Re: [PATCH] tools/libs/light: fix race in Makefile

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.10.2020 11:12, Juergen Gross wrote:
> The header $(INCLUDE)/_lixl_list.h matches two different rules, which
> can result in build breakage. Fix that.

While I don't doubt you having observed a race, I'm not sure this is
true, and hence I'm also not sure the change is going to address it:
Aiui the two rules you talk about are the one you change and

$(XEN_INCLUDE)/_%.h: _%.h
        $(call move-if-changed,_$*.h,$(XEN_INCLUDE)/_$*.h)

But a pattern rule doesn't come into play when a specific rule for
a file exists.

What I don't understand here is why this two step moving around of
headers is used: Instead of the above pattern rule, can't the rule
to generate _libxl_type%.h, _libxl_type%_json.h,
_libxl_type%_private.h, and _libxl_type%.c put the relevant header
files right into their designated place? This would allow the
pattern rule to go away, albeit I'd then still be unclear about
the specific race you did observe.

Jan

> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  tools/libs/light/Makefile | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/libs/light/Makefile b/tools/libs/light/Makefile
> index 3424fdb61b..370537ed38 100644
> --- a/tools/libs/light/Makefile
> +++ b/tools/libs/light/Makefile
> @@ -203,9 +203,9 @@ _libxl.api-for-check: $(XEN_INCLUDE)/libxl.h $(AUTOINCS)
>               >$@.new
>       mv -f $@.new $@
>  
> -$(XEN_INCLUDE)/_libxl_list.h: 
> $(XEN_INCLUDE)/xen-external/bsd-sys-queue-h-seddery 
> $(XEN_INCLUDE)/xen-external/bsd-sys-queue.h
> -     $(PERL) $^ --prefix=libxl >$(notdir $@).new
> -     $(call move-if-changed,$(notdir $@).new,$@)
> +_libxl_list.h: $(XEN_INCLUDE)/xen-external/bsd-sys-queue-h-seddery 
> $(XEN_INCLUDE)/xen-external/bsd-sys-queue.h
> +     $(PERL) $^ --prefix=libxl >$@.new
> +     $(call move-if-changed,$@.new,$@)
>  
>  _libxl_save_msgs_helper.c _libxl_save_msgs_callout.c \
>  _libxl_save_msgs_helper.h _libxl_save_msgs_callout.h: \
>