Avoid wrong results for power() with NaN input on more platforms.

Buildfarm results show that the modern POSIX rule that 1 ^ NaN = 1 is not
honored on *BSD until relatively recently, and really old platforms don't
believe that NaN ^ 0 = 1 either.  (This is unsurprising, perhaps, since
SUSv2 doesn't require either behavior.)  In hopes of getting to platform
independent behavior, let's deal with all the NaN-input cases explicitly
in dpow().

Note that numeric_power() doesn't know either of these special cases.
But since that behavior is platform-independent, I think it should be
addressed separately, and probably not back-patched.

Discussion: https://postgr.es/m/75DB81BEEA95B445AE6D576A0A5C9E936A73E741@BPXM05GP.gisp.nec.co.jp

src/backend/utils/adt/float.c

@@ -1461,6 +1461,25 @@ dpow(PG_FUNCTION_ARGS)
 	float8		arg2 = PG_GETARG_FLOAT8(1);
 	float8		result;
 
+	/*
+	 * The POSIX spec says that NaN ^ 0 = 1, and 1 ^ NaN = 1, while all other
+	 * cases with NaN inputs yield NaN (with no error).  Many older platforms
+	 * get one or more of these cases wrong, so deal with them via explicit
+	 * logic rather than trusting pow(3).
+	 */
+	if (isnan(arg1))
+	{
+		if (isnan(arg2) || arg2 != 0.0)
+			PG_RETURN_FLOAT8(get_float8_nan());
+		PG_RETURN_FLOAT8(1.0);
+	}
+	if (isnan(arg2))
+	{
+		if (arg1 != 1.0)
+			PG_RETURN_FLOAT8(get_float8_nan());
+		PG_RETURN_FLOAT8(1.0);
+	}
+
 	/*
 	 * The SQL spec requires that we emit a particular SQLSTATE error code for
 	 * certain error conditions.  Specifically, we don't return a
@@ -1482,12 +1501,11 @@ dpow(PG_FUNCTION_ARGS)
 	 * and result == NaN for negative arg1 and very large arg2 (they must be
 	 * using something different from our floor() test to decide it's
 	 * invalid).  Other platforms (HPPA) return errno == ERANGE and a large
-	 * (HUGE_VAL) but finite result to signal overflow.  Also, some versions
+	 * (HUGE_VAL) but finite result to signal overflow.
-	 * of MSVC return errno == EDOM and result == NaN for NaN inputs.
 	 */
 	errno = 0;
 	result = pow(arg1, arg2);
-	if (errno == EDOM && isnan(result) && !isnan(arg1) && !isnan(arg2))
+	if (errno == EDOM && isnan(result))
 	{
 		if ((fabs(arg1) > 1 && arg2 >= 0) || (fabs(arg1) < 1 && arg2 < 0))
 			/* The sign of Inf is not significant in this case. */

src/test/regress/expected/float8-exp-three-digits-win32.out

@@ -358,6 +358,12 @@ SELECT power(float8 'NaN', float8 'NaN');
    NaN
 (1 row)
 
+SELECT power(float8 '-1', float8 'NaN');
+ power 
+-------
+   NaN
+(1 row)
+
 SELECT power(float8 '1', float8 'NaN');
  power 
 -------

src/test/regress/expected/float8-small-is-zero.out

@@ -362,6 +362,12 @@ SELECT power(float8 'NaN', float8 'NaN');
    NaN
 (1 row)
 
+SELECT power(float8 '-1', float8 'NaN');
+ power 
+-------
+   NaN
+(1 row)
+
 SELECT power(float8 '1', float8 'NaN');
  power 
 -------

src/test/regress/expected/float8-small-is-zero_1.out

@@ -362,6 +362,12 @@ SELECT power(float8 'NaN', float8 'NaN');
    NaN
 (1 row)
 
+SELECT power(float8 '-1', float8 'NaN');
+ power 
+-------
+   NaN
+(1 row)
+
 SELECT power(float8 '1', float8 'NaN');
  power 
 -------

src/test/regress/expected/float8.out

@@ -358,6 +358,12 @@ SELECT power(float8 'NaN', float8 'NaN');
    NaN
 (1 row)
 
+SELECT power(float8 '-1', float8 'NaN');
+ power 
+-------
+   NaN
+(1 row)
+
 SELECT power(float8 '1', float8 'NaN');
  power 
 -------

src/test/regress/sql/float8.sql

@@ -111,6 +111,7 @@ SELECT power(float8 '144', float8 '0.5');
 SELECT power(float8 'NaN', float8 '0.5');
 SELECT power(float8 '144', float8 'NaN');
 SELECT power(float8 'NaN', float8 'NaN');
+SELECT power(float8 '-1', float8 'NaN');
 SELECT power(float8 '1', float8 'NaN');
 SELECT power(float8 'NaN', float8 '0');