gh-103477: Read and write gzip header and trailer with zlib

RHEL, SLES and Ubuntu for IBM zSystems (aka s390x) ship with a zlib
optimization [1] that significantly improves deflate and inflate
performance on this platform by using a specialized CPU instruction.

This instruction not only compresses the data, but also computes a
checksum. At the moment Pyhton's gzip support performs compression and
checksum calculation separately, which creates unnecessary overhead on
s390x.

The reason is that Python needs to write specific values into gzip
header; and when this support was introduced in year 1997, there was
indeed no better way to do this.

Since v1.2.2.1 (2011) zlib provides inflateGetHeader() and
deflateSetHeader() functions for that, so Python does not have to deal
with the exact header and trailer formats anymore.

Add the new interfaces to zlibmodule.c that make use of these
functions:

* Add mtime argument to zlib.compress().
* Add mtime and fname arguments to zlib.compressobj().
* Add gz_header_mtime and gz_header_done propeties to ZlibDecompressor.

In Python modules, replace raw streams with gzip streams, make use of
the new interfaces, and remove all mentions of crc32.

In addition to the new interfaces above, there is an additional
change in behavior that the users can see: for malformed gzip headers
and trailers, decompression now raises zlib.error instead of
BadGzipFile. However, this is allowed by today's spec.

📜🤖 NEWS entry added by blurb_it.

[1] madler/zlib#410

Include/internal/pycore_global_strings.h

@@ -432,6 +432,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(fix_imports)
         STRUCT_FOR_ID(flags)
         STRUCT_FOR_ID(flush)
+        STRUCT_FOR_ID(fname)
         STRUCT_FOR_ID(follow_symlinks)
         STRUCT_FOR_ID(format)
         STRUCT_FOR_ID(frequency)
@@ -548,6 +549,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(modules)
         STRUCT_FOR_ID(mro)
         STRUCT_FOR_ID(msg)
+        STRUCT_FOR_ID(mtime)
         STRUCT_FOR_ID(mycmp)
         STRUCT_FOR_ID(n)
         STRUCT_FOR_ID(n_arg)

Lib/gzip.py

@@ -217,12 +217,15 @@ def __init__(self, filename=None, mode=None,
                     FutureWarning, 2)
             self.mode = WRITE
             self._init_write(filename)
+            if mtime is None:
+                mtime = int(time.time())
             self.compress = zlib.compressobj(compresslevel,
                                              zlib.DEFLATED,
-                                             -zlib.MAX_WBITS,
+                                             16 + zlib.MAX_WBITS,
                                              zlib.DEF_MEM_LEVEL,
-                                             0)
-            self._write_mtime = mtime
+                                             0,
+                                             mtime=mtime,
+                                             fname=self._encode_fname())
             self._buffer_size = _WRITE_BUFFER_SIZE
             self._buffer = io.BufferedWriter(_WriteBufferStream(self),
                                              buffer_size=self._buffer_size)
@@ -231,9 +234,6 @@ def __init__(self, filename=None, mode=None,
 
         self.fileobj = fileobj
 
-        if self.mode == WRITE:
-            self._write_gzip_header(compresslevel)
-
     @property
     def mtime(self):
         """Last modification time read from stream, or None"""
@@ -245,7 +245,6 @@ def __repr__(self):
 
     def _init_write(self, filename):
         self.name = filename
-        self.crc = zlib.crc32(b"")
         self.size = 0
         self.writebuf = []
         self.bufsize = 0
@@ -256,9 +255,7 @@ def tell(self):
         self._buffer.flush()
         return super().tell()
 
-    def _write_gzip_header(self, compresslevel):
-        self.fileobj.write(b'\037\213')             # magic header
-        self.fileobj.write(b'\010')                 # compression method
+    def _encode_fname(self):
         try:
             # RFC 1952 requires the FNAME field to be Latin-1. Do not
             # include filenames that cannot be represented that way.
@@ -269,24 +266,7 @@ def _write_gzip_header(self, compresslevel):
                 fname = fname[:-3]
         except UnicodeEncodeError:
             fname = b''
-        flags = 0
-        if fname:
-            flags = FNAME
-        self.fileobj.write(chr(flags).encode('latin-1'))
-        mtime = self._write_mtime
-        if mtime is None:
-            mtime = time.time()
-        write32u(self.fileobj, int(mtime))
-        if compresslevel == _COMPRESS_LEVEL_BEST:
-            xfl = b'\002'
-        elif compresslevel == _COMPRESS_LEVEL_FAST:
-            xfl = b'\004'
-        else:
-            xfl = b'\000'
-        self.fileobj.write(xfl)
-        self.fileobj.write(b'\377')
-        if fname:
-            self.fileobj.write(fname + b'\000')
+        return fname
 
     def write(self,data):
         self._check_not_closed()
@@ -311,7 +291,6 @@ def _write_raw(self, data):
         if length > 0:
             self.fileobj.write(self.compress.compress(data))
             self.size += length
-            self.crc = zlib.crc32(data, self.crc)
             self.offset += length
 
         return length
@@ -355,9 +334,6 @@ def close(self):
             if self.mode == WRITE:
                 self._buffer.flush()
                 fileobj.write(self.compress.flush())
-                write32u(fileobj, self.crc)
-                # self.size may exceed 2 GiB, or even 4 GiB
-                write32u(fileobj, self.size & 0xffffffff)
             elif self.mode == READ:
                 self._buffer.close()
         finally:
@@ -439,62 +415,17 @@ def _read_exact(fp, n):
     return data
 
 
-def _read_gzip_header(fp):
-    '''Read a gzip header from `fp` and progress to the end of the header.
-
-    Returns last mtime if header was present or None otherwise.
-    '''
-    magic = fp.read(2)
-    if magic == b'':
-        return None
-
-    if magic != b'\037\213':
-        raise BadGzipFile('Not a gzipped file (%r)' % magic)
-
-    (method, flag, last_mtime) = struct.unpack("<BBIxx", _read_exact(fp, 8))
-    if method != 8:
-        raise BadGzipFile('Unknown compression method')
-
-    if flag & FEXTRA:
-        # Read & discard the extra field, if present
-        extra_len, = struct.unpack("<H", _read_exact(fp, 2))
-        _read_exact(fp, extra_len)
-    if flag & FNAME:
-        # Read and discard a null-terminated string containing the filename
-        while True:
-            s = fp.read(1)
-            if not s or s==b'\000':
-                break
-    if flag & FCOMMENT:
-        # Read and discard a null-terminated string containing a comment
-        while True:
-            s = fp.read(1)
-            if not s or s==b'\000':
-                break
-    if flag & FHCRC:
-        _read_exact(fp, 2)     # Read & discard the 16-bit header CRC
-    return last_mtime
-
-
 class _GzipReader(_compression.DecompressReader):
     def __init__(self, fp):
         super().__init__(_PaddedFile(fp), zlib._ZlibDecompressor,
-                         wbits=-zlib.MAX_WBITS)
+                         wbits=16 + zlib.MAX_WBITS)
         # Set flag indicating start of a new member
         self._new_member = True
         self._last_mtime = None
 
     def _init_read(self):
-        self._crc = zlib.crc32(b"")
         self._stream_size = 0  # Decompressed size of unconcatenated stream
 
-    def _read_gzip_header(self):
-        last_mtime = _read_gzip_header(self._fp)
-        if last_mtime is None:
-            return False
-        self._last_mtime = last_mtime
-        return True
-
     def read(self, size=-1):
         if size < 0:
             return self.readall()
@@ -508,33 +439,35 @@ def read(self, size=-1):
         while True:
             if self._decompressor.eof:
                 # Ending case: we've come to the end of a member in the file,
-                # so finish up this member, and read a new gzip header.
-                # Check the CRC and file size, and set the flag so we read
-                # a new member
+                # so finish up this member and set the flag, so that we read a
+                # new member
                 self._read_eof()
                 self._new_member = True
                 self._decompressor = self._decomp_factory(
                     **self._decomp_args)
 
-            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():
-                    self._size = self._pos
-                    return b""
-                self._new_member = False
-
             # Read a chunk of data from the file
             if self._decompressor.needs_input:
                 buf = self._fp.read(READ_BUFFER_SIZE)
+                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 buf:
+                        self._size = self._pos
+                        return b""
+                    self._new_member = False
                 uncompress = self._decompressor.decompress(buf, size)
             else:
+                assert not self._new_member
                 uncompress = self._decompressor.decompress(b"", size)
 
+            if self._decompressor.gz_header_done:
+                self._last_mtime = self._decompressor.gz_header_mtime
+
             if self._decompressor.unused_data != b"":
                 # Prepend the already read bytes to the fileobj so they can
-                # be seen by _read_eof() and _read_gzip_header()
+                # be seen by _read_eof()
                 self._fp.prepend(self._decompressor.unused_data)
 
             if uncompress != b"":
@@ -543,23 +476,12 @@ def read(self, size=-1):
                 raise EOFError("Compressed file ended before the "
                                "end-of-stream marker was reached")
 
-        self._crc = zlib.crc32(uncompress, self._crc)
         self._stream_size += len(uncompress)
         self._pos += len(uncompress)
         return uncompress
 
     def _read_eof(self):
         # We've read to the end of the file
-        # We check 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", _read_exact(self._fp, 8))
-        if crc32 != self._crc:
-            raise BadGzipFile("CRC check failed %s != %s" % (hex(crc32),
-                                                             hex(self._crc)))
-        elif isize != (self._stream_size & 0xffffffff):
-            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
         # non-zero byte. See http://www.gzip.org/#faq8
@@ -574,68 +496,32 @@ def _rewind(self):
         self._new_member = True
 
 
-def _create_simple_gzip_header(compresslevel: int,
-                               mtime = None) -> bytes:
-    """
-    Write a simple gzip header with no extra fields.
-    :param compresslevel: Compresslevel used to determine the xfl bytes.
-    :param mtime: The mtime (must support conversion to a 32-bit integer).
-    :return: A bytes object representing the gzip header.
-    """
-    if mtime is None:
-        mtime = time.time()
-    if compresslevel == _COMPRESS_LEVEL_BEST:
-        xfl = 2
-    elif compresslevel == _COMPRESS_LEVEL_FAST:
-        xfl = 4
-    else:
-        xfl = 0
-    # Pack ID1 and ID2 magic bytes, method (8=deflate), header flags (no extra
-    # fields added to header), mtime, xfl and os (255 for unknown OS).
-    return struct.pack("<BBBBLBB", 0x1f, 0x8b, 8, 0, int(mtime), xfl, 255)
-
-
 def compress(data, compresslevel=_COMPRESS_LEVEL_BEST, *, mtime=None):
     """Compress data in one shot and return the compressed string.
 
     compresslevel sets the compression level in range of 0-9.
     mtime can be used to set the modification time. The modification time is
     set to the current time by default.
     """
-    if mtime == 0:
-        # Use zlib as it creates the header with 0 mtime by default.
-        # This is faster and with less overhead.
-        return zlib.compress(data, level=compresslevel, wbits=31)
-    header = _create_simple_gzip_header(compresslevel, mtime)
-    trailer = struct.pack("<LL", zlib.crc32(data), (len(data) & 0xffffffff))
-    # Wbits=-15 creates a raw deflate block.
-    return (header + zlib.compress(data, level=compresslevel, wbits=-15) +
-            trailer)
+    if mtime is None:
+        mtime = int(time.time())
+    return zlib.compress(data, level=compresslevel, wbits=31, mtime=mtime)
 
 
 def decompress(data):
     """Decompress a gzip compressed string in one shot.
     Return the decompressed string.
     """
     decompressed_members = []
-    while True:
-        fp = io.BytesIO(data)
-        if _read_gzip_header(fp) is None:
-            return b"".join(decompressed_members)
-        # Use a zlib raw deflate compressor
-        do = zlib.decompressobj(wbits=-zlib.MAX_WBITS)
-        # Read all the data except the header
-        decompressed = do.decompress(data[fp.tell():])
-        if not do.eof or len(do.unused_data) < 8:
+    while data:
+        do = zlib.decompressobj(wbits=16 + zlib.MAX_WBITS)
+        decompressed = do.decompress(data)
+        if not do.eof:
             raise EOFError("Compressed file ended before the end-of-stream "
                            "marker was reached")
-        crc, length = struct.unpack("<II", do.unused_data[:8])
-        if crc != zlib.crc32(decompressed):
-            raise BadGzipFile("CRC check failed")
-        if length != (len(decompressed) & 0xffffffff):
-            raise BadGzipFile("Incorrect length of data produced")
         decompressed_members.append(decompressed)
-        data = do.unused_data[8:].lstrip(b"\x00")
+        data = do.unused_data.lstrip(b"\x00")
+    return b"".join(decompressed_members)
 
 
 def main():