fix crash in generator.process() with an invalid preserve_path_from

This code cleverly tried to use a fancy new pathlib.Path method to get
the os.path.commonpath of two paths and check whether one is inside the
other. It failed pretty badly, because of a hidden secret of pathlib: it
is designed to throw random exceptions at all times (except when
building os.PathLike interfaces) instead of performing useful work.
Return to `os.path`.

In particular, before this change, we wanted to check if files are NOT
in a subpath of `preserve_path_from`, and raise a meson "ERROR: xxx" in
such a case. However, the code to check for it would raise a python
ValueError if that was the case, so we never got to the properly
formatted error.

mesonbuild/build.py

@@ -1800,8 +1800,11 @@ class Generator(HoldableObject):
 
     @staticmethod
     def is_parent_path(parent: str, trial: str) -> bool:
-        relpath = pathlib.PurePath(trial).relative_to(parent)
-        return relpath.parts[0] != '..' # For subdirs we can only go "down".
+        try:
+            common = os.path.commonpath((parent, trial))
+        except ValueError: # Windows on different drives
+            return False
+        return pathlib.PurePath(common) == pathlib.PurePath(parent)
 
     def process_files(self, files: T.Iterable[T.Union[str, File, 'CustomTarget', 'CustomTargetIndex', 'GeneratedList']],
                       state: T.Union['Interpreter', 'ModuleState'],

test cases/common/168 preserve gendir/meson.build

@@ -9,5 +9,10 @@ gen = generator(gprog, \
 generated = gen.process('base.inp', 'com/mesonbuild/subbie.inp',
   preserve_path_from : meson.current_source_dir())
 
+testcase expect_error('generator.process: When using preserve_path_from, all input files must be in a subdirectory of the given dir.')
+  generated = gen.process('base.inp', 'com/mesonbuild/subbie.inp',
+    preserve_path_from : meson.current_source_dir() / 'com')
+endtestcase
+
 e = executable('testprog', 'testprog.c', generated)
 test('testprog', e)