Fix issues with timetz type I/O

For timetz an offset is applied to a time itself, it is not a time zone offset.
This was masked by a symmetrical bug in the encoder.

An offset value is still passed in a tuple (timetz_decode_tuple) as it was
received from Postgres.

asyncpg/protocol/codecs/datetime.pyx

@@ -287,7 +287,10 @@ cdef timetz_encode(ConnectionSettings settings, WriteBuffer buf, obj):
 
     buf.write_int32(12)
     _encode_time(buf, seconds, microseconds)
-    buf.write_int32(offset_sec)
+    # In Python utcoffset() is the difference between the local time
+    # and the UTC, whereas in PostgreSQL it's the opposite,
+    # so we need to flip the sign.
+    buf.write_int32(-offset_sec)
 
 
 cdef timetz_encode_tuple(ConnectionSettings settings, WriteBuffer buf, obj):
@@ -311,7 +314,8 @@ cdef timetz_encode_tuple(ConnectionSettings settings, WriteBuffer buf, obj):
 cdef timetz_decode(ConnectionSettings settings, FastReadBuffer buf):
     time = time_decode(settings, buf)
     cdef int32_t offset = <int32_t>(hton.unpack_int32(buf.read(4)) / 60)
-    return time.replace(tzinfo=datetime.timezone(timedelta(minutes=offset)))
+    # See the comment in the `timetz_encode` method.
+    return time.replace(tzinfo=datetime.timezone(timedelta(minutes=-offset)))
 
 
 cdef timetz_decode_tuple(ConnectionSettings settings, FastReadBuffer buf):

tests/test_codecs.py

@@ -1129,6 +1129,27 @@ def _decoder(value):
         finally:
             await conn.close()
 
+    async def test_timetz_encoding(self):
+        try:
+            async with self.con.transaction():
+                await self.con.execute("SET TIME ZONE 'America/Toronto'")
+                # Check decoding:
+                row = await self.con.fetchrow(
+                    'SELECT extract(epoch from now()) AS epoch, '
+                    'now()::date as date, now()::timetz as time')
+                result = datetime.datetime.combine(row['date'], row['time'])
+                expected = datetime.datetime.fromtimestamp(row['epoch'],
+                                                           tz=result.tzinfo)
+                self.assertEqual(result, expected)
+
+                # Check encoding:
+                res = await self.con.fetchval(
+                    'SELECT now() = ($1::date + $2::timetz)',
+                    row['date'], row['time'])
+                self.assertTrue(res)
+        finally:
+            await self.con.execute('RESET ALL')
+
     async def test_composites_in_arrays(self):
         await self.con.execute('''
             CREATE TYPE t AS (a text, b int);