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[PATCH v2 14/29] perf: add a GIT_PERF_MAKE_COMMAND for when *_MAKE_OPTS won't do

Add a git GIT_PERF_MAKE_COMMAND variable to compliment the existing
GIT_PERF_MAKE_OPTS facility. This allows specifying an arbitrary shell
command to execute instead of 'make'.

This is useful e.g. in cases where the name, semantics or defaults of
a Makefile flag have changed over time. It can even be used to change
the contents of the tree, useful for monkeypatching ancient versions
of git to get them to build.

This opens Pandora's box in some ways, it's now possible to
"jailbreak" the perf environment and e.g. modify the source tree via
this arbitrary instead of just issuing a custom "make" command, such a
command has to be re-entrant in the sense that subsequent perf runs
will re-use the possibly modified tree.

It would be pointless to try to mitigate or work around that caveat in
a tool purely aimed at Git developers, so this change makes no attempt
to do so.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx>
 Makefile      |  3 +++
 t/perf/README | 19 +++++++++++++++++--
 t/perf/run    | 11 +++++++++--
 3 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/Makefile b/Makefile
index eedadb8056..d1587452f1 100644
--- a/Makefile
+++ b/Makefile
@@ -2272,6 +2272,9 @@ endif
 	@echo GIT_PERF_MAKE_OPTS=\''$(subst ','\'',$(subst ','\'',$(GIT_PERF_MAKE_OPTS)))'\' >>$@+
+	@echo GIT_PERF_MAKE_COMMAND=\''$(subst ','\'',$(subst ','\'',$(GIT_PERF_MAKE_COMMAND)))'\' >>$@+
 	@echo GIT_INTEROP_MAKE_OPTS=\''$(subst ','\'',$(subst ','\'',$(GIT_INTEROP_MAKE_OPTS)))'\' >>$@+
diff --git a/t/perf/README b/t/perf/README
index 49ea4349be..b3d95042a8 100644
--- a/t/perf/README
+++ b/t/perf/README
@@ -60,8 +60,23 @@ You can set the following variables (also in your config.mak):
 	Options to use when automatically building a git tree for
-	performance testing.  E.g., -j6 would be useful.
+	performance testing. E.g., -j6 would be useful. Passed
+	directly to make as "make $GIT_PERF_MAKE_OPTS".
+	An arbitrary command that'll be run in place of the make
+	command, if set the GIT_PERF_MAKE_OPTS variable is
+	ignored. Useful in cases where source tree changes might
+	require issuing a different make command to different
+	revisions.
+	This can be (ab)used to monkeypatch or otherwise change the
+	tree about to be built. Note that the build directory can be
+	re-used for subsequent runs so the make command might get
+	executed multiple times on the same tree, but don't count on
+	any of that, that's an implementation detail that might change
+	in the future.
 	Repositories to copy for the performance tests.  The normal
diff --git a/t/perf/run b/t/perf/run
index c788d713ae..b61024a830 100755
--- a/t/perf/run
+++ b/t/perf/run
@@ -37,8 +37,15 @@ build_git_rev () {
 			cp "../../$config" "build/$rev/"
-	(cd build/$rev && make $GIT_PERF_MAKE_OPTS) ||
-	die "failed to build revision '$mydir'"
+	(
+		cd build/$rev &&
+		if test -n "$GIT_PERF_MAKE_COMMAND"
+		then
+		else
+		fi
+	) || die "failed to build revision '$mydir'"
 run_dirs_helper () {