Cache the result of python.find_installation()

This avoids running sanity checks everytime find_installation() is
called.

mesonbuild/modules/python.py

@@ -637,6 +637,7 @@ class PythonModule(ExtensionModule):
     @FeatureNew('Python Module', '0.46.0')
     def __init__(self, interpreter: 'Interpreter') -> None:
         super().__init__(interpreter)
+        self.installations: T.Dict[str, ExternalProgram] = {}
         self.methods.update({
             'find_installation': self.find_installation,
         })
@@ -658,6 +659,31 @@ class PythonModule(ExtensionModule):
         else:
             return None
 
+    def _find_installation_impl(self, state: 'ModuleState', display_name: str, name_or_path: str) -> ExternalProgram:
+        if not name_or_path:
+            python = PythonExternalProgram('python3', mesonlib.python_command)
+        else:
+            tmp_python = ExternalProgram.from_entry(display_name, name_or_path)
+            python = PythonExternalProgram(display_name, ext_prog=tmp_python)
+
+            if not python.found() and mesonlib.is_windows():
+                pythonpath = self._get_win_pythonpath(name_or_path)
+                if pythonpath is not None:
+                    name_or_path = pythonpath
+                    python = PythonExternalProgram(name_or_path)
+
+            # Last ditch effort, python2 or python3 can be named python
+            # on various platforms, let's not give up just yet, if an executable
+            # named python is available and has a compatible version, let's use
+            # it
+            if not python.found() and name_or_path in ['python2', 'python3']:
+                python = PythonExternalProgram('python')
+
+        if python.found() and not python.sanity(state):
+            python = NonExistingExternalProgram()
+
+        return python
+
     @disablerIfNotFound
     @typed_pos_args('python.find_installation', optargs=[str])
     @typed_kwargs(
@@ -670,9 +696,6 @@ class PythonModule(ExtensionModule):
                           kwargs: 'FindInstallationKw') -> ExternalProgram:
         feature_check = FeatureNew('Passing "feature" option to find_installation', '0.48.0')
         disabled, required, feature = extract_required_kwarg(kwargs, state.subproject, feature_check)
-        want_modules = kwargs['modules']
-        found_modules: T.List[str] = []
-        missing_modules: T.List[str] = []
 
         # FIXME: this code is *full* of sharp corners. It assumes that it's
         # going to get a string value (or now a list of length 1), of `python2`
@@ -691,25 +714,14 @@ class PythonModule(ExtensionModule):
             mlog.log('Program', name_or_path or 'python', 'found:', mlog.red('NO'), '(disabled by:', mlog.bold(feature), ')')
             return NonExistingExternalProgram()
 
-        if not name_or_path:
-            python = PythonExternalProgram('python3', mesonlib.python_command)
-        else:
-            tmp_python = ExternalProgram.from_entry(display_name, name_or_path)
-            python = PythonExternalProgram(display_name, ext_prog=tmp_python)
-
-            if not python.found() and mesonlib.is_windows():
-                pythonpath = self._get_win_pythonpath(name_or_path)
-                if pythonpath is not None:
-                    name_or_path = pythonpath
-                    python = PythonExternalProgram(name_or_path)
-
-            # Last ditch effort, python2 or python3 can be named python
-            # on various platforms, let's not give up just yet, if an executable
-            # named python is available and has a compatible version, let's use
-            # it
-            if not python.found() and name_or_path in ['python2', 'python3']:
-                python = PythonExternalProgram('python')
+        python = self.installations.get(name_or_path)
+        if not python:
+            python = self._find_installation_impl(state, display_name, name_or_path)
+            self.installations[name_or_path] = python
 
+        want_modules = kwargs['modules']
+        found_modules: T.List[str] = []
+        missing_modules: T.List[str] = []
         if python.found() and want_modules:
             for mod in want_modules:
                 p, *_ = mesonlib.Popen_safe(
@@ -743,14 +755,7 @@ class PythonModule(ExtensionModule):
                 raise mesonlib.MesonException('{} is missing modules: {}'.format(name_or_path or 'python', ', '.join(missing_modules)))
             return NonExistingExternalProgram()
         else:
-            sane = python.sanity(state)
-
-            if sane:
-                return python
-            else:
-                if required:
-                    raise mesonlib.MesonException(f'{python} is not a valid python or it is missing distutils')
-                return NonExistingExternalProgram()
+            return python
 
         raise mesonlib.MesonBugException('Unreachable code was reached (PythonModule.find_installation).')