Fix power_var_int() for large integer exponents.

The code for raising a NUMERIC value to an integer power wasn't very
careful about large powers.  It got an outright wrong answer for an
exponent of INT_MIN, due to failure to consider overflow of the Abs(exp)
operation; which is fixable by using an unsigned rather than signed
exponent value after that point.  Also, even though the number of
iterations of the power-computation loop is pretty limited, it's easy for
the repeated squarings to result in ridiculously enormous intermediate
values, which can take unreasonable amounts of time/memory to process,
or even overflow the internal "weight" field and so produce a wrong answer.
We can forestall misbehaviors of that sort by bailing out as soon as the
weight value exceeds what will fit in int16, since then the final answer
must overflow (if exp > 0) or underflow (if exp < 0) the packed numeric
format.

Per off-list report from Pavel Stehule.  Back-patch to all supported
branches.

2 files changed, 28 insertions, 0 deletions

diff --git a/src/test/regress/expected/numeric.out b/src/test/regress/expected/numeric.out
index 94cb0ed551..5fafdaf13f 100644
--- a/src/test/regress/expected/numeric.out
+++ b/src/test/regress/expected/numeric.out
@@ -1390,3 +1390,22 @@ select div(12345678901234567890, 123) * 123 + 12345678901234567890 % 123;
  12345678901234567890
 (1 row)
 
+--
+-- Test code path for raising to integer powers
+--
+select 10.0 ^ -2147483648 as rounds_to_zero;
+   rounds_to_zero   
+--------------------
+ 0.0000000000000000
+(1 row)
+
+select 10.0 ^ -2147483647 as rounds_to_zero;
+   rounds_to_zero   
+--------------------
+ 0.0000000000000000
+(1 row)
+
+select 10.0 ^ 2147483647 as overflows;
+ERROR:  value overflows numeric format
+select 117743296169.0 ^ 1000000000 as overflows;
+ERROR:  value overflows numeric format
diff --git a/src/test/regress/sql/numeric.sql b/src/test/regress/sql/numeric.sql
index b2dc46fae2..5c08717e7a 100644
--- a/src/test/regress/sql/numeric.sql
+++ b/src/test/regress/sql/numeric.sql
@@ -828,3 +828,12 @@ select 12345678901234567890 % 123;
 select 12345678901234567890 / 123;
 select div(12345678901234567890, 123);
 select div(12345678901234567890, 123) * 123 + 12345678901234567890 % 123;
+
+--
+-- Test code path for raising to integer powers
+--
+
+select 10.0 ^ -2147483648 as rounds_to_zero;
+select 10.0 ^ -2147483647 as rounds_to_zero;
+select 10.0 ^ 2147483647 as overflows;
+select 117743296169.0 ^ 1000000000 as overflows;