pgsql: Fix roundoff problems in float8_timestamptz() and make_interval(

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

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 e511d878f (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

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/7786b984825ea720aed3a11ee465dc3d6cfc8d96

Modified Files
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src/backend/utils/adt/timestamp.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)