Makefiles: add and use wildcard "mkdir -p" template

Add a template to do the "mkdir -p" of $(@D) (the parent dir of $@)
for us, and use it for the "make lint-docs" targets I added in
8650c6298c1 (doc lint: make "lint-docs" non-.PHONY, 2021-10-15).

As seen in 4c64fb5aad9 (Documentation/Makefile: fix lint-docs mkdir
dependency, 2021-10-26) maintaining these manual lists of parent
directory dependencies is fragile, in addition to being obviously
verbose.

I used this pattern at the time because I couldn't find another method
than "order-only" prerequisites to avoid doing a "mkdir -p $(@D)" for
every file being created, which as noted in [1] would be significantly
slower.

But as it turns out we can use this neat trick of only doing a "mkdir
-p" if the $(wildcard) macro tells us the path doesn't exist. A re-run
of a performance test similar to that noted downthread of [1] in [2]
shows that this is faster, in addition to being less verbose and more
reliable (this uses my "git-hyperfine" thin wrapper for "hyperfine"[3]):

    $ git -c hyperfine.hook.setup= hyperfine -L rev HEAD~1,HEAD~0 -s 'make -C Documentation lint-docs' -p 'rm -rf Documentation/.build' 'make -C Documentation -j1 lint-docs'
    Benchmark 1: make -C Documentation -j1 lint-docs' in 'HEAD~1
      Time (mean ± σ):      2.914 s ±  0.062 s    [User: 2.449 s, System: 0.489 s]
      Range (min … max):    2.834 s …  3.020 s    10 runs

    Benchmark 2: make -C Documentation -j1 lint-docs' in 'HEAD~0
      Time (mean ± σ):      2.315 s ±  0.062 s    [User: 1.950 s, System: 0.386 s]
      Range (min … max):    2.229 s …  2.397 s    10 runs

    Summary
      'make -C Documentation -j1 lint-docs' in 'HEAD~0' ran
        1.26 ± 0.04 times faster than 'make -C Documentation -j1 lint-docs' in 'HEAD~1'

So let's use that pattern both for the "lint-docs" target, and a few
miscellaneous other targets.

This method of creating parent directories is explicitly racy in that
we don't know if we're going to say always create a "foo" followed by
a "foo/bar" under parallelism, or skip the "foo" because we created
"foo/bar" first. In this case it doesn't matter for anything except
that we aren't guaranteed to get the same number of rules firing when
running make in parallel.

1. https://lore.kernel.org/git/211028.861r45y3pt.gmgdl@evledraar.gmail.com/
2. https://lore.kernel.org/git/211028.86o879vvtp.gmgdl@evledraar.gmail.com/
3. https://gitlab.com/avar/git-hyperfine/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/shared.mak b/shared.mak
index c45b2812eb..4e1b62ee99 100644
--- a/shared.mak
+++ b/shared.mak
@@ -53,6 +53,8 @@ ifndef V
 	QUIET          = @
 	QUIET_GEN      = @echo '   ' GEN $@;
 
+	QUIET_MKDIR_P_PARENT  = @echo $(wspfx_SQ) MKDIR -p $(@D);
+
 ## Used in "Makefile"
 	QUIET_CC       = @echo '   ' CC $@;
 	QUIET_AR       = @echo '   ' AR $@;
@@ -84,3 +86,18 @@ ifndef V
 	export V
 endif
 endif
+
+### Templates
+
+## mkdir_p_parent: lazily "mkdir -p" the path needed for a $@
+## file. Uses $(wildcard) to avoid the "mkdir -p" if it's not
+## needed.
+##
+## Is racy, but in a good way; we might redundantly (and safely)
+## "mkdir -p" when running in parallel, but won't need to exhaustively create
+## individual rules for "a" -> "prefix" -> "dir" -> "file" if given a
+## "a/prefix/dir/file". This can instead be inserted at the start of
+## the "a/prefix/dir/file" rule.
+define mkdir_p_parent_template
+$(if $(wildcard $(@D)),,$(QUIET_MKDIR_P_PARENT)$(shell mkdir -p $(@D)))
+endef