Re: [nft PATCH v2] parser_bison: Fix for bison < 3.6

[Phil Sutter <phil@nwl.cc>]

On Thu, Jun 11, 2026 at 02:52:25PM +0200, Phil Sutter wrote:
> Support for 'custom' parse.error value was added in bison-3.6. Fall back
> to previous value for earlier versions.
> 
> This is harder to get right than it seems: On one hand, preprocessor
> macros can't be used in parser_bison.y's declaration section and
> automake forbids conditional changes to AM_YFLAGS on the other.
> 
> Another aspect complicating things is compiling with (an up to date)
> parser_bison.c in place vs. without: Dist tarballs generally have it in
> place, relieving users from having to provide a YACC when compiling. The
> existing parser_bison.c either uses parse.error=custom or not which does
> not (should not) change when compiling. Hiding yyreport_syntax_error()
> behind a CPPFLAG which may be set or not depending on bison presence and
> version then causes trouble if it doesn't match how parser_bison.c was
> created.
> 
> Avoid these pitfalls by:
> - Not relying upon a preprocessor define to control parser_bison.c
>   compilation, instead check existence of the bison-internal
>   YY_LAC_ESTABLISH macro
> - Exporting the above macro existence check in a variable for use by
>   main.c (thereby crossing the libnftables library boundary)
> 
> Also:
> - Introduce have_prebuilt_bison variable in configure.ac, unifying the
>   parser_bison.c existence check and solidify the latter by also
>   comparing its timestamp
> - Report extended parser errors enableval in configure only if
>   parser_bison.c will be recreated, otherwise we can't quite tell if it
>   will turn out to be enabled or not
> 
> Suggested-by: Jan Kończak <jan.konczak@cs.put.poznan.pl>
> Fixes: 67b822f2b2624 ("parser_bison: on syntax errors, output expected tokens")
> Signed-off-by: Phil Sutter <phil@nwl.cc>

Patch applied.