Re: [PATCHv2 2/5] gdb/doc: allow for version.subst in the source tree

[Andrew Burgess <aburgess@redhat.com>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Andrew Burgess <aburgess@redhat.com>
>> Cc: gdb-patches@sourceware.org
>> Date: Mon, 03 Jun 2024 15:22:41 +0100
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> 
>> >>   make: Entering directory '/tmp/build/build/gdb/doc'
>> >>     GEN    GDBvn.texi
>> >>   sed: can't read version.subst: No such file or directory
>> >>   make: Leaving directory '/tmp/build/build/gdb/doc'
>> >> 
>> >> The problem is that every reference to version.subst in GDB's Makefile
>> >> assumes that the version.subst file will always be in the build
>> >> directory.
>> >> 
>> >> In this commit I replace direct references to version.subst with a
>> >> small shell snippet which returns the path to the version.subst file
>> >> to use.  If there is a version.subst file in the build directory then
>> >> that file is selected, otherwise we return the path to a version.subst
>> >> file in the source directory.
>> >
>> > Why isn't VPATH or vpath enough to solve this nuisance?
>> 
>> If anything VPATH is the _cause_ of this nuisance!  With the VPATH set
>> as it is, make will check both the build directory and the source
>> directory.  So when we have a rule like:
>> 
>>   some_target : version.subst
>>      some_command version.subst
>> 
>> make is going to look for the version.subst dependency in both
>> locations, however, where 'some_command' looks is entirely up to
>> 'some_command'.  In our case for example, the 'sed' command doesn't (to
>> my knowledge) have a search path command line option, so a command like:
>> 
>>   sed q version.subst
>> 
>> is only going to search in the build directory.
>
> My reading of the node "Recipes/Search" in the GNU Make manual is that
> if you use automatic variables, such as $^, in the recipe, Make will
> expand the variable into the full file name including the directory in
> which it found the file via VPATH search.  Can't we use that feature
> here?  IOW, instead of
>
>   some_target : version.subst
>      some_command version.subst
>
> you should use something like
>
>   some_target : version.subst
>      some_command $<

Thanks.  I was completely unaware that $< (and friends) worked like
this... but I guess it makes sense when you think about it.

This is indeed a better way to solve the problem I had.

While working on this change I ended up adding a whole extra patch to
the series, so I've followed up with a V3 series.

Thanks for your patience.

Andrew