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

On Thu, Nov 05, 2020 at 09:03:49PM +0000, Ramsay Jones wrote:

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).