sense, and on the
> command line having it flattened makes more sense.
>
> So accepting "path" for "device" in ChardevHostdev is probably a case
> where switching both QMP and CLI to "modern" use and remove the
> "blemish" eventually makes sense to me. The nesting for "addr" isn't.
>
> We may even add more differences to make things human friendly. My
> standard for this is whether the difference serves only for
> compatibility (should go away eventually) or usability (should stay).

In theory, cleaning up QMP is nice.  In practice, we don't have a good
way to rename or move members.  When we do, we have to make both old and
new optional, even when you actually have to specify exactly one.  This
isn't the end of the world, but it is trading one kind of blemish for
another.

Improving on that would involve a non-trivial extension to
introspection.  Pain right away, gain only when all management
applications that matter grok it.

I think the realistic assumption is that QMP blemishes stay.

Of course, we'd prefer not to copy existing QMP blemishes into new
interfaces.  That's why we have SocketAddress for new code, and
SocketAddressLegacy for old code.  Workable for simple utility types.  I
don't have a good solution for "bigger" types.

> Now with this, it's still open whether the "standard" syntax should only
> be supported in QMP or also in the CLI, and whether "alternate" syntax
> should only be supported in the CLI or also in QMP.
>
> Is usability actually improved by rejecting the "standard" syntax on the
> command line, or by rejecting "alternate" syntax in QMP? Hardly so. It's
> also not compatibility. So what is the justification for the difference?
> Maintainability? I honestly don't see the improvement there either.
>
> So I don't really see a reason for differences, but at the same time
> it's also not a very important question to me. If you prefer restricting
> things, so be it.

I dislike adding "alternate" to machine-friendly QMP / CLI poses,
because:

* Adding "alternate" poses documentation and introspection problems.
  The introspection solutions will then necessitate management
  application updates.  Anything we do becomes ABI immediately, so we
  better get it right on the first try.

* Ignoring the documentation problems is a bad idea, because
  undocumented interfaces are.

* If we ignore the introspection problems, then machines are well
  advised to stick to introspectable "standard".  Why add "alternate"
  then?  Complexity for nothing.

* Adding "alternate" to existing interfaces is pretty much pointless.
  To use "alternate", machines first have to check whether this QEMU
  provides it, and if not, fall back to "standard".  Just use "standard"
  and call it a day.

* For new interfaces, the differences between "standard" and "alternate"
  are hopefully smaller than for old interfaces.  Machines will likely
  not care for improvements "alternate" may provide over "standard".

I haven't made up my mind on adding "standard" to human-friendly CLI.

Too many ways to do the same thing are bound to confuse.  We could end
up with the standard way, the alternate way we provide for backward
compatibility, the alternate way we provide for usability, and any
combination thereof.

Each of the three ways has its uses, though.  Combinations not so much.

Adding "standard" to human-friendly CLI only poses documentation
problems (there is no introspection).  If we limit the backward
compatibility promise to machine-friendly, getting things wrong is no
big deal.

>> I start at "human-friendly syntax should be as friendly as we can make
>> it, except where we have to compromise, say for backward compatibility".
>> 
>> This approach embraces differences.  Mind, not differences just for
>> differences sake, only where they help users.
>> 
>> Additionally accepting the "standard" abstract syntax is useful only
>> where it helps users.
>> 
>> "Alternate" should be limited to human-friendly.
>
> I think there is a small inconsistency in this reasoning: You say that
> differences must help users, but then this is not the measuring stick
> you use in the next paragraph. If it were, you would be asking whether
> rejecting "standard" abstract syntax helps users, rather than whether
> adding it does. (Even more so because rejecting it is more work! Not
> much more work, but it adds a little bit of complexity.)

Additionally accepting "standard" complicates the interface and its
documentation.  Whether this helps users more than it hurts them is not
obvious.

> So it seems that in practice your approach is more like "different by
> default, making it the same needs justification", whereas I am leaning
> more towards "same by default, making it different needs justification".
>
> Your idea of "human-friendly syntax should be as friendly as we can make
> it" isn't in conflict with either approach. The thing that the idea
> might actually conflict with is our time budget.

Reasonable people often have quite different ideas on "human-friendly".

>> Different approaches, without doubt.  Yet both have to deal with
>> differences, and both could use similar techniques and machinery for
>> that.  You're proposing to do the bulk of the work with aliases, and to
>> have a tree-transforming hook for the remainder.  Makes sense to me.
>> However, in sufficiently gnarly cases, we might have to bypass all this
>> and keep using handwritten code until the backward compatibility promise
>> is history: see 2. above.
>
> Possibly. I honestly can't tell how many of these cases we will have.
> In all of -object, we had exactly one problematic option. This could
> easily be handled in a tree-transforming hook.
>
>> In addition each approach has its own, special needs.
>> 
>> Yours adds "alternate" syntax to QMP.  This poses documentation and
>> introspection problems.  The introspection solutions will then
>> necessitate management application updates.
>> 
>> Mine trades these problems for others, namely how to generate parsers
>> for two different syntaxes from one QAPI schema.
>> 
>> Do I make sense?
>
> In the long run, won't we need documentation and introspection even for
> things that are limited to the command line? Introspection is one of the
> big features enabled by CLI QAPIfication.

CLI introspection is about the machine-friendly syntax, not the
human-friendly one, just like monitor introspection is about QMP, not
HMP.

> But otherwise yes.
>
>> >> Both -chardev and QMP chardev-add use the same helpers (there are minor
>> >> differences that don't matter here).  The former basically translates
>> >> its flat argument into the latter's arguments.  HMP chardev-add is just
>> >> like -chardev.
>> >> 
>> >> The quickest way to QAPIfy existing -chardev is probably the one we
>> >> chose for -object: if the argument is JSON, use the new, QAPI-based
>> >> interface, else use the old interface.
>> >
>> > -chardev doesn't quite translate into chardev-add arguments. Converting
>> > into the input of a QMP command normally means providing a QDict that
>> > can be accepted by the QAPI-generated visitor code, and then using that
>> > QAPI parser to create the C object. What -chardev does instead is using
>> > an entirely separate parser to create the C object directly, without
>> > going through any QAPI code.
>> 
>> Yes, and that's quite some extra code, with plenty of opportunity for
>> inconsistency.
>
> Needless to say, opportunity that we happily made use of.

Hah!

>> > After QAPIfication, both code paths go through the QAPI code and undergo
>> > the same validations etc.
>> >
>> > If we still have code paths that completely bypass QAPI, it's hard to
>> > call the command line option fully QAPIfied. Of course, if you don't
>> > care about duplication and inconsistencies between the interfaces,
>> > though, and just want to achieve the initially stated goal of providing
>> > at least one interface consistent between QMP and CLI (besides others)
>> > and a config file, then it might be QAPIfied enough.
>> 
>> Reworking human-friendly -chardev to target QMP instead of a C interface
>> shared with QMP would be nice.  Just because prior attempts to replace
>> gnarly stuff compatibly have failed doesn't mean this one must fail.
>
> I mean, for -chardev specifically, you don't even have to take a guess.
> The patches exist, a git tag with them is mentioned in the cover letter
> of this series, and they have just been waiting for about a year for
> their dependency (QAPI aliases) to be merged.
>
> If we don't do aliases, I'll have to rework them to do everything in
> code instead. It's doable, even if the chardev code wouldn't shrink as
> nicely as with the current patches. I just need to know whether it has
> to be done or not.
>
>> >> Now the question that matters for this series: how can QAPI aliases
>> >> help us with the things discussed above?
>> >
>> > The translation needs to be implemented somehow. The obvious way is just
>> > writing, reviewing and maintaining code that manually translates. (We
>> > don't have this code yet for a fully QAPIfied -chardev. We only have
>> > code that bypasses QAPI, i.e. translates to the wrong target format.)
>> >
>> > Aliases help because they allow reducing the amount of code in favour of
>> > data, the alias declarations in the schema.
>> 
>> Understood.
>> 
>> >> If we use the flat variation just internally, say for -chardev's dotted
>> >> keys argument, then introspection is not needed.  We'd use "with
>> >> aliases" just for translating -chardev's dotted keys argument.
>> >
>> > Note that we're doing more translations that just flattening with
>> > aliases, some options have different names in QMP and the CLI.
>> >
>> > But either way, yes, alias support could be optional so that you need to
>> > explicitly enable it when creating a visitor, and then only the CLI code
>> > could actually enable it.
>> >
>> > Limits the possible future uses for the new tool in our toolbox, but is
>> > sufficient for -chardev. We can always extend support for it later
>> > if/when we actually want to make use of it outside of the CLI.
>> 
>> Yes.
>> 
>> >> Valid argument.  CLI with JSON argument should match QMP.  Even when
>> >> that means both are excessively nested.
>> >> 
>> >> CLI with dotted keys is a different matter, in my opinion.
>> >
>> > I'm skipping quite a bit of text here, mainly because I think we need an
>> > answer first if we really want to switch the keyval options to be
>> > HMP-like in more than just the support status (see above).
>> >
>> > Obviously, the other relevant question would then be if it also ends up
>> > like HMP in that most interesting functionality isn't even available and
>> > you're forced to use QMP/JSON when you're serious.
>> 
>> I figure this is because we have let "QMP first" degenerate into "QMP
>> first, HMP never" sometimes, and we did so mostly because HMP is extra
>> work that was hard to justify.
>> 
>> What if you could get the 1:1 HMP command basically for free?  It may
>> not be the most human-friendly command possible.  But it should be a
>> start.
>
> How would you do this?
>
> The obvious way is mapping QMP 1:1 like we do for some of the command
> line options. But you just argued that this is not what you would prefer
> for the command line, so it's probably not what you have in mind for HMP
> either?

I think there are QMP commands where a 1:1 HMP command would be just
fine.

For others, it would be awkward, but that could still better than
nothing.

Infrastructure we build to QAPIfy the command line could probably be
used to get less awkward HMP commands without writing so much code.

Note I'm not talking about HMP commands somebody *wants* to write, say
to provide convenience magic that is inappropriate for QMP, or
specialized higher level commands where QMP only provides general
building blocks.

>> > I guess I can go into more details once we have answered these
>> > fundamental questions.
>> >
>> > (We could and should have talked about them and about whether you want
>> > to have aliases at all a year ago, before going into detailed code
>> > review and making error messages perfect in code that we might throw
>> > away anyway...)
>> 
>> I doubt you could possibly be more right than that!
>
> So what questions need to be answered before we can make a decision?
>
> You talked a lot about how you like QMP to be different from the command
> line. So is restricting aliases to the command line enough to make them
> acceptable? Or do we need larger changes? Should I just throw them away
> and write translation code for -chardev manually?

Fair question, but I'm out of time for right now.  Let's continue
tomorrow.