gh-94772: Fix off-by-one error in Windows launcher (GH-94779)

Lib/test/test_launcher.py

@@ -515,6 +515,30 @@ def test_py3_shebang(self):
         self.assertEqual("3.100-arm64", data["SearchInfo.tag"])
         self.assertEqual(f"X.Y-arm64.exe -X fake_arg_for_test -prearg {script} -postarg", data["stdout"].strip())
 
+    def test_py_shebang_nl(self):
+        with self.py_ini(TEST_PY_COMMANDS):
+            with self.script("#! /usr/bin/env python -prearg\n") as script:
+                data = self.run_py([script, "-postarg"])
+        self.assertEqual("PythonTestSuite", data["SearchInfo.company"])
+        self.assertEqual("3.100", data["SearchInfo.tag"])
+        self.assertEqual(f"X.Y.exe -prearg {script} -postarg", data["stdout"].strip())
+
+    def test_py2_shebang_nl(self):
+        with self.py_ini(TEST_PY_COMMANDS):
+            with self.script("#! /usr/bin/env python2 -prearg\n") as script:
+                data = self.run_py([script, "-postarg"])
+        self.assertEqual("PythonTestSuite", data["SearchInfo.company"])
+        self.assertEqual("3.100-32", data["SearchInfo.tag"])
+        self.assertEqual(f"X.Y-32.exe -prearg {script} -postarg", data["stdout"].strip())
+
+    def test_py3_shebang_nl(self):
+        with self.py_ini(TEST_PY_COMMANDS):
+            with self.script("#! /usr/bin/env python3 -prearg\n") as script:
+                data = self.run_py([script, "-postarg"])
+        self.assertEqual("PythonTestSuite", data["SearchInfo.company"])
+        self.assertEqual("3.100-arm64", data["SearchInfo.tag"])
+        self.assertEqual(f"X.Y-arm64.exe -X fake_arg_for_test -prearg {script} -postarg", data["stdout"].strip())
+
     def test_install(self):
         data = self.run_py(["-V:3.10"], env={"PYLAUNCHER_ALWAYS_INSTALL": "1"}, expect_returncode=111)
         cmd = data["stdout"].strip()

Misc/NEWS.d/next/Windows/2022-07-12-20-45-43.gh-issue-94772.uNMmdG.rst

+Fix incorrect handling of shebang lines in py.exe launcher

PC/launcher2.c

@@ -874,7 +874,9 @@ checkShebang(SearchInfo *search)
     while (--bytesRead > 0 && *++b != '\r' && *b != '\n') { }
     wchar_t *shebang;
     int shebangLength;
-    int exitCode = _decodeShebang(search, start, (int)(b - start + 1), onlyUtf8, &shebang, &shebangLength);
+    // We add 1 when bytesRead==0, as in that case we hit EOF and b points
+    // to the last character in the file, not the newline
+    int exitCode = _decodeShebang(search, start, (int)(b - start + (bytesRead == 0)), onlyUtf8, &shebang, &shebangLength);
     if (exitCode) {
         return exitCode;
     }