Fix decoding of fractional timestamps before Postgres epoch

The datetime decoder currently incorrectly uses an unsigned integer type
for microseconds, which leads to incorrect decoding of timestamps with
fractional seconds which represent dates before Jan 1 2000.  Same issue
affects negative intervals with fractional seconds.

Fixes: #363

asyncpg/protocol/codecs/datetime.pyx

@@ -79,20 +79,17 @@ cdef inline _encode_time(WriteBuffer buf, int64_t seconds,
 
 
 cdef inline int32_t _decode_time(FastReadBuffer buf, int64_t *seconds,
-                                 uint32_t *microseconds):
-    # XXX: add support for double timestamps
-    # int64 timestamps,
+                                 int32_t *microseconds):
     cdef int64_t ts = hton.unpack_int64(buf.read(8))
 
     if ts == pg_time64_infinity:
         return 1
     elif ts == pg_time64_negative_infinity:
         return -1
-
-    seconds[0] = <int64_t>(ts / 1000000)
-    microseconds[0] = <uint32_t>(ts % 1000000)
-
-    return 0
+    else:
+        seconds[0] = ts // 1000000
+        microseconds[0] = <int32_t>(ts % 1000000)
+        return 0
 
 
 cdef date_encode(ConnectionSettings settings, WriteBuffer buf, obj):
@@ -181,7 +178,7 @@ cdef timestamp_encode_tuple(ConnectionSettings settings, WriteBuffer buf, obj):
 cdef timestamp_decode(ConnectionSettings settings, FastReadBuffer buf):
     cdef:
         int64_t seconds = 0
-        uint32_t microseconds = 0
+        int32_t microseconds = 0
         int32_t inf = _decode_time(buf, &seconds, &microseconds)
 
     if inf > 0:
@@ -242,7 +239,7 @@ cdef timestamptz_encode(ConnectionSettings settings, WriteBuffer buf, obj):
 cdef timestamptz_decode(ConnectionSettings settings, FastReadBuffer buf):
     cdef:
         int64_t seconds = 0
-        uint32_t microseconds = 0
+        int32_t microseconds = 0
         int32_t inf = _decode_time(buf, &seconds, &microseconds)
 
     if inf > 0:
@@ -285,7 +282,7 @@ cdef time_encode_tuple(ConnectionSettings settings, WriteBuffer buf, obj):
 cdef time_decode(ConnectionSettings settings, FastReadBuffer buf):
     cdef:
         int64_t seconds = 0
-        uint32_t microseconds = 0
+        int32_t microseconds = 0
 
     _decode_time(buf, &seconds, &microseconds)
 
@@ -400,9 +397,10 @@ cdef interval_decode(ConnectionSettings settings, FastReadBuffer buf):
         int32_t months
         int32_t years
         int64_t seconds = 0
-        uint32_t microseconds = 0
+        int32_t microseconds = 0
 
     _decode_time(buf, &seconds, &microseconds)
+
     days = hton.unpack_int32(buf.read(4))
     months = hton.unpack_int32(buf.read(4))
 

tests/test_codecs.py

@@ -169,7 +169,11 @@ def _system_timezone():
         {'textinput': 'infinity', 'output': infinity_datetime},
         {'textinput': '-infinity', 'output': negative_infinity_datetime},
         {'input': datetime.date(2000, 1, 1),
-         'output': datetime.datetime(2000, 1, 1)}
+         'output': datetime.datetime(2000, 1, 1)},
+        {'textinput': '1970-01-01 20:31:23.648',
+         'output': datetime.datetime(1970, 1, 1, 20, 31, 23, 648000)},
+        {'input': datetime.datetime(1970, 1, 1, 20, 31, 23, 648000),
+         'textoutput': '1970-01-01 20:31:23.648'},
     ]),
     ('date', 'date', [
         datetime.date(3000, 5, 20),
@@ -215,12 +219,29 @@ def _system_timezone():
         datetime.time(22, 30, 0, tzinfo=_timezone(0)),
     ]),
     ('interval', 'interval', [
-        # no months :(
         datetime.timedelta(40, 10, 1234),
         datetime.timedelta(0, 0, 4321),
         datetime.timedelta(0, 0),
         datetime.timedelta(-100, 0),
         datetime.timedelta(-100, -400),
+        {
+            'textinput': '-2 years -11 months -10 days '
+                         '-2 hours -800 milliseconds',
+            'output': datetime.timedelta(
+                days=(-2 * 365) + (-11 * 30) - 10,
+                seconds=(-2 * 3600),
+                milliseconds=-800
+            ),
+        },
+        {
+            'query': 'SELECT justify_hours($1::interval)::text',
+            'input': datetime.timedelta(
+                days=(-2 * 365) + (-11 * 30) - 10,
+                seconds=(-2 * 3600),
+                milliseconds=-800
+            ),
+            'textoutput': '-1070 days -02:00:00.8',
+        },
     ]),
     ('uuid', 'uuid', [
         uuid.UUID('38a4ff5a-3a56-11e6-a6c2-c8f73323c6d4'),
@@ -458,6 +479,9 @@ async def test_standard_codecs(self):
                             stmt = text_out
                         else:
                             outputval = sample['output']
+
+                        if sample.get('query'):
+                            stmt = await self.con.prepare(sample['query'])
                     else:
                         inputval = outputval = sample