python: Fix using overrides when not building PyGObject

Since 547570cd790f2b2e390edc1dfb5df4c7a33de45c we do not always build
PyGObject and our development environment is broken when trying to use
GStreamer python when built against system PyGObject with the following
error importing Gst in there:

```
12345678** (gst-plugin-scanner:710617): CRITICAL **: 11:45:02.343: can't find gi.repository.Gst
Traceback (most recent call last):
  File "/usr/lib/python3.9/site-packages/gi/repository/__init__.py", line 23, in <module>
    from ..importer import DynamicImporter
  File "/usr/lib64/python3.9/site-packages/gi/importer.py", line 33, in <module>
    from .overrides import load_overrides
ImportError: cannot import name 'load_overrides' from 'gi.overrides' (/var/home/thiblahute/devel/gstreamer/gstreamer/subprojects/gst-editing-services/bindings/python/gi/overrides/__init__.py)
Factory Details:
```

The approach to fixing it is to implement override `gi` in
`gst-python/gi/` which we add to `PYTHONPATH`) and in there reset the
`gi` module to the right place and we get overrides from paths from
`_GI_OVERRIDES_PATH` we set in `gst-env.py` which points to all the
overrides that will be installed.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1155>

1 files changed, 12 insertions, 1 deletions

diff --git a/gst-env.py b/gst-env.py
index 7b7d3ae7f7..c64042d92b 100755
--- a/gst-env.py
+++ b/gst-env.py
@@ -386,6 +386,7 @@ def get_subprocess_env(options, gst_version):
     presets = set()
     encoding_targets = set()
     python_dirs = setup_gdb(options)
+    overrides_dirs = set()
     if '--installed' in subprocess.check_output(meson + ['introspect', '-h']).decode():
         installed_s = subprocess.check_output(meson + ['introspect', options.builddir, '--installed'])
         for path, installpath in json.loads(installed_s.decode()).items():
@@ -406,7 +407,10 @@ def get_subprocess_env(options, gst_version):
             if 'site-packages' in installpath_parts:
                 install_subpath = os.path.join(*installpath_parts[installpath_parts.index('site-packages') + 1:])
                 if path.endswith(install_subpath):
-                    python_dirs.add(path[:len (install_subpath) * -1])
+                    if os.path.commonprefix(["gi/overrides", install_subpath]):
+                        overrides_dirs.add(os.path.dirname(path))
+                    else:
+                        python_dirs.add(path[:len (install_subpath) * -1])
 
             if path.endswith('.prs'):
                 presets.add(os.path.dirname(path))
@@ -432,11 +436,18 @@ def get_subprocess_env(options, gst_version):
     for python_dir in sorted(python_dirs):
         prepend_env_var(env, 'PYTHONPATH', python_dir, options.sysroot)
 
+    for python_dir in sorted(overrides_dirs):
+        prepend_env_var(env, '_GI_OVERRIDES_PATH', python_dir, options.sysroot)
+
     mesonpath = os.path.join(SCRIPTDIR, "meson")
     if os.path.join(mesonpath):
         # Add meson/ into PYTHONPATH if we are using a local meson
         prepend_env_var(env, 'PYTHONPATH', mesonpath, options.sysroot)
 
+    # Ensure that gst-python/gi is used first
+    prepend_env_var(env, "PYTHONPATH", os.path.join(SCRIPTDIR, 'subprojects', 'gst-python'),
+                    options.sysroot)
+
     # For devhelp books
     if 'XDG_DATA_DIRS' not in env or not env['XDG_DATA_DIRS']:
         # Preserve default paths when empty