Merge pull request dpkp#127 from GregBowyer/master

Make it possible to read and write xerial snappy

Author: dpkp

kafka/codec.py

@@ -1,5 +1,9 @@
 from cStringIO import StringIO
 import gzip
+import struct
+
+_XERIAL_V1_HEADER = (-126, 'S', 'N', 'A', 'P', 'P', 'Y', 0, 1, 1)
+_XERIAL_V1_FORMAT = 'bccccccBii'
 
 try:
     import snappy
@@ -36,13 +40,101 @@ def gzip_decode(payload):
     return result
 
 
-def snappy_encode(payload):
+def snappy_encode(payload, xerial_compatible=False, xerial_blocksize=32 * 1024):
+    """Encodes the given data with snappy if xerial_compatible is set then the
+       stream is encoded in a fashion compatible with the xerial snappy library
+
+       The block size (xerial_blocksize) controls how frequent the blocking occurs
+       32k is the default in the xerial library.
+
+       The format winds up being
+        +-------------+------------+--------------+------------+--------------+
+        |   Header    | Block1 len | Block1 data  | Blockn len | Blockn data  |
+        |-------------+------------+--------------+------------+--------------|
+        |  16 bytes   |  BE int32  | snappy bytes |  BE int32  | snappy bytes |
+        +-------------+------------+--------------+------------+--------------+
+
+        It is important to not that the blocksize is the amount of uncompressed
+        data presented to snappy at each block, whereas the blocklen is the
+        number of bytes that will be present in the stream, that is the
+        length will always be <= blocksize.
+    """
+
     if not _has_snappy:
         raise NotImplementedError("Snappy codec is not available")
-    return snappy.compress(payload)
+
+    if xerial_compatible:
+        def _chunker():
+            for i in xrange(0, len(payload), xerial_blocksize):
+                yield payload[i:i+xerial_blocksize]
+
+        out = StringIO()
+
+        header = ''.join([struct.pack('!' + fmt, dat) for fmt, dat
+            in zip(_XERIAL_V1_FORMAT, _XERIAL_V1_HEADER)])
+
+        out.write(header)
+        for chunk in _chunker():
+            block = snappy.compress(chunk)
+            block_size = len(block)
+            out.write(struct.pack('!i', block_size))
+            out.write(block)
+
+        out.seek(0)
+        return out.read()
+
+    else:
+        return snappy.compress(payload)
+
+
+def _detect_xerial_stream(payload):
+    """Detects if the data given might have been encoded with the blocking mode
+        of the xerial snappy library.
+
+        This mode writes a magic header of the format:
+            +--------+--------------+------------+---------+--------+
+            | Marker | Magic String | Null / Pad | Version | Compat |
+            |--------+--------------+------------+---------+--------|
+            |  byte  |   c-string   |    byte    |  int32  | int32  |
+            |--------+--------------+------------+---------+--------|
+            |  -126  |   'SNAPPY'   |     \0     |         |        |
+            +--------+--------------+------------+---------+--------+
+
+        The pad appears to be to ensure that SNAPPY is a valid cstring
+        The version is the version of this format as written by xerial,
+        in the wild this is currently 1 as such we only support v1.
+
+        Compat is there to claim the miniumum supported version that
+        can read a xerial block stream, presently in the wild this is
+        1.
+    """
+
+    if len(payload) > 16:
+        header = header = struct.unpack('!' + _XERIAL_V1_FORMAT, bytes(payload)[:16])
+        return header == _XERIAL_V1_HEADER
+    return False
 
 
 def snappy_decode(payload):
     if not _has_snappy:
         raise NotImplementedError("Snappy codec is not available")
-    return snappy.decompress(payload)
+
+    if _detect_xerial_stream(payload):
+        # TODO ? Should become a fileobj ?
+        out = StringIO()
+        byt = buffer(payload[16:])
+        length = len(byt)
+        cursor = 0
+
+        while cursor < length:
+            block_size = struct.unpack_from('!i', byt[cursor:])[0]
+            # Skip the block size
+            cursor += 4
+            end = cursor + block_size
+            out.write(snappy.decompress(byt[cursor:end]))
+            cursor = end
+
+        out.seek(0)
+        return out.read()
+    else:
+        return snappy.decompress(payload)

test/test_unit.py

@@ -70,6 +70,49 @@ def test_snappy(self):
             s2 = snappy_decode(snappy_encode(s1))
             self.assertEquals(s1, s2)
 
+    @unittest.skipUnless(has_snappy(), "Snappy not available")
+    def test_snappy_detect_xerial(self):
+        import kafka as kafka1
+        _detect_xerial_stream = kafka1.codec._detect_xerial_stream
+
+        header = b'\x82SNAPPY\x00\x00\x00\x00\x01\x00\x00\x00\x01Some extra bytes'
+        false_header = b'\x01SNAPPY\x00\x00\x00\x01\x00\x00\x00\x01'
+        random_snappy = snappy_encode('SNAPPY' * 50)
+        short_data = b'\x01\x02\x03\x04'
+
+        self.assertTrue(_detect_xerial_stream(header))
+        self.assertFalse(_detect_xerial_stream(b''))
+        self.assertFalse(_detect_xerial_stream(b'\x00'))
+        self.assertFalse(_detect_xerial_stream(false_header))
+        self.assertFalse(_detect_xerial_stream(random_snappy))
+        self.assertFalse(_detect_xerial_stream(short_data))
+
+    @unittest.skipUnless(has_snappy(), "Snappy not available")
+    def test_snappy_decode_xerial(self):
+        header = b'\x82SNAPPY\x00\x00\x00\x00\x01\x00\x00\x00\x01'
+        random_snappy = snappy_encode('SNAPPY' * 50)
+        block_len = len(random_snappy)
+        random_snappy2 = snappy_encode('XERIAL' * 50)
+        block_len2 = len(random_snappy2)
+
+        to_test = header \
+            + struct.pack('!i', block_len) + random_snappy \
+            + struct.pack('!i', block_len2) + random_snappy2 \
+
+        self.assertEquals(snappy_decode(to_test), ('SNAPPY' * 50) + ('XERIAL' * 50))
+
+    @unittest.skipUnless(has_snappy(), "Snappy not available")
+    def test_snappy_encode_xerial(self):
+        to_ensure = b'\x82SNAPPY\x00\x00\x00\x00\x01\x00\x00\x00\x01' + \
+            '\x00\x00\x00\x18' + \
+            '\xac\x02\x14SNAPPY\xfe\x06\x00\xfe\x06\x00\xfe\x06\x00\xfe\x06\x00\x96\x06\x00' + \
+            '\x00\x00\x00\x18' + \
+            '\xac\x02\x14XERIAL\xfe\x06\x00\xfe\x06\x00\xfe\x06\x00\xfe\x06\x00\x96\x06\x00'
+
+        to_test = ('SNAPPY' * 50) + ('XERIAL' * 50)
+
+        compressed = snappy_encode(to_test, xerial_compatible=True, xerial_blocksize=300)
+        self.assertEquals(compressed, to_ensure)
 
 class TestProtocol(unittest.TestCase):