Fix roundoff problems in float8_timestamptz() and make_interval().

tglsfdc · tglsfdc · commit 8f93bd851246 · 2017-02-08T18:04:59.000-05:00
When converting a float value to integer microseconds, we should be careful to round the value to the nearest integer, typically with rint(); simply assigning to an int64 variable will truncate, causing apparently off-by-one values in cases that should work. Most places in the datetime code got this right, but not these two. float8_timestamptz() is new as of commit e511d87 (9.6). Previous versions effectively depended on interval_mul() to do roundoff correctly, which it does, so this fixes an accuracy regression in 9.6. The problem in make_interval() dates to its introduction in 9.4. Aside from being careful to round not truncate, let's incorporate the hours and minutes inputs into the result with exact integer arithmetic, rather than risk introducing roundoff error where there need not have been any. float8_timestamptz() problem reported by Erik Nordström, though this is not his proposed patch. make_interval() problem found by me. Discussion: https://postgr.es/m/CAHuQZDS76jTYk3LydPbKpNfw9KbACmD=49dC4BrzHcfPv6yA1A@mail.gmail.com
diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
@@ -788,7 +788,7 @@ float8_timestamptz(PG_FUNCTION_ARGS)
 		seconds -= ((POSTGRES_EPOCH_JDATE - UNIX_EPOCH_JDATE) * SECS_PER_DAY);
 
 #ifdef HAVE_INT64_TIMESTAMP
-		result = seconds * USECS_PER_SEC;
+		result = rint(seconds * USECS_PER_SEC);
 #else
 		result = seconds;
 #endif
@@ -1623,12 +1623,14 @@ make_interval(PG_FUNCTION_ARGS)
 	result->month = years * MONTHS_PER_YEAR + months;
 	result->day = weeks * 7 + days;
 
-	secs += hours * (double) SECS_PER_HOUR + mins * (double) SECS_PER_MINUTE;
-
 #ifdef HAVE_INT64_TIMESTAMP
-	result->time = (int64) (secs * USECS_PER_SEC);
+	result->time = hours * ((int64) SECS_PER_HOUR * USECS_PER_SEC) +
+		mins * ((int64) SECS_PER_MINUTE * USECS_PER_SEC) +
+		(int64) rint(secs * USECS_PER_SEC);
 #else
-	result->time = secs;
+	result->time = hours * (double) SECS_PER_HOUR +
+		mins * (double) SECS_PER_MINUTE +
+		secs;
 #endif
 
 	PG_RETURN_INTERVAL_P(result);
