ENH: make the Meson that is used user-configurable

Allow using a MESON environment variable, analogous to the NINJA one,
or a string to an executable or Python script in the
`[tool.meson-python]` section of pyproject.toml

Closes mesonbuildgh-458

mesonpy/__init__.py

@@ -528,7 +528,13 @@ def _bool(value: Any, name: str) -> bool:
             raise ConfigError(f'Configuration entry "{name}" must be a boolean')
         return value
 
+    def _str(value: Any, name: str) -> bool:
+        if not isinstance(value, str):
+            raise ConfigError(f'Configuration entry "{name}" must be a string')
+        return value
+
     scheme = _table({
+        'cli': _str,
         'limited-api': _bool,
         'args': _table({
             name: _strings for name in _MESON_ARGS_KEYS
@@ -607,7 +613,22 @@ def __init__(  # noqa: C901
         self._meson_args: MesonArgs = collections.defaultdict(list)
         self._limited_api = False
 
-        _check_meson_version()
+        # load pyproject.toml
+        pyproject = tomllib.loads(self._source_dir.joinpath('pyproject.toml').read_text())
+
+        # load meson args from pyproject.toml
+        pyproject_config = _validate_pyproject_config(pyproject)
+        for key, value in pyproject_config.get('args', {}).items():
+            self._meson_args[key].extend(value)
+
+        # meson arguments from the command line take precedence over
+        # arguments from the configuration file thus are added later
+        if meson_args:
+            for key, value in meson_args.items():
+                self._meson_args[key].extend(value)
+
+        # determine command to invoke meson
+        self._meson = _get_meson_command(pyproject_config)
 
         self._ninja = _env_ninja_command()
         if self._ninja is None:
@@ -645,20 +666,6 @@ def __init__(  # noqa: C901
                     self._meson_cross_file.write_text(cross_file_data)
                     self._meson_args['setup'].extend(('--cross-file', os.fspath(self._meson_cross_file)))
 
-        # load pyproject.toml
-        pyproject = tomllib.loads(self._source_dir.joinpath('pyproject.toml').read_text())
-
-        # load meson args from pyproject.toml
-        pyproject_config = _validate_pyproject_config(pyproject)
-        for key, value in pyproject_config.get('args', {}).items():
-            self._meson_args[key].extend(value)
-
-        # meson arguments from the command line take precedence over
-        # arguments from the configuration file thus are added later
-        if meson_args:
-            for key, value in meson_args.items():
-                self._meson_args[key].extend(value)
-
         # write the native file
         native_file_data = textwrap.dedent(f'''
             [binaries]
@@ -740,7 +747,7 @@ def _configure(self, reconfigure: bool = False) -> None:
         ]
         if reconfigure:
             setup_args.insert(0, '--reconfigure')
-        self._run(['meson', 'setup', *setup_args])
+        self._run(self._meson + ['setup', *setup_args])
 
     @property
     def _build_command(self) -> List[str]:
@@ -750,7 +757,7 @@ def _build_command(self) -> List[str]:
             # environment. Using the --ninja-args option allows to
             # provide the exact same semantics for the compile arguments
             # provided by the users.
-            cmd = ['meson', 'compile']
+            cmd = self._meson + ['compile']
             args = list(self._meson_args['compile'])
             if args:
                 cmd.append(f'--ninja-args={args!r}')
@@ -824,7 +831,7 @@ def version(self) -> str:
     def sdist(self, directory: Path) -> pathlib.Path:
         """Generates a sdist (source distribution) in the specified directory."""
         # generate meson dist file
-        self._run(['meson', 'dist', '--allow-dirty', '--no-tests', '--formats', 'gztar', *self._meson_args['dist']])
+        self._run(self._meson + ['dist', '--allow-dirty', '--no-tests', '--formats', 'gztar', *self._meson_args['dist']])
 
         # move meson dist file to output path
         dist_name = f'{self.name}-{self.version}'
@@ -919,6 +926,31 @@ def _parse_version_string(string: str) -> Tuple[int, ...]:
         return (0, )
 
 
+def _get_meson_command(
+        meson_tool: str | None = None, /, *, version: str = _MESON_REQUIRED_VERSION
+    ) -> List[str, ...]:
+    """Return the command to invoke meson.
+    Uses the MESON env var if set, else the `cli` entry in pyproject.toml if given,
+    else the standard `meson` executable.
+    """
+    _meson = os.environ.get('MESON')
+    if _meson is None:
+        _meson = meson_tool if meson_tool else 'meson'
+
+    # If the specified Meson string ends in `.py`, we run it with the current
+    # Python executable. This avoids problems for users on Windows, where
+    # making a script executable isn't enough to get it to run when invoked
+    # directly. For packages that vendor a forked Meson, the `meson.py` in the
+    # root of the Meson repo can be used this way.
+    if _meson.endswith(".py"):
+        meson_cli = [sys.executable, _meson]
+    else:
+        meson_cli = [_meson]
+
+    _check_meson_version(meson_cli, version=version)
+    return meson_cli
+
+
 def _env_ninja_command(*, version: str = _NINJA_REQUIRED_VERSION) -> Optional[str]:
     """Returns the path to ninja, or None if no ninja found."""
     required_version = _parse_version_string(version)
@@ -933,7 +965,7 @@ def _env_ninja_command(*, version: str = _NINJA_REQUIRED_VERSION) -> Optional[st
     return None
 
 
-def _check_meson_version(*, version: str = _MESON_REQUIRED_VERSION) -> None:
+def _check_meson_version(meson_cli : List[str, ...], /, *, version: str = _MESON_REQUIRED_VERSION) -> None:
     """Check that the meson executable in the path has an appropriate version.
 
     The meson Python package is a dependency of the meson-python
@@ -944,7 +976,7 @@ def _check_meson_version(*, version: str = _MESON_REQUIRED_VERSION) -> None:
 
     """
     required_version = _parse_version_string(version)
-    meson_version = subprocess.run(['meson', '--version'], check=False, text=True, capture_output=True).stdout
+    meson_version = subprocess.run(meson_cli + ['--version'], check=False, text=True, capture_output=True).stdout
     if _parse_version_string(meson_version) < required_version:
         raise ConfigError(f'Could not find meson version {version} or newer, found {meson_version}.')