Re: [Qemu-devel] [PATCH v6 01/15] qapi: add 'if' to top-level expressions

[Markus Armbruster <armbru@redhat.com>]

Marc-André Lureau <marcandre.lureau@redhat.com> writes:

> Accept 'if' key in top-level elements, accepted as string or list of
> string type. The following patches will modify the test visitor to
> check the value is correctly saved, and generate #if/#endif code (as a
> single #if/endif line or a series for a list).
>
> Example of 'if' key:
> { 'struct': 'TestIfStruct', 'data': { 'foo': 'int' },
>   'if': 'defined(TEST_IF_STRUCT)' }
>
> The generated code is for now *unconditional*. Later patches generate
> the conditionals.
>
> A following patch for qapi-code-gen.txt will provide more complete
> documentation for 'if' usage.

This paragraph looks obsolete now, because...

> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> Reviewed-by: Markus Armbruster <armbru@redhat.com>
> ---
>  scripts/qapi/common.py                   | 35 ++++++++++++++++++++----
>  tests/test-qmp-cmds.c                    |  6 ++++
>  docs/devel/qapi-code-gen.txt             | 22 +++++++++++++++

... you update documentation right in this patch (I like that).

[...]
> diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt
> index 88a70e4d45..7af60b48f3 100644
> --- a/docs/devel/qapi-code-gen.txt
> +++ b/docs/devel/qapi-code-gen.txt
> @@ -739,6 +739,28 @@ Example: Red Hat, Inc. controls redhat.com, and may therefore add a
>  downstream command __com.redhat_drive-mirror.
>  
>  
> +=== Configuring the schema ===
> +
> +Top-level QAPI expressions.

This sentence no verb :)  I suspect an editing accident ate "can take an
'if' key".

>                               The value must be a string or a list of
> +string.

s/string/strings/

>           The corresponding generated code will then guard the inclusion
> +of that member in the larger struct or function with #if IFCOND
> +(or several #if lines for a list), where IFCOND is the value of the
> +'if' key.

"that member" makes little sense; top-level expressions aren't members.

> +
> +'struct', 'enum', 'union', 'alternate', 'command' and 'event'
> +top-level QAPI expressions can take an 'if' keyword like:
> +
> +{ 'struct': 'IfStruct', 'data': { 'foo': 'int' },
> +  'if': 'defined(IFCOND)' }

Repetitive.

Moreover, I'd like to see the example spell out how conditions affect
generated code.

Here's my try:

=== Configuring the schema ===

The 'struct', 'enum', 'union', 'alternate', 'command' and 'event'
top-level expressions can take an 'if' key.  Its value must be a string
or a list of strings.  A string is shorthand for a list containing just
that string.  The code generated for the top-level expression will then
be guarded by #if COND for each COND in the list.

Example: a conditional struct

 { 'struct': 'IfStruct', 'data': { 'foo': 'int' },
   'if': ['defined(CONFIG_FOO)', 'defined(HAVE_BAR)'] }

gets its generated code guarded like this:

 #if defined(CONFIG_FOO)
 #if defined(HAVE_BAR)
 ... generated code ...
 #endif /* defined(HAVE_BAR) */
 #endif /* defined(CONFIG_FOO) */

> +
> +Please note that you are responsible to ensure that the C code will
> +compile with an arbitrary combination of conditions, since the
> +generators are unable to check it at this point.
> +
> +The presence of 'if' keys in the schema is reflected through to the
> +introspection output depending on the build configuration.
> +
> +
>  == Client JSON Protocol introspection ==
>  
>  Clients of a Client JSON Protocol commonly need to figure out what
[...]

I'm happy to apply these proposals without a respin, if you like them.