setuptools3: check for a PEP517 build system selection

Some recipes use setuptools3 but should be using a PEP517-compliant
backend, be it the setuptools itself via python_setuptools_build_meta or
an alternative backend such as flit/hatch/poetry.

As we can't currently assume Python 3.11 on the build host we need to
parse the pyproject.toml manually, but this should be sufficient for the
limited parsing needed.

This task emits a QA error if a build backend is set, and can be ignored
by using INSANE_SKIP if needed.

[ YOCTO #14736 ]

(From OE-Core rev: a9d800f6cdfcedbe3de8366d935ee5f0c8557c06)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes-recipe/setuptools3.bbclass

@@ -12,6 +12,20 @@ SETUPTOOLS_BUILD_ARGS ?= ""
 
 SETUPTOOLS_SETUP_PATH ?= "${S}"
 
+python do_check_backend() {
+    import re
+    filename = d.expand("${SETUPTOOLS_SETUP_PATH}/pyproject.toml")
+    if os.path.exists(filename):
+        for line in open(filename):
+            match = re.match(r"build-backend\s*=\s*\W([\w.]+)\W", line)
+            if not match: continue
+
+            msg = f"inherits setuptools3 but has pyproject.toml with {match[1]}, use the correct class"
+            if "pep517-backend" not in (d.getVar("INSANE_SKIP") or "").split():
+                oe.qa.handle_error("pep517-backend", msg, d)
+}
+addtask check_backend after do_patch before do_configure
+
 setuptools3_do_configure() {
     :
 }