[PATCH] Makefile: avoid running curl-config unnecessarily

On Sat, Apr 04, 2020 at 03:38:00PM +0200, Johannes Schindelin wrote:

Hmm, yeah. It's an unfortunate side effect of the ":=" assignment to stop repeatedly invoking curl-config. Now it's only invoked once, but it's _always_ once, even if we're not building anything that needs it.

I wonder if there's a way to expand a Makefile variable lazily, but only once...aha, with some help from the Internet, I came up with the patch below.

Yes, this works, but it's rather unfortunate that we're invoking curl-config when it's not needed. Perhaps using NO_CURL in the documentation job would be better. But if the patch below isn't too disgusting, I think I prefer that approach (because it helps _any_ top-level make invocation that isn't actually building http binaries).

Junio, you may want to hold off on moving jk/build-with-right-curl to next until we resolve this one way or the other.

-- >8 --
Subject: [PATCH] Makefile: avoid running curl-config unnecessarily

Commit 94a88e2524 (Makefile: avoid running curl-config multiple times, 2020-03-26) put the call to $(CURL_CONFIG) into a "simple" variable which is expanded immediately, rather than expanding it each time it's needed. However, that also means that we expand it whenever the Makefile is parsed, whether we need it or not.

This is wasteful, but also breaks the ci/test-documentation.sh job, as it does not have curl at all and complains about the extra messages to stderr. An easy way to see it is just:

  $ make CURL_CONFIG=does-not-work check-builtins
  make: does-not-work: Command not found
  make: does-not-work: Command not found
  GIT_VERSION = 2.26.0.108.gb3f3f45f29
  make: does-not-work: Command not found
  make: does-not-work: Command not found
  ./check-builtins.sh

We can get the best of both worlds if we're willing to accept a little Makefile hackery. Courtesy of the article at:

  http://make.mad-scientist.net/deferred-simple-variable-expansion/

this patch uses a lazily-evaluated recursive variable which replaces its contents with an immediately assigned simple one on first use.

Reported-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Jeff King <peff@peff.net>
---
This is our first use of eval in the Makefile, but that goes back to GNU
make v3.80, which is from 2002. I think that should be OK.

If this inlining is too gross, we could probably contain it in a function where callers would do something like:

  $(eval lazy-shell-var, CURL_LDFLAGS, $(CURL_CONFIG) --libs)

That's better in the sense that there's less gobbledygook at each callsite, but worse in that it obscures the fact that it's a variable assignment. I'd probably consider going that direction if we started growing more use-cases than these two.

We could probably also stuff this into a sh
 Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)