bpo-6584: Add a BadGzipFile exception to the gzip module

Doc/library/gzip.rst

@@ -59,6 +59,12 @@ The module defines the following items:
  .. versionchanged:: 3.6
  Accepts a :term:`path-like object`.
 
+.. exception:: BadGzipFile
+
+ The exception raised for invalid gzip files. It inherits :class:`OSError`.
+
+ .. versionadded:: 3.8
+
 .. class:: GzipFile(filename=None, mode=None, compresslevel=9, fileobj=None, mtime=None)
 
  Constructor for the :class:`GzipFile` class, which simulates most of the

Doc/whatsnew/3.8.rst

@@ -289,6 +289,11 @@ gzip
 Added the *mtime* parameter to :func:`gzip.compress` for reproducible output.
 (Contributed by Guo Ci Teo in :issue:`34898`.)
 
+A :exc:`~gzip.BadGzipFile` exception is now raised in case of invalid or
+corrupt gzip files instead of :exc:`OSError`.
+(Contributed by Filip Gruszczyński, Michele Orrù, and Zackery Spytz in
+:issue:`6584`.)
+
 
 idlelib and IDLE
 ----------------

Lib/gzip.py

@@ -11,7 +11,7 @@
 import io
 import _compression
 
-__all__ = ["GzipFile", "open", "compress", "decompress"]
+__all__ = ["BadGzipFile", "GzipFile", "open", "compress", "decompress"]
 
 FTEXT, FHCRC, FEXTRA, FNAME, FCOMMENT = 1, 2, 4, 8, 16
 
@@ -112,6 +112,11 @@ def seek(self, off):
  def seekable(self):
  return True # Allows fast-forwarding even in unseekable streams
 
+
+class BadGzipFile(OSError):
+ """Exception raised if the processed gzip file is not valid."""
+
+
 class GzipFile(_compression.BaseStream):
  """The GzipFile class simulates most of the methods of a file object with
  the exception of the truncate() method.
@@ -413,12 +418,12 @@ def _read_gzip_header(self):
  return False
 
  if magic != b'\037\213':
- raise OSError('Not a gzipped file (%r)' % magic)
+ raise BadGzipFile('Not a gzipped file (%r)' % magic)
 
  (method, flag,
  self._last_mtime) = struct.unpack("<BBIxx", self._read_exact(8))
  if method != 8:
- raise OSError('Unknown compression method')
+ raise BadGzipFile('Unknown compression method')
 
  if flag & FEXTRA:
  # Read & discard the extra field, if present
@@ -502,10 +507,10 @@ def _read_eof(self):
  # stored is the true file size mod 2**32.
  crc32, isize = struct.unpack("<II", self._read_exact(8))
  if crc32 != self._crc:
- raise OSError("CRC check failed %s != %s" % (hex(crc32),
- hex(self._crc)))
+ raise BadGzipFile("CRC check failed %s != %s" % (hex(crc32),
+  hex(self._crc)))
  elif isize != (self._stream_size & 0xffffffff):
- raise OSError("Incorrect length of data produced")
+ raise BadGzipFile("Incorrect length of data produced")
 
  # Gzip files can be padded with zeroes and still have archives.
  # Consume all zero bytes and set the file position to the first

Lib/test/test_gzip.py

@@ -391,6 +391,15 @@ def test_zero_padded_file(self):
  d = f.read()
  self.assertEqual(d, data1 * 50, "Incorrect data in file")
 
+ def test_gzip_BadGzipFile_exception(self):
+ self.assertTrue(issubclass(gzip.BadGzipFile, OSError))
+
+ def test_bad_gzip_file(self):
+ with open(self.filename, 'wb') as file:
+ file.write(data1 * 50)
+ with gzip.GzipFile(self.filename, 'r') as file:
+ self.assertRaises(gzip.BadGzipFile, file.readlines)
+
  def test_non_seekable_file(self):
  uncompressed = data1 * 50
  buf = UnseekableIO()

Misc/NEWS.d/next/Library/2019-04-30-04-34-53.bpo-6584.Hzp9-P.rst

@@ -0,0 +1 @@
+Add a :exc:`~gzip.BadGzipFile` exception to the :mod:`gzip` module.