bpo-43316: gzip: CLI uses non-zero return code on error. (GH-24647)

Exit code is now 1 instead of 0. A message is printed to stderr instead of stdout. This is
the proper behaviour for a tool that can be used in scripts.
(cherry picked from commit cc3df63)

Co-authored-by: Ruben Vorderman <r.h.p.vorderman@lumc.nl>

Author: miss-islington

Lib/gzip.py

@@ -575,8 +575,7 @@ def main():
                 g = sys.stdout.buffer
             else:
                 if arg[-3:] != ".gz":
-                    print("filename doesn't end in .gz:", repr(arg))
-                    continue
+                    sys.exit("filename doesn't end in .gz:", repr(arg))
                 f = open(arg, "rb")
                 g = builtins.open(arg[:-3], "wb")
         else:

Lib/test/test_gzip.py

@@ -760,10 +760,10 @@ def test_decompress_infile_outfile(self):
         self.assertEqual(err, b'')
 
     def test_decompress_infile_outfile_error(self):
-        rc, out, err = assert_python_ok('-m', 'gzip', '-d', 'thisisatest.out')
-        self.assertIn(b"filename doesn't end in .gz:", out)
-        self.assertEqual(rc, 0)
-        self.assertEqual(err, b'')
+        rc, out, err = assert_python_failure('-m', 'gzip', '-d', 'thisisatest.out')
+        self.assertIn(b"filename doesn't end in .gz:", err)
+        self.assertEqual(rc, 1)
+        self.assertEqual(out, b'')
 
     @create_and_remove_directory(TEMPDIR)
     def test_compress_stdin_outfile(self):

Misc/NEWS.d/next/Library/2021-02-25-09-44-36.bpo-43316.k9Gyqn.rst

@@ -0,0 +1,3 @@
+The ``python -m gzip`` command line application now properly fails when
+detecting an unsupported extension. It exits with a non-zero exit code and
+prints an error message to stderr.