Set the meson command to use when we know what it isnirbheek/rework-meson-script-handling

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 abbac6f4a..dc84b5134 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())