Issue #1159051: Back out a fix for handling corrupted gzip files that

broke backwards compatibility.

Lib/gzip.py

@@ -33,6 +33,9 @@ def write32u(output, value):
     # or unsigned.
     output.write(struct.pack("<L", value))
 
+def read32(input):
+    return struct.unpack("<I", input.read(4))[0]
+
 def open(filename, mode="rb", compresslevel=9):
     """Shorthand for GzipFile(filename, mode, compresslevel).
 
@@ -256,32 +259,27 @@ class GzipFile(io.BufferedIOBase):
         self.crc = zlib.crc32(b"") & 0xffffffff
         self.size = 0
 
-    def _read_exact(self, n):
-        data = self.fileobj.read(n)
-        while len(data) < n:
-            b = self.fileobj.read(n - len(data))
-            if not b:
-                raise EOFError("Compressed file ended before the "
-                               "end-of-stream marker was reached")
-            data += b
-        return data
-
     def _read_gzip_header(self):
         magic = self.fileobj.read(2)
         if magic == b'':
-            return False
+            raise EOFError("Reached EOF")
 
         if magic != b'\037\213':
             raise IOError('Not a gzipped file')
-
-        method, flag, self.mtime = struct.unpack("<BBIxx", self._read_exact(8))
+        method = ord( self.fileobj.read(1) )
         if method != 8:
             raise IOError('Unknown compression method')
+        flag = ord( self.fileobj.read(1) )
+        self.mtime = read32(self.fileobj)
+        # extraflag = self.fileobj.read(1)
+        # os = self.fileobj.read(1)
+        self.fileobj.read(2)
 
         if flag & FEXTRA:
             # Read & discard the extra field, if present
-            extra_len, = struct.unpack("<H", self._read_exact(2))
-            self._read_exact(extra_len)
+            xlen = ord(self.fileobj.read(1))
+            xlen = xlen + 256*ord(self.fileobj.read(1))
+            self.fileobj.read(xlen)
         if flag & FNAME:
             # Read and discard a null-terminated string containing the filename
             while True:
@@ -295,13 +293,12 @@ class GzipFile(io.BufferedIOBase):
                 if not s or s==b'\000':
                     break
         if flag & FHCRC:
-            self._read_exact(2)     # Read & discard the 16-bit header CRC
+            self.fileobj.read(2)     # Read & discard the 16-bit header CRC
 
         unused = self.fileobj.unused()
         if unused:
             uncompress = self.decompress.decompress(unused)
             self._add_read_data(uncompress)
-        return True
 
     def write(self,data):
         self._check_closed()
@@ -335,16 +332,20 @@ class GzipFile(io.BufferedIOBase):
 
         readsize = 1024
         if size < 0:        # get the whole thing
-            while self._read(readsize):
+            try:
+                while True:
+                    self._read(readsize)
                     readsize = min(self.max_read_chunk, readsize * 2)
+            except EOFError:
                 size = self.extrasize
         else:               # just get some more of it
+            try:
                 while size > self.extrasize:
-                if not self._read(readsize):
+                    self._read(readsize)
+                    readsize = min(self.max_read_chunk, readsize * 2)
+            except EOFError:
                 if size > self.extrasize:
                     size = self.extrasize
-                    break
-                readsize = min(self.max_read_chunk, readsize * 2)
 
         offset = self.offset - self.extrastart
         chunk = self.extrabuf[offset: offset + size]
@@ -365,9 +366,12 @@ class GzipFile(io.BufferedIOBase):
         if self.extrasize == 0:
             if self.fileobj is None:
                 return b''
+            try:
                 # Ensure that we don't return b"" if we haven't reached EOF.
+                while self.extrasize == 0:
                     # 1024 is the same buffering heuristic used in read()
-            while self.extrasize == 0 and self._read(max(n, 1024)):
+                    self._read(max(n, 1024))
+            except EOFError:
                 pass
         offset = self.offset - self.extrastart
         remaining = self.extrasize
@@ -380,14 +384,13 @@ class GzipFile(io.BufferedIOBase):
 
     def _read(self, size=1024):
         if self.fileobj is None:
-            return False
+            raise EOFError("Reached EOF")
 
         if self._new_member:
             # If the _new_member flag is set, we have to
             # jump to the next member, if there is one.
             self._init_read()
-            if not self._read_gzip_header():
-                return False
+            self._read_gzip_header()
             self.decompress = zlib.decompressobj(-zlib.MAX_WBITS)
             self._new_member = False
 
@@ -404,7 +407,7 @@ class GzipFile(io.BufferedIOBase):
             self.fileobj.prepend(self.decompress.unused_data, True)
             self._read_eof()
             self._add_read_data( uncompress )
-            return False
+            raise EOFError('Reached EOF')
 
         uncompress = self.decompress.decompress(buf)
         self._add_read_data( uncompress )
@@ -420,7 +423,6 @@ class GzipFile(io.BufferedIOBase):
             # a new member on the next call
             self._read_eof()
             self._new_member = True
-        return True
 
     def _add_read_data(self, data):
         self.crc = zlib.crc32(data, self.crc) & 0xffffffff
@@ -435,7 +437,8 @@ class GzipFile(io.BufferedIOBase):
         # We check the that the computed CRC and size of the
         # uncompressed data matches the stored values.  Note that the size
         # stored is the true file size mod 2**32.
-        crc32, isize = struct.unpack("<II", self._read_exact(8))
+        crc32 = read32(self.fileobj)
+        isize = read32(self.fileobj)  # may exceed 2GB
         if crc32 != self.crc:
             raise IOError("CRC check failed %s != %s" % (hex(crc32),
                                                          hex(self.crc)))

Lib/test/test_bz2.py

@@ -292,24 +292,6 @@ class BZ2FileTest(BaseTest):
             self.assertRaises(ValueError, f.readline)
             self.assertRaises(ValueError, f.readlines)
 
-    def test_read_truncated(self):
-        # Drop the eos_magic field (6 bytes) and CRC (4 bytes).
-        truncated = self.DATA[:-10]
-        with open(self.filename, 'wb') as f:
-            f.write(truncated)
-        with BZ2File(self.filename) as f:
-            self.assertRaises(EOFError, f.read)
-        with BZ2File(self.filename) as f:
-            self.assertEqual(f.read(len(self.TEXT)), self.TEXT)
-            self.assertRaises(EOFError, f.read, 1)
-        # Incomplete 4-byte file header, and block header of at least 146 bits.
-        for i in range(22):
-            with open(self.filename, 'wb') as f:
-                f.write(truncated[:i])
-            with BZ2File(self.filename) as f:
-                self.assertRaises(EOFError, f.read, 1)
-
-
 class BZ2CompressorTest(BaseTest):
     def testCompress(self):
         # "Test BZ2Compressor.compress()/flush()"

Lib/test/test_gzip.py

@@ -365,19 +365,6 @@ class TestGzip(unittest.TestCase):
             datac = gzip.compress(data)
             self.assertEqual(gzip.decompress(datac), data)
 
-    def test_read_truncated(self):
-        data = data1*50
-        # Drop the CRC (4 bytes) and file size (4 bytes).
-        truncated = gzip.compress(data)[:-8]
-        with gzip.GzipFile(fileobj=io.BytesIO(truncated)) as f:
-            self.assertRaises(EOFError, f.read)
-        with gzip.GzipFile(fileobj=io.BytesIO(truncated)) as f:
-            self.assertEqual(f.read(len(data)), data)
-            self.assertRaises(EOFError, f.read, 1)
-        # Incomplete 10-byte header.
-        for i in range(2, 10):
-            with gzip.GzipFile(fileobj=io.BytesIO(truncated[:i])) as f:
-                self.assertRaises(EOFError, f.read, 1)
 
     def test_read_with_extra(self):
         # Gzip data with an extra field

Misc/NEWS

@@ -14,11 +14,12 @@ Library
   which were omitted in 3.2.4 when updating the bundled version of
   libffi used by ctypes.
 
-- Issue #17666: Fix reading gzip files with an extra field.
-
 - Issue #15535: Fix namedtuple pickles which were picking up the OrderedDict
   instead of just the underlying tuple.
 
+- Issue #1159051: Back out a fix for handling corrupted gzip files that
+  broke backwards compatibility.
+
 Build
 -----