Fix simply running testsuite in meson

- Make sure to never have root folder in sys.path when running meson,
  as pythondetector won't be able to access gi._overridesdir

- Generate a mesonconfig.py file that will be used by the testsuite to
  know where meson generated files, making `python -m unittest` working.

gi/overrides/meson.build

@@ -12,7 +12,7 @@ gstpython = shared_library('_gi_gst',
 
 # Workaround to get uninstalled working.
 foreach source: pysources
-    run_command(python, '-c', 'import os; os.symlink("@0@/@1@", "@2@/@3@")'.format(
+    run_command(python3, '-c', 'import os; os.symlink("@0@/@1@", "@2@/@3@")'.format(
         meson.current_source_dir(), source,
         meson.current_build_dir(), source))
 endforeach

meson.build

@@ -21,7 +21,7 @@ gmodule_dep = dependency('gmodule-2.0')
 pygobject_dep = dependency('pygobject-3.0', version : '>= 3.0')
 python_dep = dependency('python3')
 
-python = find_program('python3')
+python3 = import('python3').find_python()
 pythondetector = find_program('scripts/pythondetector')
 
 cres = run_command(pythondetector, '--sosuffix')
@@ -83,5 +83,4 @@ subdir('gi')
 subdir('plugin')
 subdir('testsuite')
 
-python3 = find_program('python3')
 run_command(python3, '-c', 'import shutil; shutil.copy("hooks/pre-commit.hook", ".git/hooks/pre-commit")')

scripts/pythondetector

@@ -7,6 +7,12 @@ import sys
 from distutils import sysconfig
 
 
+try:
+    sys.path.remove(os.path.abspath(os.path.join(os.path.dirname(os.path.realpath(__file__)),
+                                                 '..')))
+except ValueError:
+    pass
+
 def get_python_abiflags():
     return sysconfig.get_config_var("ABIFLAGS")
 

testsuite/meson.build

@@ -17,6 +17,14 @@ if not meson.is_subproject()
   endif
 endif
 
+runcmd = run_command(python3, '-c', '''with open("@0@/mesonconfig.py", "w") as f:
+    f.write("path='@1@'")'''.format(
+  meson.current_source_dir(), join_paths(meson.current_build_dir(), '../')))
+
+if runcmd.returncode() != 0
+    error('Could not configure testsuite config file.' + runcmd.stderr())
+endif
+
 foreach i: tests
     test_name = i.get(0)
     env = environment()

testsuite/overrides_hack.py

@@ -1,6 +1,12 @@
 import os
 import gi.overrides
 
+try:
+    import mesonconfig
+except ImportError:
+    mesonconfig = None
+    pass
+
 FILE = os.path.realpath(__file__)
 if not gi.overrides.__path__[0].endswith("gst-python/gi/overrides"):
     local_overrides = None
@@ -16,6 +22,8 @@ if not gi.overrides.__path__[0].endswith("gst-python/gi/overrides"):
 
     gi.overrides.__path__.insert(0, local_overrides)
 
+if mesonconfig:
+    gi.overrides.__path__.insert(0, os.path.abspath(os.path.join(mesonconfig.path, "gi", "overrides")))
 # Execute previously set sitecustomize.py script if it existed
 if os.environ.get("GST_ENV"):
     old_sitecustomize = os.path.join(os.path.dirname(__file__),