Re: [RFC PATCH-for-9.1] qapi: Do not generate commands/events/introspect code for user emulation

[Markus Armbruster <armbru@redhat.com>]

Philippe Mathieu-Daudé <philmd@linaro.org> writes:

> User emulation requires the QAPI types. Due to the command
> line processing, some visitor code is also used. The rest
> is irrelevant (no QMP socket).
>
> Add an option to the qapi-gen script to allow generating
> the minimum when only user emulation is being built.
>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
> RFC: Quick PoC for Markus. It is useful for user-only builds.
> ---
>  qapi/meson.build     |  6 +++++-
>  scripts/qapi/main.py | 16 +++++++++++-----
>  2 files changed, 16 insertions(+), 6 deletions(-)
>
> diff --git a/qapi/meson.build b/qapi/meson.build
> index 375d564277..5e02621145 100644
> --- a/qapi/meson.build
> +++ b/qapi/meson.build
> @@ -115,10 +115,14 @@ foreach module : qapi_all_modules
>    endif
>  endforeach
>  
> +qapi_gen_cmd = [ qapi_gen, '-o', 'qapi', '-b', '@INPUT0@' ]
> +if not (have_system or have_tools)
> +  qapi_gen_cmd += [ '--types-only' ]
> +endif
>  qapi_files = custom_target('shared QAPI source files',
>    output: qapi_util_outputs + qapi_specific_outputs + qapi_nonmodule_outputs,
>    input: [ files('qapi-schema.json') ],
> -  command: [ qapi_gen, '-o', 'qapi', '-b', '@INPUT0@' ],
> +  command: qapi_gen_cmd,
>    depend_files: [ qapi_inputs, qapi_gen_depends ])
>  
>  # Now go through all the outputs and add them to the right sourceset.
> diff --git a/scripts/qapi/main.py b/scripts/qapi/main.py
> index 316736b6a2..925af5841b 100644
> --- a/scripts/qapi/main.py
> +++ b/scripts/qapi/main.py
> @@ -33,7 +33,8 @@ def generate(schema_file: str,
>               prefix: str,
>               unmask: bool = False,
>               builtins: bool = False,
> -             gen_tracing: bool = False) -> None:
> +             gen_tracing: bool = False,
> +             gen_types_only: bool = False) -> None:
>      """
>      Generate C code for the given schema into the target directory.
>  
> @@ -50,9 +51,10 @@ def generate(schema_file: str,
>      schema = QAPISchema(schema_file)
>      gen_types(schema, output_dir, prefix, builtins)
>      gen_visit(schema, output_dir, prefix, builtins)
> -    gen_commands(schema, output_dir, prefix, gen_tracing)
> -    gen_events(schema, output_dir, prefix)
> -    gen_introspect(schema, output_dir, prefix, unmask)
> +    if not gen_types_only:
> +        gen_commands(schema, output_dir, prefix, gen_tracing)
> +        gen_events(schema, output_dir, prefix)
> +        gen_introspect(schema, output_dir, prefix, unmask)

This is the behavior change, everything else is plumbing.  You suppress
generation of source code for commands, events, and introspection, i.e.

    qapi-commands*.[ch]
    qapi-init-commands.[ch]
    qapi-events*[ch]
    qapi-introspect.[ch]

and the associated .trace-events.

But none of these .c get compiled for a user-only build.

So, all we save is a bit of build time and disk space: less than 0.1s on
my machine, ~1.6MiB in ~220 files.  My linux-user-only build tree clocks
in at 317MiB in ~4900 files, a full build takes me around 30s (real
time, -j 14 with ccache), so we're talking about 0.5% in disk space and
0.3% in build time.

Moreover, the patch needs work:

    FAILED: tests/unit/test-qobject-input-visitor.p/test-qobject-input-visitor.c.o 
    cc [...] -c ../tests/unit/test-qobject-input-visitor.c
    ../tests/unit/test-qobject-input-visitor.c:27:10: fatal error: qapi/qapi-introspect.h: No such file or directory
       27 | #include "qapi/qapi-introspect.h"
          |          ^~~~~~~~~~~~~~~~~~~~~~~~
    FAILED: libqemuutil.a.p/stubs_monitor-core.c.o 
    cc [...] -c ../stubs/monitor-core.c
    ../stubs/monitor-core.c:3:10: fatal error: qapi/qapi-emit-events.h: No such file or directory
        3 | #include "qapi/qapi-emit-events.h"
          |          ^~~~~~~~~~~~~~~~~~~~~~~~~

I don't think it's worth the bother.

>  
>  
>  def main() -> int:
> @@ -75,6 +77,9 @@ def main() -> int:
>      parser.add_argument('-u', '--unmask-non-abi-names', action='store_true',
>                          dest='unmask',
>                          help="expose non-ABI names in introspection")
> +    parser.add_argument('-t', '--types-only', action='store_true',
> +                        dest='gen_types_only',
> +                        help="Only generate QAPI types")
>  
>      # Option --suppress-tracing exists so we can avoid solving build system
>      # problems.  TODO Drop it when we no longer need it.
> @@ -96,7 +101,8 @@ def main() -> int:
>                   prefix=args.prefix,
>                   unmask=args.unmask,
>                   builtins=args.builtins,
> -                 gen_tracing=not args.suppress_tracing)
> +                 gen_tracing=not args.suppress_tracing,
> +                 gen_types_only=args.gen_types_only)
>      except QAPIError as err:
>          print(err, file=sys.stderr)
>          return 1