[PATCH 15/16] Makefiles: add and use wildcard "mkdir -p" template

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

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 the performance test I noted downthread of [1] in [2] shows that
this is faster, in addition to being less verbose and more reliable:

    $ hyperfine --warmup 5 -m 30 -L s ",~" -p 'git checkout HEAD{s} -- Makefile; rm -rf .build' 'make -j8 lint-docs R={s}'
     -c 'git checkout HEAD -- Makefile'
    Benchmark #1: make -j8 lint-docs R=
      Time (mean ± σ):     537.5 ms ±   7.1 ms    [User: 2.412 s, System: 0.460 s]
      Range (min … max):   530.2 ms … 564.9 ms    30 runs

    Benchmark #2: make -j8 lint-docs R=~
      Time (mean ± σ):     645.7 ms ±   7.1 ms    [User: 3.083 s, System: 0.630 s]
      Range (min … max):   637.4 ms … 675.2 ms    30 runs

    Summary
      'make -j8 lint-docs R=' ran
        1.20 ± 0.02 times faster than 'make -j8 lint-docs R=~'

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/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Documentation/Makefile | 25 +++----------------------
 Makefile               | 12 +++++++-----
 shared.mak             |  7 +++++++
 3 files changed, 17 insertions(+), 27 deletions(-)

diff --git a/Documentation/Makefile b/Documentation/Makefile
index 69a9af35397..d16b653394c 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -428,25 +428,11 @@ quick-install-html: require-htmlrepo
 print-man1:
 	@for i in $(MAN1_TXT); do echo $$i; done
 
-## Lint: Common
-.build:
-	$(QUIET)mkdir $@
-.build/lint-docs: | .build
-	$(QUIET)mkdir $@
-
 ## Lint: gitlink
-.build/lint-docs/gitlink: | .build/lint-docs
-	$(QUIET)mkdir $@
-.build/lint-docs/gitlink/howto: | .build/lint-docs/gitlink
-	$(QUIET)mkdir $@
-.build/lint-docs/gitlink/config: | .build/lint-docs/gitlink
-	$(QUIET)mkdir $@
 LINT_DOCS_GITLINK = $(patsubst %.txt,.build/lint-docs/gitlink/%.ok,$(HOWTO_TXT) $(DOC_DEP_TXT))
-$(LINT_DOCS_GITLINK): | .build/lint-docs/gitlink
-$(LINT_DOCS_GITLINK): | .build/lint-docs/gitlink/howto
-$(LINT_DOCS_GITLINK): | .build/lint-docs/gitlink/config
 $(LINT_DOCS_GITLINK): lint-gitlink.perl
 $(LINT_DOCS_GITLINK): .build/lint-docs/gitlink/%.ok: %.txt
+	$(call mkdir_p_parent_template)
 	$(QUIET_LINT_GITLINK)$(PERL_PATH) lint-gitlink.perl \
 		$< \
 		$(HOWTO_TXT) $(DOC_DEP_TXT) \
@@ -457,23 +443,18 @@ $(LINT_DOCS_GITLINK): .build/lint-docs/gitlink/%.ok: %.txt
 lint-docs-gitlink: $(LINT_DOCS_GITLINK)
 
 ## Lint: man-end-blurb
-.build/lint-docs/man-end-blurb: | .build/lint-docs
-	$(QUIET)mkdir $@
 LINT_DOCS_MAN_END_BLURB = $(patsubst %.txt,.build/lint-docs/man-end-blurb/%.ok,$(MAN_TXT))
-$(LINT_DOCS_MAN_END_BLURB): | .build/lint-docs/man-end-blurb
 $(LINT_DOCS_MAN_END_BLURB): lint-man-end-blurb.perl
 $(LINT_DOCS_MAN_END_BLURB): .build/lint-docs/man-end-blurb/%.ok: %.txt
+	$(call mkdir_p_parent_template)
 	$(QUIET_LINT_MANEND)$(PERL_PATH) lint-man-end-blurb.perl $< >$@
 .PHONY: lint-docs-man-end-blurb
-lint-docs-man-end-blurb: $(LINT_DOCS_MAN_END_BLURB)
 
 ## Lint: man-section-order
-.build/lint-docs/man-section-order: | .build/lint-docs
-	$(QUIET)mkdir $@
 LINT_DOCS_MAN_SECTION_ORDER = $(patsubst %.txt,.build/lint-docs/man-section-order/%.ok,$(MAN_TXT))
-$(LINT_DOCS_MAN_SECTION_ORDER): | .build/lint-docs/man-section-order
 $(LINT_DOCS_MAN_SECTION_ORDER): lint-man-section-order.perl
 $(LINT_DOCS_MAN_SECTION_ORDER): .build/lint-docs/man-section-order/%.ok: %.txt
+	$(call mkdir_p_parent_template)
 	$(QUIET_LINT_MANSEC)$(PERL_PATH) lint-man-section-order.perl $< >$@
 .PHONY: lint-docs-man-section-order
 lint-docs-man-section-order: $(LINT_DOCS_MAN_SECTION_ORDER)
diff --git a/Makefile b/Makefile
index cd7727ebe39..141a87371fe 100644
--- a/Makefile
+++ b/Makefile
@@ -2622,7 +2622,8 @@ all:: $(MOFILES)
 endif
 
 po/build/locale/%/LC_MESSAGES/git.mo: po/%.po
-	$(QUIET_MSGFMT)mkdir -p $(dir $@) && $(MSGFMT) -o $@ $<
+	$(call mkdir_p_parent_template)
+	$(QUIET_MSGFMT)$(MSGFMT) -o $@ $<
 
 ifndef NO_PERL
 LIB_PERL := $(wildcard perl/Git.pm perl/Git/*.pm perl/Git/*/*.pm perl/Git/*/*/*.pm)
@@ -2631,7 +2632,8 @@ LIB_CPAN := $(wildcard perl/FromCPAN/*.pm perl/FromCPAN/*/*.pm)
 LIB_CPAN_GEN := $(patsubst perl/%.pm,perl/build/lib/%.pm,$(LIB_CPAN))
 
 perl/build/lib/%.pm: perl/%.pm GIT-PERL-DEFINES
-	$(QUIET_GEN)mkdir -p $(dir $@) && \
+	$(call mkdir_p_parent_template)
+	$(QUIET_GEN) \
 	sed -e 's|@@LOCALEDIR@@|$(perl_localedir_SQ)|g' \
 	    -e 's|@@NO_GETTEXT@@|$(NO_GETTEXT_SQ)|g' \
 	    -e 's|@@NO_PERL_CPAN_FALLBACKS@@|$(call shq,$(NO_PERL_CPAN_FALLBACKS))|g' \
@@ -2645,8 +2647,8 @@ endif
 
 # install-man depends on Git.3pm even with NO_PERL=Y
 perl/build/man/man3/Git.3pm: perl/Git.pm
-	$(QUIET_GEN)mkdir -p $(dir $@) && \
-	pod2man $< $@
+	$(call mkdir_p_parent_template)
+	$(QUIET_GEN)pod2man $< $@
 
 FIND_SOURCE_FILES = ( \
 	git ls-files \
@@ -2770,7 +2772,7 @@ test_bindir_programs := $(patsubst %,bin-wrappers/%,$(BINDIR_PROGRAMS_NEED_X) $(
 all:: $(TEST_PROGRAMS) $(test_bindir_programs)
 
 bin-wrappers/%: wrap-for-bin.sh
-	@mkdir -p bin-wrappers
+	$(call mkdir_p_parent_template)
 	$(QUIET_GEN)sed -e '1s|#!.*/sh|#!$(SHELL_PATH_SQ)|' \
 	     -e 's|@@BUILD_DIR@@|$(shell pwd)|' \
 	     -e 's|@@PROG@@|$(patsubst test-%,t/helper/test-%$(X),$(@F))$(patsubst git%,$(X),$(filter $(@F),$(BINDIR_PROGRAMS_NEED_X)))|' < $< > $@ && \
diff --git a/shared.mak b/shared.mak
index 566eb70cf2a..ced88924ea3 100644
--- a/shared.mak
+++ b/shared.mak
@@ -73,6 +73,8 @@ ifndef V
 	QUIET          = @
 	QUIET_GEN      = @echo $(wspfx_sq) GEN $@;
 
+	QUIET_MKDIR_P_PARENT  = @echo $(wspfx_sq) MKDIR -p $(@D);
+
 ## Used in "Makefile"
 	QUIET_CC       = @echo $(wspfx_sq) CC $@;
 	QUIET_AR       = @echo $(wspfx_sq) AR $@;
@@ -104,3 +106,8 @@ ifndef V
 	export V
 endif
 endif
+
+## Helpers
+define mkdir_p_parent_template
+$(if $(wildcard $(@D)),,$(QUIET_MKDIR_P_PARENT)$(shell mkdir -p $(@D)))
+endef
-- 
2.34.0.rc1.741.gab7bfd97031