Use shell command substitution correctly when choosing the test job count

tests/Makefile

@@ -25,16 +25,7 @@ help-recurse-list:
 	@echo " which are ones that have been selected as useful tests.)"
 	@echo ""
 
-# To produce incremental output, invoke make as: make MPARG=-Otarget ...
-# To not run in parallel, invoke make as: make MPARG=-j1 ...
-ifeq (4.0,$(firstword $(sort ${MAKE_VERSION} 4.0)))
-  # Version 4.0 or higher
-  MPARG ?= -Orecurse
-else
-  MPARG ?=
-endif
-
-PLUME_SCRIPTS ?= ${DAIKONDIR}/.utils/plume-scripts
+include $(dir $(lastword $(MAKEFILE_LIST)))Makefile.jobs
 
 all diffs: txt-diff non-txt-diff dyncomp-tests results
 
@@ -51,12 +42,7 @@ esc jml inv txt-daikon-diff txt-esc-diff txt-jml-diff txt-csharpcontract-diff tx
 txt-chicory-diff everything-txt-diff stats dyncomp-diff txt-jaif-diff txt-jaif:
 	${MAKE} pat-daikon-tests-$@
 pat-daikon-tests-%:
-# Under CI, there are two CPUs, but limit to 1 to avoid out-of-memory error.
-	if [ -n "$("${PLUME_SCRIPTS}"/is-ci.sh)" ]; then \
-	  ${MAKE} -C daikon-tests --jobs=1 ${MPARG} $*; \
-	else \
-	  ${MAKE} -C daikon-tests --jobs=4 ${MPARG} $*; \
-	fi
+	${MAKE} -C daikon-tests --jobs=${DAIKON_TEST_JOBS} ${MPARG} $*
 
 list-targets:
 	${MAKE} -C daikon-tests list-targets

tests/Makefile.jobs

@@ -0,0 +1,28 @@
+# This is a -*- makefile -*-, Emacs
+#
+# Settings shared by the Makefiles that drive a whole test suite:
+# tests/Makefile, tests/kvasir-tests/Makefile, and tests/dyncomp-tests/Makefile.
+# The including Makefile locates this file via $(lastword $(MAKEFILE_LIST)),
+# so its include line must precede any other include in that Makefile.
+
+# To produce incremental output, invoke make as: make MPARG=-Otarget ...
+# To not run in parallel, invoke make as: make MPARG=-j1 ...
+ifeq (4.0,$(firstword $(sort ${MAKE_VERSION} 4.0)))
+  # Version 4.0 or higher
+  MPARG ?= -Orecurse
+else
+  MPARG ?=
+endif
+
+ifndef PLUME_SCRIPTS
+  PLUME_SCRIPTS := $(or ${DAIKONDIR},$(realpath $(dir $(lastword $(MAKEFILE_LIST)))..))/.utils/plume-scripts
+endif
+
+# DAIKON_TEST_JOBS is the number of tests to run in parallel.
+# Under CI, limit to 1 to avoid out-of-memory errors.
+# To override it, set DAIKON_TEST_JOBS on the command line or in the environment.
+# The default is computed only when a recipe uses it, so targets such as
+# "clean" and "help" work even if plume-scripts has not been cloned.
+ifeq ($(strip ${DAIKON_TEST_JOBS}),)
+  override DAIKON_TEST_JOBS = $(if $(wildcard ${PLUME_SCRIPTS}/is-ci.sh),$(if $(shell sh "${PLUME_SCRIPTS}/is-ci.sh"),1,4),$(error ${PLUME_SCRIPTS}/is-ci.sh not found; clone plume-scripts there, or set DAIKON_TEST_JOBS))
+endif

tests/dyncomp-tests/Makefile

@@ -18,7 +18,7 @@
 
 # Forked from $DAIKONDIR/tests/kvasir-tests/Makefile
 
-PLUME_SCRIPTS ?= ${DAIKONDIR}/.utils/plume-scripts
+include $(dir $(lastword $(MAKEFILE_LIST)))../Makefile.jobs
 
 default: summary
 
@@ -64,11 +64,7 @@ update-all:
 
 .PHONY: clean-all
 clean-all:
-	if [ -n "$("${PLUME_SCRIPTS}"/is-ci.sh)" ]; then \
-	  ${MAKE} --jobs=1 ${MPARG} nightly-tests__clean; \
-	else \
-	  ${MAKE} --jobs=4 ${MPARG} nightly-tests__clean; \
-	fi
+	${MAKE} --jobs=${DAIKON_TEST_JOBS} ${MPARG} nightly-tests__clean
 
 
 default-projects__%: pgbovine-tests__%

tests/kvasir-tests/Makefile

@@ -13,24 +13,13 @@
 #    category to contain it.
 #
 
-ifeq (4.0,$(firstword $(sort ${MAKE_VERSION} 4.0)))
-  # Version 4.0 or higher
-  MPARG ?= -Orecurse
-else
-  MPARG ?=
-endif
-
-PLUME_SCRIPTS ?= ${DAIKONDIR}/.utils/plume-scripts
+include $(dir $(lastword $(MAKEFILE_LIST)))../Makefile.jobs
 
 default: summary-w-daikon
 
 .PHONY: regression-tests
 regression-tests:
-	if [ -n "$("${PLUME_SCRIPTS}"/is-ci.sh)" ]; then \
-	  ${MAKE} --jobs=1 ${MPARG} nightly-tests__diffs-w-daikon-if-working; \
-	else \
-	  ${MAKE} --jobs=4 ${MPARG} nightly-tests__diffs-w-daikon-if-working; \
-	fi
+	${MAKE} --jobs=${DAIKON_TEST_JOBS} ${MPARG} nightly-tests__diffs-w-daikon-if-working
 	${MAKE} nightly-summary-only-w-daikon
 	@${MAKE} --no-print-directory results
 
@@ -80,11 +69,7 @@ update-all:
 
 .PHONY: clean-all
 clean-all:
-	if [ -n "$("${PLUME_SCRIPTS}"/is-ci.sh)" ]; then \
-	  ${MAKE} --jobs=1 ${MPARG} nightly-tests__clean; \
-	else \
-	  ${MAKE} --jobs=4 ${MPARG} nightly-tests__clean; \
-	fi
+	${MAKE} --jobs=${DAIKON_TEST_JOBS} ${MPARG} nightly-tests__clean
 
 default-projects__%: pgbovine-tests__% smcc-tests__% tiny-real-tests__% tws-tests__%
 	@