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Re: Proposal for handling .hx files with Sphinx

From: Markus Armbruster
Subject: Re: Proposal for handling .hx files with Sphinx
Date: Tue, 21 Jan 2020 07:40:28 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux)

John Snow <address@hidden> writes:

> On 1/17/20 12:30 PM, Peter Maydell wrote:
>> Currently our manual creation includes some .texi files which
>> are autogenerated from .hx files by running scripts/hxtool.
>> .hx files are a simple format, where where a line is either a
>> directive or literal text to be output:
>>  -- comment lines, ignored
>>  -- mark start/end of chunks of text to put into the texinfo output only
>>  -- appear in the .h file output verbatim (they are defined as C macros);
>>     for texi output they are parsed to add in header sections
>> For Sphinx, rather than creating a file to include, the most
>> natural way to handle this is to have a small custom Sphinx
>> extension which will read the .hx file and process it. So
>> instead of "makefile produces foo.texi from foo.hx, qemu-doc.texi
>> says '@include foo.texi'", we have "qemu-doc.rst says
>> 'hxtool-doc:: foo.hx', the Sphinx extension for hxtool has
>> code that runs to handle that Sphinx directive, it reads the .hx
>> file and emits the appropriate documentation contents". (This is
>> pretty much the same way the kerneldoc extension works right now.
>> It also has the advantage that it should work for third-party
>> services like readthedocs that expect to build the docs directly
>> with sphinx rather than by invoking our makefiles.)
>> We'll need to update what the markup is to handle having rST
>> fragments in it. A very minimalist approach to this would
>> simply define a new pair of SRST/ERST directives marking the
>> start/end of chunks of rST text to go into the rST only.
>> (We might be able to do better than that later, as there's
>> some repetition currently going on. But we'll probably get
>> a better idea of how easy it is to avoid the repetition if
>> we start with a simple conversion.)
>> Here's what we do with hx files at the moment. We have four:
>>  hmp-commands.hx
>>    -- defines monitor commands used by monitor.c; generates
>>       qemu-monitor.texi, used by qemu-doc.texi
>>  hmp-commands-info.hx
>>    -- ditto, for the "info" command's subcommand;
>>       generates qemu-monitor-info.texi, used by qemu-doc.texi
>> These two use only the "put this in the texi or in the .h file"
>> functionality, alternating "raw C code defining an entry for the
>> monitor command array" with "lump of raw texi for the docs".
>>  qemu-img-cmds.hx
>>    -- defines options for qemu-img, used by qemu-img.texi
>> This uses the STEXI/ETEXI directives to alternate C and texi.
>> In the for-the-h-file section the only content is always a DEF()
>> macro invocation defining the option; in the texi is only the
>> synopsis of the command. This means there's a lot of repetition,
>> as the DEF macro includes an argument giving the text of the
>> option synopsis, and then the texi also has that synopsis with
>> some extra markup. Finally the main qemu-img.texi repeats the
>> marked-up synopsis later on when it has the actual main documentation
>> of each command.
>>  qemu-options.hx
>>    -- options for qemu proper, used by qemu-doc.texi
>> This uses only the DEF, DEFHEADING, ARCHHEADING macros
>> in the for-the-h-file sections (and the DEFHEADING/ARCHHEADING
>> are read also for texi generation). This also repeats the
>> synopsis in the DEF macro and in the texi fragment.
>> So I think my current view is that we should do the very
>> simple "add SRST/ERST directives" to start with:
>>  * scripts/hxtool needs to recognize them and just ignore
>>    the text inside them
>>  * write the hxtool sphinx extension (shouldn't be too hard)
>>  * conversion of any particular .hx file then involves
>>    replacing the STEXI ...texi stuff... ETEXI sections with
>>    SRST ...rst stuff... ERST. There's no need for any
>>    particular .hx file to support both texi and rst output
>>    at the same time
>> I would initially start with qemu-img-cmds.hx, since that's
>> pulled in by qemu-img.texi, which we can convert in the
>> same way I've been doing qemu-nbd and other standalone-ish
>> manpages. The others are part of the big fat qemu-doc.texi,
>> which is probably going to be the very last thing we convert...
> At one point I did a quick mockup of turning qemu-img-cmds.hx into json
> and wrote a small tool I called "pxtool" that was used for generating
> all the rest of the subsequent information -- an attempt at getting rid
> of .hx files *entirely*.
> The idea at heart was: "Can we remove .hx files and describe everything
> in terms of the QAPI schema instead?"
> I'm still a bit partial to that idea, but realize there are some nasty
> complexities when it comes to describing the QEMU CLI as a schema. One
> of those is that I doubt we even have a full understanding of what the
> CLI syntax is at all.

My CLI QAPIfication prototype[*] gets rid of qemu-options.hx.  Three
more are left: hmp-commands.hx, hmp-commands-info.hx, qemu-img-cmds.hx.
No idea whether these could and should be QAPIfied.

Going beyond prototype is hard, not least for the reason you mentioned.

> Still, I do want to ask: Are we sure we want to double-down on keeping
> the .hx files around instead of trying to move to a more generic data
> format?

One the one hand, I'd prefer to invest as little as practical into .hx.
On the other hand, adding more hard dependencies on QAPIfication is not
a good idea.

What's the stupidest solution that could possibly work now?  Is it the
one Peter sketched?

[*] https://lists.nongnu.org/archive/html/qemu-devel/2017-10/msg00209.html
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