sanitize_dir: use pathlib to handle case-insensitive filesystems

fixes mesonbuild#6395

mesonbuild/coredata.py

@@ -466,7 +466,7 @@ def sanitize_prefix(self, prefix):
                 prefix = prefix[:-1]
         return prefix
 
-    def sanitize_dir_option_value(self, prefix, option, value):
+    def sanitize_dir_option_value(self, prefix: str, option: str, value: PurePath) -> str:
         '''
         If the option is an installation directory option and the value is an
         absolute path, check that it resides within prefix and return the value
@@ -475,21 +475,25 @@ def sanitize_dir_option_value(self, prefix, option, value):
         This way everyone can do f.ex, get_option('libdir') and be sure to get
         the library directory relative to prefix.
         '''
-        if option.endswith('dir') and os.path.isabs(value) and \
+        value = PurePath(value)
+        if option.endswith('dir') and value.is_absolute() and \
            option not in builtin_dir_noprefix_options:
             # Value must be a subdir of the prefix
             # commonpath will always return a path in the native format, so we
             # must use pathlib.PurePath to do the same conversion before
             # comparing.
-            if os.path.commonpath([value, prefix]) != str(PurePath(prefix)):
-                m = 'The value of the {!r} option is {!r} which must be a ' \
-                    'subdir of the prefix {!r}.\nNote that if you pass a ' \
-                    'relative path, it is assumed to be a subdir of prefix.'
-                raise MesonException(m.format(option, value, prefix))
-            # Convert path to be relative to prefix
-            skip = len(prefix) + 1
-            value = value[skip:]
-        return value
+            msg = ('The value of the {!r} option is {!r} which must be a '
+                   'subdir of the prefix {!r}.\nNote that if you pass a '
+                   'relative path, it is assumed to be a subdir of prefix.')
+            # os.path.commonpath doesn't understand case-insensitive filesystems,
+            # but PurePath().relative_to() does.
+            try:
+                value = value.relative_to(prefix)
+            except ValueError:
+                raise MesonException(msg.format(option, value, prefix))
+            if '..' in str(value):
+                raise MesonException(msg.format(option, value, prefix))
+        return str(value)
 
     def init_builtins(self):
         # Create builtin options with default values