Disallow NaN as a value for floating-point GUCs.

None of the code that uses GUC values is really prepared for them to
hold NaN, but parse_real() didn't have any defense against accepting
such a value.  Treat it the same as a syntax error.

I haven't attempted to analyze the exact consequences of setting any
of the float GUCs to NaN, but since they're quite unlikely to be good,
this seems like a back-patchable bug fix.

Note: we don't need an explicit test for +-Infinity because those will
be rejected by existing range checks.  I added a regression test for
that in HEAD, but not older branches because the spelling of the value
in the error message will be platform-dependent in branches where we
don't always use port/snprintf.c.

Discussion: https://postgr.es/m/1798.1552165479@sss.pgh.pa.us

src/backend/utils/misc/guc.c

@@ -5503,6 +5503,10 @@ parse_real(const char *value, double *result)
 	if (endptr == value || errno == ERANGE)
 		return false;
 
+	/* reject NaN (infinities will fail range checks later) */
+	if (isnan(val))
+		return false;
+
 	/* allow whitespace after number */
 	while (isspace((unsigned char) *endptr))
 		endptr++;

src/test/regress/expected/guc.out

@@ -506,6 +506,11 @@ SELECT '2006-08-13 12:34:56'::timestamptz;
  Sun Aug 13 12:34:56 2006 PDT
 (1 row)
 
+-- Test some simple error cases
+SET seq_page_cost TO 'NaN';
+ERROR:  parameter "seq_page_cost" requires a numeric value
+SET vacuum_cost_delay TO '10s';
+ERROR:  10000 is outside the valid range for parameter "vacuum_cost_delay" (0 .. 100)
 --
 -- Test DISCARD TEMP
 --

src/test/regress/sql/guc.sql

@@ -144,6 +144,10 @@ RESET datestyle;
 SHOW datestyle;
 SELECT '2006-08-13 12:34:56'::timestamptz;
 
+-- Test some simple error cases
+SET seq_page_cost TO 'NaN';
+SET vacuum_cost_delay TO '10s';
+
 --
 -- Test DISCARD TEMP
 --