python: Fix using overrides when not building PyGObject

Since 547570cd79 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>

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():
@@ -406,6 +407,9 @@ 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):
+                    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'):
@@ -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

meson.build

@@ -368,14 +368,12 @@ endforeach
 
 message('Building subprojects: ' + ', '.join(subprojects_names))
 
-subdir('tests')
-
 setenv = find_program('gst-env.py')
-
 devenv_cmd = [setenv, '--builddir=@0@'.format(meson.build_root()),
               '--gstbuilddir=@0@'.format(meson.current_build_dir()),
               '--srcdir=@0@'.format(meson.source_root())]
 
+subdir('tests')
 if meson.has_exe_wrapper() and build_machine.system() == 'linux' and host_machine.system() == 'windows'
   # FIXME: Ideally we could get the wrapper directly from meson
   devenv_cmd += ['--wine', host_machine.cpu_family() == 'x86_64' ? 'wine64' : 'wine32']

subprojects/gst-python/gi/__init__.py

@@ -0,0 +1,45 @@
+import gi
+import os
+import sys
+import imp
+from pathlib import Path
+
+# Remove this dummy module the python path and
+# try to import the actual gi module
+sys.path.remove(str(Path(__file__).parents[1]))
+del sys.modules["gi"]
+import gi
+
+
+class GstOverrideImport:
+    def find_module(self, fullname, path=None, target=None):
+        if fullname.startswith('gi.overrides'):
+            fp = None
+            try:
+                fp, _, _ = imp.find_module(fullname.split(
+                    '.')[-1], os.environ.get('_GI_OVERRIDES_PATH', '').split(os.pathsep),)
+            except ImportError:
+                return None
+            finally:
+                if fp:
+                    fp.close()
+            return self
+        return None
+
+    def load_module(self, name):
+        if name in sys.modules:
+            return sys.modules[name]
+
+        fp, pathname, description = imp.find_module(name.split(
+            '.')[-1], os.environ.get('_GI_OVERRIDES_PATH', '').split(os.pathsep),)
+
+        try:
+            module = imp.load_module(name, fp, pathname, description)
+        finally:
+            if fp:
+                fp.close()
+        sys.modules[name] = module
+        return module
+
+
+sys.meta_path.insert(0, GstOverrideImport())

tests/meson.build

@@ -2,3 +2,4 @@ if get_option('tests').disabled()
 	subdir_done()
 endif
 subdir('static-plugins')
+subdir('python')

tests/python/meson.build

@@ -0,0 +1,19 @@
+gst_python = subproject('gst-python', required: false)
+gir = find_program('g-ir-scanner', required : get_option('introspection'))
+if not gst_python.found() or not gir.found()
+    message('Not running python devenv tests: gst_python: @0@ gir: @1@'.format(gst_python.found(), gir.found()))
+    subdir_done()
+endif
+
+root_rel = '../..'
+python = import('python').find_installation()
+
+if run_command(python, '-c', 'import gi').returncode() != 0
+    message('PyGObject not found, not running PyGObject tests')
+    subdir_done()
+endif
+
+test('python-overrides-devenv', setenv, args: ['--builddir=@0@'.format(meson.build_root()),
+              '--gstbuilddir=@0@'.format(meson.current_build_dir() / '..' / '..'),
+              '--srcdir=@0@'.format(meson.source_root()),
+              meson.current_source_dir() / 'python-devenv-overrides.py'])

tests/python/python-devenv-overrides.py

@@ -0,0 +1,20 @@
+#!/usr/bin/python3
+import unittest
+from pathlib import Path
+from unittest import TestCase
+from gi.repository import Gst
+
+
+class TestBin(TestCase):
+    def test_overrides(self):
+        from gi.overrides import Gst
+        self.assertEqual(Path(Gst.__file__), Path(__file__).parents[2] / "subprojects/gst-python/gi/overrides/Gst.py")
+
+    def simple_functional_test(self):
+        Gst.init(None)
+        self.assertEqual(Gst.ElementFactory.make("bin", None).sinkpads, [])
+        self.assertEqual(float(Gst.Fraction(1, 2)), 0.5)
+
+
+if __name__ == "__main__":
+    unittest.main()