Update txid_snapshot codec to handle pg_snapshot

Recent Postgres versions have `txid_snapshot` deprecated and now use
`pg_snapshot` instead.  The difference is in the signedness of the
returned transaction id values.

Author: elprans

codecs/__init__.pxd

@@ -141,6 +141,6 @@ cdef inet_encode(CodecContext settings, WriteBuffer buf, obj)
 cdef inet_decode(CodecContext settings, FRBuffer * buf)
 
 
-# txid
-cdef txid_snapshot_encode(CodecContext settings, WriteBuffer buf, obj)
-cdef txid_snapshot_decode(CodecContext settings, FRBuffer * buf)
+# pg_snapshot
+cdef pg_snapshot_encode(CodecContext settings, WriteBuffer buf, obj)
+cdef pg_snapshot_decode(CodecContext settings, FRBuffer * buf)

codecs/txid.pyx renamed to codecs/pg_snapshot.pyx

@@ -5,11 +5,11 @@
 # the Apache 2.0 License: http://www.apache.org/licenses/LICENSE-2.0
 
 
-cdef txid_snapshot_encode(CodecContext settings, WriteBuffer buf, obj):
+cdef pg_snapshot_encode(CodecContext settings, WriteBuffer buf, obj):
     cdef:
         ssize_t nxip
-        int64_t xmin
-        int64_t xmax
+        uint64_t xmin
+        uint64_t xmax
         int i
         WriteBuffer xip_buf = WriteBuffer.new()
 
@@ -29,32 +29,34 @@ cdef txid_snapshot_encode(CodecContext settings, WriteBuffer buf, obj):
     xmax = obj[1]
 
     for i in range(nxip):
-        xip_buf.write_int64(obj[2][i])
+        xip_buf.write_int64(
+            <int64_t>cpython.PyLong_AsUnsignedLongLong(obj[2][i]))
 
     buf.write_int32(20 + xip_buf.len())
 
     buf.write_int32(<int32_t>nxip)
-    buf.write_int64(obj[0])
-    buf.write_int64(obj[1])
+    buf.write_int64(<int64_t>xmin)
+    buf.write_int64(<int64_t>xmax)
     buf.write_buffer(xip_buf)
 
 
-cdef txid_snapshot_decode(CodecContext settings, FRBuffer *buf):
+cdef pg_snapshot_decode(CodecContext settings, FRBuffer *buf):
     cdef:
         int32_t nxip
-        int64_t xmin
-        int64_t xmax
+        uint64_t xmin
+        uint64_t xmax
         tuple xip_tup
         int32_t i
         object xip
 
     nxip = hton.unpack_int32(frb_read(buf, 4))
-    xmin = hton.unpack_int64(frb_read(buf, 8))
-    xmax = hton.unpack_int64(frb_read(buf, 8))
+    xmin = <uint64_t>hton.unpack_int64(frb_read(buf, 8))
+    xmax = <uint64_t>hton.unpack_int64(frb_read(buf, 8))
 
     xip_tup = cpython.PyTuple_New(nxip)
     for i in range(nxip):
-        xip = cpython.PyLong_FromLongLong(hton.unpack_int64(frb_read(buf, 8)))
+        xip = cpython.PyLong_FromUnsignedLongLong(
+            <uint64_t>hton.unpack_int64(frb_read(buf, 8)))
         cpython.Py_INCREF(xip)
         cpython.PyTuple_SET_ITEM(xip_tup, i, xip)
 

pgproto.pyx

@@ -45,4 +45,4 @@ include "./codecs/hstore.pyx"
 include "./codecs/misc.pyx"
 include "./codecs/network.pyx"
 include "./codecs/tid.pyx"
-include "./codecs/txid.pyx"
+include "./codecs/pg_snapshot.pyx"