Fix contrib/seg to be more wary of long input numbers.

seg stores the number of significant digits in an input number
in a "char" field.  If char is signed, and the input is more than
127 digits long, the count can read out as negative causing
seg_out() to print garbage (or, if you're really unlucky,
even crash).

To fix, clamp the digit count to be not more than FLT_DIG.
(In theory this loses some information about what the original
input was, but it doesn't seem like useful information; it would
not survive dump/restore in any case.)

Also, in case there are stored values of the seg type containing
bad data, add a clamp in seg_out's restore() subroutine.

Per bug #17725 from Robins Tharakan.  It's been like this
forever, so back-patch to all supported branches.

Discussion: https://postgr.es/m/17725-0a09313b67fbe86e@postgresql.org

contrib/seg/expected/seg.out

@@ -256,6 +256,13 @@ SELECT '12.34567890123456'::seg AS seg;
  12.3457
 (1 row)
 
+-- Same, with a very long input
+SELECT '12.3456789012345600000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000'::seg AS seg;
+   seg   
+---------
+ 12.3457
+(1 row)
+
 -- Numbers with certainty indicators
 SELECT '~6.5'::seg AS seg;
  seg  

contrib/seg/seg.c

@@ -928,9 +928,13 @@ restore(char *result, float val, int n)
 
 	/*
 	 * Put a cap on the number of significant digits to avoid garbage in the
-	 * output and ensure we don't overrun the result buffer.
+	 * output and ensure we don't overrun the result buffer.  (n should not be
+	 * negative, but check to protect ourselves against corrupted data.)
 	 */
-	n = Min(n, FLT_DIG);
+	if (n <= 0)
+		n = FLT_DIG;
+	else
+		n = Min(n, FLT_DIG);
 
 	/* remember the sign */
 	sign = (val < 0 ? 1 : 0);

contrib/seg/segparse.y

@@ -3,6 +3,7 @@
 
 #include "postgres.h"
 
+#include <float.h>
 #include <math.h>
 
 #include "fmgr.h"
@@ -20,6 +21,8 @@
 
 static float seg_atof(const char *value);
 
+static int sig_digits(const char *value);
+
 static char strbuf[25] = {
 	'0', '0', '0', '0', '0',
 	'0', '0', '0', '0', '0',
@@ -62,9 +65,9 @@ range: boundary PLUMIN deviation
 		result->lower = $1.val - $3.val;
 		result->upper = $1.val + $3.val;
 		sprintf(strbuf, "%g", result->lower);
-		result->l_sigd = Max(Min(6, significant_digits(strbuf)), Max($1.sigd, $3.sigd));
+		result->l_sigd = Max(sig_digits(strbuf), Max($1.sigd, $3.sigd));
 		sprintf(strbuf, "%g", result->upper);
-		result->u_sigd = Max(Min(6, significant_digits(strbuf)), Max($1.sigd, $3.sigd));
+		result->u_sigd = Max(sig_digits(strbuf), Max($1.sigd, $3.sigd));
 		result->l_ext = '\0';
 		result->u_ext = '\0';
 	}
@@ -121,7 +124,7 @@ boundary: SEGFLOAT
 		float		val = seg_atof($1);
 
 		$$.ext = '\0';
-		$$.sigd = significant_digits($1);
+		$$.sigd = sig_digits($1);
 		$$.val = val;
 	}
 	| EXTENSION SEGFLOAT
@@ -130,7 +133,7 @@ boundary: SEGFLOAT
 		float		val = seg_atof($2);
 
 		$$.ext = $1[0];
-		$$.sigd = significant_digits($2);
+		$$.sigd = sig_digits($2);
 		$$.val = val;
 	}
 	;
@@ -141,7 +144,7 @@ deviation: SEGFLOAT
 		float		val = seg_atof($1);
 
 		$$.ext = '\0';
-		$$.sigd = significant_digits($1);
+		$$.sigd = sig_digits($1);
 		$$.val = val;
 	}
 	;
@@ -157,3 +160,12 @@ seg_atof(const char *value)
 	datum = DirectFunctionCall1(float4in, CStringGetDatum(value));
 	return DatumGetFloat4(datum);
 }
+
+static int
+sig_digits(const char *value)
+{
+	int			n = significant_digits(value);
+
+	/* Clamp, to ensure value will fit in sigd fields */
+	return Min(n, FLT_DIG);
+}

contrib/seg/sql/seg.sql

@@ -60,6 +60,9 @@ SELECT '3.400e5'::seg AS seg;
 -- Digits truncated
 SELECT '12.34567890123456'::seg AS seg;
 
+-- Same, with a very long input
+SELECT '12.3456789012345600000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000'::seg AS seg;
+
 -- Numbers with certainty indicators
 SELECT '~6.5'::seg AS seg;
 SELECT '<6.5'::seg AS seg;