Re: [Qemu-devel] [PATCH v2 10/29] qapi: Touch generated files only when they change

[Michael Roth]

Quoting Markus Armbruster (2018-02-11 03:35:48)
> A massive number of objects depends on QAPI-generated headers.  In my
> "build everything" tree, it's roughly 4800 out of 5100.  This is
> particularly annoying when only some of the generated files change,
> say for a doc fix.
> 
> Improve qapi-gen.py to touch its output files only if they actually
> change.  Rebuild time for a QAPI doc fix drops from many minutes to a
> few seconds.  Rebuilds get faster for certain code changes, too.  For
> instance, adding a simple QMP event now recompiles less than 200
> instead of 4800 objects.  But adding a QAPI type is as bad as ever;
> we've clearly got more work to do.
> 
> Signed-off-by: Markus Armbruster <address@hidden>
> Reviewed-by: Eric Blake <address@hidden>

Reviewed-by: Michael Roth <address@hidden>

> ---
>  scripts/qapi/common.py | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/scripts/qapi/common.py b/scripts/qapi/common.py
> index 8290795dc1..2e58573a39 100644
> --- a/scripts/qapi/common.py
> +++ b/scripts/qapi/common.py
> @@ -1951,9 +1951,16 @@ class QAPIGen(object):
>              except os.error as e:
>                  if e.errno != errno.EEXIST:
>                      raise
> -        f = open(os.path.join(output_dir, fname), 'w')
> -        f.write(self._top(fname) + self._preamble + self._body
> +        fd = os.open(os.path.join(output_dir, fname),
> +                     os.O_RDWR | os.O_CREAT, 0666)
> +        f = os.fdopen(fd, 'r+')
> +        text = (self._top(fname) + self._preamble + self._body
>                  + self._bottom(fname))
> +        oldtext = f.read(len(text) + 1)
> +        if text != oldtext:
> +            f.seek(0)
> +            f.truncate(0)
> +            f.write(text)
>          f.close()
> 
> 
> -- 
> 2.13.6
>