Re: [PATCH 2/8] Documentation/Makefile: conditionally include ../GIT-VERSION-FILE

> On Thu, Nov 05, 2020 at 09:03:49PM +0000, Ramsay Jones wrote:
> 
>> diff --git a/Documentation/Makefile b/Documentation/Makefile
>> index 652d57a1b6..5c680024eb 100644
>> --- a/Documentation/Makefile
>> +++ b/Documentation/Makefile
>> @@ -272,7 +272,9 @@ install-html: html
>>  ../GIT-VERSION-FILE: FORCE
>>  	$(QUIET_SUBDIR0)../ $(QUIET_SUBDIR1) GIT-VERSION-FILE
>>  
>> +ifneq ($(MAKECMDGOALS),clean)
>>  -include ../GIT-VERSION-FILE
>> +endif
> 
> Calling out "clean" here specially feels somewhat backwards to me, in
> terms of Makefile design. In an ideal world we provide all of the
> dependencies to "make", and based on the targets we are building, it
> decides what needs to be done.
> 
> This works with normal targets, obviously, but also with variables. If
> we do:
> 
>   FOO = $(shell do-the-thing)
> 
> then we execute that command only when $(FOO) is needed[1].
> 
> But "include" here is tricky. It is loaded regardless of whether the
> values it contains are needed or not. I wonder if we could do better by
> giving make more information about what we're expecting to get from it.
> I.e., if we wrote:
> 
>   GIT_VERSION = $(shell awk '{print $3}' GIT-VERSION-FILE)
> 
> Then "make clean", not needing the value of $(GIT_VERSION), wouldn't run
> that shell snippet at all. Of course there's a catch; we are also
> relying on the include to trigger the dependency. So it is really more
> like:
> 
>   GIT_VERSION = $(shell make GIT-VERSION-FILE && awk '{print $3}' GIT-VERSION-FILE)
> 
> I'm not sure how bad that is. Re-invoking make seems like it could get
> expensive, especially for the common case that we're building actual
> binaries and we _do_ need the version. But we could probably cut "make"
> out of the loop entirely. Generating GIT-VERSION-FILE is already a FORCE
> target, so really:
> 
>   GIT_VERSION = $(shell ./GIT-VERSION-GEN)
> 
> would be equivalent-ish (with some output changes, and possibly we'd
> want to stash the value in a file for any other scripts to make use of).
> 
> This is all just stuff I've written in my editor and not tried. I won't
> be surprised if there are some gotchas. But it at least seems like a
> conceptually cleaner path.
> 
> -Peff
> 
> [1] Variable assignment actually has a slight problem in the opposite
>     direction: it wants to run the shell snippet every time the variable
>     is referenced. There's a trick to get around that described in
>     0573831950 (Makefile: avoid running curl-config unnecessarily,
>     2020-04-04).
> 
>     It's built around evals. In fact, I suspect you could build a
>     function around eval that actually works similar to include, but
>     lazy-loads the file only when one of its stubs is referenced. I.e.,
>     something like:
> 
>       GIT_VERSION = $(eval include GIT-VERSION-FILE)
> 
>     would probably work (and for other includes, multiple variables
>     could mention the same file; as soon as it gets included, it
>     overwrites the stubs).
>