Port regen: hand the tools a Python that runs

python() returned shutil.which("python3"), which on Git for Windows is the
Microsoft Store stub (prints "Python", exits 49): every tool it launched
failed before its first line. Probe python3, python and py and keep the first
that is Python 3, once per process.

Co-Authored-By: Hermes Agent (Nous Research) <[email protected]>

tests/test_port_regen.py

@@ -1,3 +1,4 @@
+import subprocess
 import unittest
 
 from tools import port_regen as regen
@@ -48,5 +49,13 @@ class CompileErrorTests(unittest.TestCase):
         self.assertEqual(regen.compile_errors("expected the root's order to differ"), [])
 
 
+class InterpreterTests(unittest.TestCase):
+    def test_python_is_an_interpreter_that_runs(self):
+        found = regen.python()
+        done = subprocess.run([found, "-c", "import sys; sys.exit(sys.version_info < (3,))"],
+                              capture_output=True)
+        self.assertEqual(done.returncode, 0, f"{found} is not a Python 3 that runs")
+
+
 if __name__ == "__main__":
     unittest.main()

tools/port_regen.py

@@ -124,8 +124,23 @@ def prepare_local(root, version, found, store):
         run([python(), "tools/lab.py", "prepare", "--minecraft-dir", tree, "--server-jar", found["server"]], root)
 
 
+_PYTHON = None
+
+
 def python():
-    return shutil.which("python3") or "python3"
+    """The interpreter to run the other tools with. Git for Windows puts a
+    `python3` on PATH that is only the Store stub: it prints "Python" and exits
+    49, so probe the names and keep the first one that runs Python 3."""
+    global _PYTHON
+    if _PYTHON is None:
+        _PYTHON = "python3"
+        for name in ("python3", "python", "py"):
+            found = shutil.which(name)
+            probe = [found, "-c", "import sys; sys.exit(sys.version_info < (3,))"] if found else None
+            if probe and subprocess.run(probe, capture_output=True).returncode == 0:
+                _PYTHON = found
+                break
+    return _PYTHON
 
 
 class Failed(RuntimeError):