[3.13] pythongh-130655: Add a test for bad magic numbers in .mo files parsed by gettext (pythonGH-131909) (python#132077)

pythongh-130655: Add a test for bad magic numbers in `.mo` files parsed by `gettext` (pythonGH-131909)
(cherry picked from commit 16a6270)

Co-authored-by: Tomas R <[email protected]>

Author: miss-islington

Lib/test/test_gettext.py

@@ -39,6 +39,9 @@
 bmsgd2luayAoaW4gIm15IG90aGVyIGNvbnRleHQiKQB3aW5rIHdpbmsA
 '''
 
+# .mo file with an invalid magic number
+GNU_MO_DATA_BAD_MAGIC_NUMBER = base64.b64encode(b'ABCD')
+
 # This data contains an invalid major version number (5)
 # An unexpected major version number should be treated as an error when
 # parsing a .mo file
@@ -111,6 +114,7 @@
 
 LOCALEDIR = os.path.join('xx', 'LC_MESSAGES')
 MOFILE = os.path.join(LOCALEDIR, 'gettext.mo')
+MOFILE_BAD_MAGIC_NUMBER = os.path.join(LOCALEDIR, 'gettext_bad_magic_number.mo')
 MOFILE_BAD_MAJOR_VERSION = os.path.join(LOCALEDIR, 'gettext_bad_major_version.mo')
 MOFILE_BAD_MINOR_VERSION = os.path.join(LOCALEDIR, 'gettext_bad_minor_version.mo')
 UMOFILE = os.path.join(LOCALEDIR, 'ugettext.mo')
@@ -131,6 +135,8 @@ def setUpClass(cls):
             os.makedirs(LOCALEDIR)
         with open(MOFILE, 'wb') as fp:
             fp.write(base64.decodebytes(GNU_MO_DATA))
+        with open(MOFILE_BAD_MAGIC_NUMBER, 'wb') as fp:
+            fp.write(base64.decodebytes(GNU_MO_DATA_BAD_MAGIC_NUMBER))
         with open(MOFILE_BAD_MAJOR_VERSION, 'wb') as fp:
             fp.write(base64.decodebytes(GNU_MO_DATA_BAD_MAJOR_VERSION))
         with open(MOFILE_BAD_MINOR_VERSION, 'wb') as fp:
@@ -249,6 +255,16 @@ def test_bindtextdomain(self):
     def test_textdomain(self):
         self.assertEqual(gettext.textdomain(), 'gettext')
 
+    def test_bad_magic_number(self):
+        with open(MOFILE_BAD_MAGIC_NUMBER, 'rb') as fp:
+            with self.assertRaises(OSError) as cm:
+                gettext.GNUTranslations(fp)
+
+            exception = cm.exception
+            self.assertEqual(exception.errno, 0)
+            self.assertEqual(exception.strerror, "Bad magic number")
+            self.assertEqual(exception.filename, MOFILE_BAD_MAGIC_NUMBER)
+
     def test_bad_major_version(self):
         with open(MOFILE_BAD_MAJOR_VERSION, 'rb') as fp:
             with self.assertRaises(OSError) as cm: