gh-119070: Fix py.exe handling of /usr/bin/env commands missing extension (GH-119426)

(cherry picked from commit df93f5d4bf)

Co-authored-by: Steve Dower <steve.dower@python.org>

Lib/test/test_launcher.py

@@ -764,3 +764,11 @@ class TestLauncher(unittest.TestCase, RunPyMixin):
             with self.script(f'#! /usr/bin/env {exe.stem} arg1') as script:
                 data = self.run_py([script], env=env)
             self.assertEqual(data["stdout"].strip(), f"{quote(exe)} arg1 {quote(script)}")
+
+    def test_shebang_executable_extension(self):
+        with self.script('#! /usr/bin/env python3.12') as script:
+            data = self.run_py([script])
+        expect = "# Search PATH for python3.12.exe"
+        actual = [line.strip() for line in data["stderr"].splitlines()
+                  if line.startswith("# Search PATH")]
+        self.assertEqual([expect], actual)

Misc/NEWS.d/next/Windows/2024-05-22-19-43-29.gh-issue-119070._enton.rst

@@ -0,0 +1,3 @@
+Fixes ``py.exe`` handling of shebangs like ``/usr/bin/env python3.12``,
+which were previously interpreted as ``python3.exe`` instead of
+``python3.12.exe``.

PC/launcher2.c

@@ -853,7 +853,7 @@ searchPath(SearchInfo *search, const wchar_t *shebang, int shebangLength)
     }
 
     wchar_t filename[MAXLEN];
-    if (wcsncpy_s(filename, MAXLEN, command, lastDot)) {
+    if (wcsncpy_s(filename, MAXLEN, command, commandLength)) {
         return RC_BAD_VIRTUAL_PATH;
     }