`make sharedinstall` does not create lib-dynload in DESTDIR if lib-dynload exists in system already

[mgorny]

Bug report

The Makefile.pre.in rule for creating directories is specified as:

$(DESTSHARED):
		@for i in $(DESTDIRS); \
		do \
			if test ! -d $(DESTDIR)$$i; then \
				echo "Creating directory $$i"; \
				$(INSTALL) -d -m $(DIRMODE) $(DESTDIR)$$i; \
			else    true; \
			fi; \
		done

This means that if $(DESTSHARED) exists already (note: missing $(DESTDIR)!), i.e. Python 3.12 has been installed into the system, the directories in $(DESTDIR) won't be created and the successive sharedinstall rules fail due to the missing destination, e.g.:

/usr/bin/install -c -m 755 Modules/array.cpython-312-x86_64-linux-gnu.so /usr/lib/python3.12/lib-dynload/array.cpython-312-x86_64-linux-gnu.so
/usr/bin/install: cannot create regular file '/var/tmp/portage/dev-lang/python-3.12.0_alpha3/image/usr/lib/python3.12/lib-dynload/array.cpython-312-x86_64-linux-gnu.so': No such file or directory
/usr/bin/install -c -m 755 Modules/_asyncio.cpython-312-x86_64-linux-gnu.so /usr/lib/python3.12/lib-dynload/_asyncio.cpython-312-x86_64-linux-gnu.so
/usr/bin/install: cannot create regular file '/var/tmp/portage/dev-lang/python-3.12.0_alpha3/image/usr/lib/python3.12/lib-dynload/_asyncio.cpython-312-x86_64-linux-gnu.so': No such file or directory
/usr/bin/install -c -m 755 Modules/_bisect.cpython-312-x86_64-linux-gnu.so /usr/lib/python3.12/lib-dynload/_bisect.cpython-312-x86_64-linux-gnu.so
/usr/bin/install: cannot create regular file '/var/tmp/portage/dev-lang/python-3.12.0_alpha3/image/usr/lib/python3.12/lib-dynload/_bisect.cpython-312-x86_64-linux-gnu.so': No such file or directory

Full log: dev-lang:python-3.12.0_alpha3:20221207-142002.log

Your environment

• CPython versions tested on: 3.12.0a3
• Operating system and architecture: Gentoo Linux/amd64

Linked PRs

• gh-100221: Fix creating dirs in make sharedinstall #100329

[zware]

Is this only an issue on 3.12, or is 3.11 also affected?

[mgorny]

I think the relevant code technically exists in 3.11 too but it's not being used since setup.py is used to install the extensions.

[zware]

Ok, we'll leave 3.11 as is for now, then.

Thanks for the patch!