test-guard.mk: Add file

Running "make clean" in some directories can break "make test" in
others. That notably happens by running clean somewhere in or below
$(TOPDIR)/src/python, which removes __init__.py files potentially needed
by unit and integration tests in $(TOPDIR)/test.

This commit makes the failure easily visible by introducing the notion
of "testabiltity" - if the tree is not testable, "make test" logs
exactly that in a clear error message and gives up.

Makefiles which need to check "testability" can use it as a
prerequisite. This commit does that with py-run.mk and jw-py-test.mk.

Testability is asserted by running "make", "make all" or "make test"
from $(TOPDIR). A successful toplevel build coincides with testability,
and leaves a $(TOPDIR)/dirs-all.done behind, which is why that file is
the perfect testability marker.

It is automatically created by a toplevel build and cleared by using the
"invalidate-testability" prerequisite. This commit makes target "clean"
depend on it in py-mod.mk. To be extended to other use cases / makefile
snippets as needed.

Signed-off-by: Jan Lindemann <jan@janware.com>

make/test-guard.mk

@@ -0,0 +1,21 @@
+# Testability guard
+#
+# Tree testability is a single top-level property tracked by
+# $(TOPDIR)/dirs-all.done: created by a full `make all` from the topdir,
+# removed by `make clean`. A per-dir `make test` only makes sense when the tree
+# is importable, i.e. testable. So `test` requires testability, and a `make
+# clean` that drops artifacts crucial for testing (a Python module's
+# __init__.py, ...) invalidates it.
+
+.PHONY: testability invalidate-testability
+
+TEST_GUARD                   = $(TOPDIR)/dirs-all.done
+
+all:
+
+testability:
+	@test -f "$(TEST_GUARD)" || \
+		{ echo 'Error: Tree not testable -- run "make all" (or "make test") from the project\'s root to fix' >&2; exit 1; }
+
+invalidate-testability:
+	$(RM) -f "$(TEST_GUARD)"