py-ns-dir.mk: Support __init__.py generation

For in-tree tests, the jw package is split across several projects, and
each directory containing a py-ns-dir.mk contributes its subtree. So far,
it's up to that directory how the contribution is handled. Usually an
__init__.py with pkgutil.extend_path() is present in version control to
glue these parts together. This commit makes py-ns-dir.mk handle the
contribution method centrally by default unless PY_UPDATE_INIT_PY is set to
false.

As of this commit, this is the case by default, i.e. PY_UPDATE_INIT_PY is
set to false in py-ns-dir.mk, maintaining the current behaviour.

Downstream projects or modules can decide to have __init__.py centrally
maintained. For this, they need to remove __init__.py from version control
and set PY_UPDATE_INIT_PY to true in the respective Makefile.

PY_UPDATE_INIT_PY = false is also set explicitly in src/python/jw/Makefile,
because that file should never be generated. Bootstrapping the entire
workspace hinges on it.

Pending better testing, most notably of consistent in-tree testing
functionality, PY_UPDATE_INIT_PY = true might become a global default in
the future.

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

make/py-ns-dir.mk

@@ -1,7 +1,26 @@
+define INIT_PY
+from pkgutil import extend_path
+
+__path__ = extend_path(__path__, __name__)
+endef
+
+# The __init__.py machinery duplicates (but is independent from) py-mod.mk
+PY_UPDATE_INIT_PY           ?= true
+PY_INSTALL_INIT_PY          ?= false
+
+ifneq ($(PY_UPDATE_INIT_PY),false)
+  PY_GENERATED_PY           += __init__.py
+endif
+
 include $(JWBDIR)/make/dirs.mk
 
-all:
+all: $(PY_GENERATED_PY)
 
 clean: py.clean
 py.clean:
-	rm -rf __pycache__
+	rm -rf __pycache__ $(PY_GENERATED_PY)
+
+__init__.py:
+	@echo "Creating default namespace stub $@.tmp"
+	$(file >$@.tmp,$(INIT_PY))
+	mv $@.tmp $@

src/python/jw/Makefile

@@ -1,4 +1,6 @@
 TOPDIR = ../../..
 
+PY_UPDATE_INIT_PY = false
+
 include $(TOPDIR)/make/proj.mk
 include $(JWBDIR)/make/py-ns-dir.mk