modules: python: better handling of the Python paths for Debian

Hardcoding the name is fragile, and enabling it based on the existence of
/etc/debian_version (as the is_debianlike helper does) will result in
incorrect paths if the Python binary is not provided by Debian.

Using the deb_system sysconfig scheme instead makes sure we use the
install path from the current interpreter, which Debian could change
between releases, and gives us the correct value on Python installations
that are not provided by Debian (eg. deadsnakes, Github Action Python,
etc.)

Do notice, though, that there is still no guarantee that these are the
correct paths, as they assume all schemes paths have the install prefix
as a base, see mesonbuild#9284.

Signed-off-by: Filipe Laíns <lains@riseup.net>

mesonbuild/modules/python.py

@@ -280,7 +280,18 @@ def set_env(name, value):
 import json
 import sys
 
-install_paths = sysconfig.get_paths(vars={'base': '', 'platbase': '', 'installed_base': ''})
+# On Debian derivatives, the Python interpreter shipped by the distribution uses
+# a custom install scheme, deb_system, for the system install, and changes the
+# default scheme to a custom one pointing to /usr/local and replacing
+# site-packages with dist-packages.
+# See https://github.com/mesonbuild/meson/issues/8739.
+empty_vars = {'base': '', 'platbase': '', 'installed_base': ''}
+if 'deb_system' in sysconfig.get_scheme_names():
+    paths = sysconfig.get_paths('deb_system')
+    install_paths = sysconfig.get_paths('deb_system', vars=empty_vars)
+else:
+    paths = sysconfig.get_paths()
+    install_paths = sysconfig.get_paths(vars=empty_vars)
 
 def links_against_libpython():
     from distutils.core import Distribution, Extension
@@ -290,7 +301,7 @@ def links_against_libpython():
 
 print(json.dumps({
   'variables': sysconfig.get_config_vars(),
-  'paths': sysconfig.get_paths(),
+  'paths': paths,
   'install_paths': install_paths,
   'sys_paths': sys.path,
   'version': sysconfig.get_python_version(),
@@ -373,14 +384,9 @@ def _get_path(self, key: str) -> None:
         sys_paths = self.info['sys_paths']
         rel_path = self.info['install_paths'][key][1:]
         if not any(p.endswith(rel_path) for p in sys_paths if not p.startswith(user_dir)):
-            # On Debian derivatives sysconfig install path is broken and is not
-            # included in the locations python actually lookup.
-            # See https://github.com/mesonbuild/meson/issues/8739.
             mlog.warning('Broken python installation detected. Python files',
                          'installed by Meson might not be found by python interpreter.',
                          once=True)
-            if mesonlib.is_debianlike():
-                rel_path = 'lib/python3/dist-packages'
         return rel_path