Fix power() for infinity inputs some more.

Buildfarm results for commit decbe2bf show that AIX and illumos
have non-POSIX-compliant pow() functions, as do ancient NetBSD
and HPUX releases.  While it's dubious how much we should care
about the latter two platforms, the former two are probably enough
reason to put in manual handling of infinite-input cases.  Hence,
do so, and clean up the post-pow() error handling to reflect its
now-more-limited scope.  (Notably, while we no longer expect to
ever see EDOM from pow(), report it as a domain error if we do.
The former coding had the net effect of expensively converting the
error to ERANGE, which seems highly questionable: if pow() wanted
to report ERANGE, it would have done so.)

Patch by me; thanks to Michael Paquier for review.

Discussion: https://postgr.es/m/E1jkU7H-00024V-NZ@gemulon.postgresql.org

src/backend/utils/adt/float.c

@@ -1540,33 +1540,101 @@ dpow(PG_FUNCTION_ARGS)
 				 errmsg("a negative number raised to a non-integer power yields a complex result")));
 
 	/*
-	 * pow() sets errno only on some platforms, depending on whether it
-	 * follows _IEEE_, _POSIX_, _XOPEN_, or _SVID_, so we try to avoid using
-	 * errno.  However, some platform/CPU combinations return errno == EDOM
-	 * 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.
+	 * We don't trust the platform's pow() to handle infinity cases per POSIX
+	 * spec either, so deal with those explicitly too.  It's easier to handle
+	 * infinite y first, so that it doesn't matter if x is also infinite.
 	 */
-	errno = 0;
-	result = pow(arg1, arg2);
-	if (errno == EDOM && isnan(result))
+	if (isinf(arg2))
 	{
-		if ((fabs(arg1) > 1 && arg2 >= 0) || (fabs(arg1) < 1 && arg2 < 0))
-			/* The sign of Inf is not significant in this case. */
-			result = get_float8_infinity();
-		else if (fabs(arg1) != 1)
-			result = 0;
-		else
-			result = 1;
+		double		absx = fabs(arg1);
+
+		if (absx == 1.0)
+			result = 1.0;
+		else if (arg2 > 0.0)	/* y = +Inf */
+		{
+			if (absx > 1.0)
+				result = arg2;
+			else
+				result = 0.0;
+		}
+		else					/* y = -Inf */
+		{
+			if (absx > 1.0)
+				result = 0.0;
+			else
+				result = -arg2;
+		}
 	}
-	else if (errno == ERANGE && result != 0 && !isinf(result))
-		result = get_float8_infinity();
+	else if (isinf(arg1))
+	{
+		if (arg2 == 0.0)
+			result = 1.0;
+		else if (arg1 > 0.0)	/* x = +Inf */
+		{
+			if (arg2 > 0.0)
+				result = arg1;
+			else
+				result = 0.0;
+		}
+		else					/* x = -Inf */
+		{
+			bool		yisoddinteger = false;
 
-	if (unlikely(isinf(result)) && !isinf(arg1) && !isinf(arg2))
-		float_overflow_error();
-	if (unlikely(result == 0.0) && arg1 != 0.0 && !isinf(arg1) && !isinf(arg2))
-		float_underflow_error();
+			if (arg2 == floor(arg2))
+			{
+				/* y is integral; it's odd if y/2 is not integral */
+				double		halfy = arg2 * 0.5; /* should be computed exactly */
+
+				if (halfy != floor(halfy))
+					yisoddinteger = true;
+			}
+			if (arg2 > 0.0)
+				result = yisoddinteger ? arg1 : -arg1;
+			else
+				result = yisoddinteger ? -0.0 : 0.0;
+		}
+	}
+	else
+	{
+		/*
+		 * pow() sets errno on only some platforms, depending on whether it
+		 * follows _IEEE_, _POSIX_, _XOPEN_, or _SVID_, so we must check both
+		 * errno and invalid output values.  (We can't rely on just the
+		 * latter, either; some old platforms return a large-but-finite
+		 * HUGE_VAL when reporting overflow.)
+		 */
+		errno = 0;
+		result = pow(arg1, arg2);
+		if (errno == EDOM || isnan(result))
+		{
+			/*
+			 * We eliminated all the possible domain errors above, or should
+			 * have; but if pow() has a more restrictive test for "is y an
+			 * integer?" than we do, we could get here anyway.  Historical
+			 * evidence suggests that some platforms once implemented the test
+			 * as "y == (long) y", which of course misbehaves beyond LONG_MAX.
+			 * There's not a lot of choice except to accept the platform's
+			 * conclusion that we have a domain error.
+			 */
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_ARGUMENT_FOR_POWER_FUNCTION),
+					 errmsg("a negative number raised to a non-integer power yields a complex result")));
+		}
+		else if (errno == ERANGE)
+		{
+			if (result != 0.0)
+				float_overflow_error();
+			else
+				float_underflow_error();
+		}
+		else
+		{
+			if (unlikely(isinf(result)))
+				float_overflow_error();
+			if (unlikely(result == 0.0) && arg1 != 0.0)
+				float_underflow_error();
+		}
+	}
 
 	PG_RETURN_FLOAT8(result);
 }

src/test/regress/expected/float8.out

@@ -525,6 +525,8 @@ SELECT power(float8 '-inf', float8 '3');
  -Infinity
 (1 row)
 
+SELECT power(float8 '-inf', float8 '3.5');
+ERROR:  a negative number raised to a non-integer power yields a complex result
 SELECT power(float8 '-inf', float8 'inf');
   power   
 ----------

src/test/regress/sql/float8.sql

@@ -144,6 +144,7 @@ SELECT power(float8 '-inf', float8 '-2');
 SELECT power(float8 '-inf', float8 '-3');
 SELECT power(float8 '-inf', float8 '2');
 SELECT power(float8 '-inf', float8 '3');
+SELECT power(float8 '-inf', float8 '3.5');
 SELECT power(float8 '-inf', float8 'inf');
 SELECT power(float8 '-inf', float8 '-inf');