Set the meson command to use when we know what it is

Instead of using fragile guessing to figure out how to invoke meson,
set the value when meson is run. Also rework how we pass of
meson_script_launcher to regenchecker.py -- it wasn't even being used

With this change, we only need to guess the meson path when running
the tests, and in that case:

1. If MESON_EXE is set in the env, we know how to run meson
   for project tests.
2. MESON_EXE is not set, which means we run the configure in-process
   for project tests and need to guess what meson to run, so either
   - meson.py is found next to run_tests.py, or
   - meson, meson.py, or meson.exe is in PATH

Otherwise, you can invoke meson in the following ways:

1. meson is installed, and mesonbuild is available in PYTHONPATH:
   - meson, meson.py, meson.exe from PATH
   - python3 -m mesonbuild.mesonmain
   - python3 /path/to/meson.py
   - meson is a shell wrapper to meson.real
2. meson is not installed, and is run from git:
   - Absolute path to meson.py
   - Relative path to meson.py
   - Symlink to meson.py

All these are tested in test_meson_commands.py, except meson.exe since
that involves building the meson msi and installing it.

1 files changed, 10 insertions, 7 deletions

diff --git a/meson.py b/meson.py
index abbac6f..dc84b51 100755
--- a/meson.py
+++ b/meson.py
@@ -14,13 +14,16 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from mesonbuild import mesonmain
-import sys, os
+import sys
+from pathlib import Path
+
+# If we're run uninstalled, add the script directory to sys.path to ensure that
+# we always import the correct mesonbuild modules even if PYTHONPATH is mangled
+meson_exe = Path(sys.argv[0]).resolve()
+if (meson_exe.parent / 'mesonbuild').is_dir():
+    sys.path.insert(0, meson_exe.parent)
 
-def main():
-    # Always resolve the command path so Ninja can find it for regen, tests, etc.
-    launcher = os.path.realpath(sys.argv[0])
-    return mesonmain.run(sys.argv[1:], launcher)
+from mesonbuild import mesonmain
 
 if __name__ == '__main__':
-    sys.exit(main())
+    sys.exit(mesonmain.main())