Bug 1748383 - In configure, make e.g. os.path.join mean the same thing whether importing os or not. r=firefox-build-system-reviewers,mhentges

Currently, using any of the functions defined in
`ConfigureSandbox.OS.path` in a `@depends` function will have a different
behavior if the function has an `@imports("os")` or not. In the former
case, we get plain `os.path.$function`, while in the latter we get the
function from `ConfigureSandbox.OS.path`, which handles path separators
differently, which makes a significant difference on Windows.

With this change, we now consistently use the versions from
`ConfigureSandbox.OS.path`.

Differential Revision: https://phabricator.services.mozilla.com/D135003

python/mozbuild/mozbuild/configure/__init__.py

@@ -1004,6 +1004,11 @@ class ConfigureSandbox(dict):
         # Special case os and os.environ so that os.environ is our copy of
         # the environment.
         wrapped_os["environ"] = self._environ
+        # Also override some os.path functions with ours.
+        wrapped_path = {}
+        exec_("from os.path import *", {}, wrapped_path)
+        wrapped_path.update(self.OS.path.__dict__)
+        wrapped_os["path"] = ReadOnlyNamespace(**wrapped_path)
         return ReadOnlyNamespace(**wrapped_os)
 
     @memoized_property

python/mozbuild/mozbuild/test/configure/test_configure.py

@@ -278,6 +278,12 @@ class TestConfigure(unittest.TestCase):
 
         self.assertIs(sandbox["foo"](), sys)
 
+        # os.path after an import is a mix of vanilla os.path and sandbox os.path.
+        os_path = {}
+        exec_("from os.path import *", {}, os_path)
+        os_path.update(sandbox.OS.path.__dict__)
+        os_path = ReadOnlyNamespace(**os_path)
+
         exec_(
             textwrap.dedent(
                 """
@@ -289,7 +295,7 @@ class TestConfigure(unittest.TestCase):
             sandbox,
         )
 
-        self.assertIs(sandbox["foo"](), os.path)
+        self.assertEquals(sandbox["foo"](), os_path)
 
         exec_(
             textwrap.dedent(
@@ -302,7 +308,7 @@ class TestConfigure(unittest.TestCase):
             sandbox,
         )
 
-        self.assertIs(sandbox["foo"](), os.path)
+        self.assertEquals(sandbox["foo"](), os_path)
 
         exec_(
             textwrap.dedent(