Protect GIST logic that assumes penalty values can't be negative.

Apparently sane-looking penalty code might return small negative values,
for example because of roundoff error.  This will confuse places like
gistchoose().  Prevent problems by clamping negative penalty values to
zero.  (Just to be really sure, I also made it force NaNs to zero.)
Back-patch to all supported branches.

Alexander Korotkov

2 files changed, 11 insertions, 2 deletions

diff --git a/doc/src/sgml/gist.sgml b/doc/src/sgml/gist.sgml
index f6e31092aa..78171cfa3f 100644
--- a/doc/src/sgml/gist.sgml
+++ b/doc/src/sgml/gist.sgml
@@ -378,6 +378,8 @@ my_decompress(PG_FUNCTION_ARGS)
        Returns a value indicating the <quote>cost</quote> of inserting the new
        entry into a particular branch of the tree.  Items will be inserted
        down the path of least <function>penalty</function> in the tree.
+       Values returned by <function>penalty</function> should be non-negative.
+       If a negative value is returned, it will be treated as zero.
       </para>
 
       <para>
diff --git a/src/backend/access/gist/gistutil.c b/src/backend/access/gist/gistutil.c
index e61b676628..1aabcc527a 100644
--- a/src/backend/access/gist/gistutil.c
+++ b/src/backend/access/gist/gistutil.c
@@ -13,6 +13,8 @@
  */
 #include "postgres.h"
 
+#include <math.h>
+
 #include "access/gist_private.h"
 #include "access/reloptions.h"
 #include "storage/freespace.h"
@@ -526,16 +528,21 @@ gistpenalty(GISTSTATE *giststate, int attno,
 
 	if (giststate->penaltyFn[attno].fn_strict == FALSE ||
 		(isNullOrig == FALSE && isNullAdd == FALSE))
+	{
 		FunctionCall3Coll(&giststate->penaltyFn[attno],
 						  giststate->supportCollation[attno],
 						  PointerGetDatum(orig),
 						  PointerGetDatum(add),
 						  PointerGetDatum(&penalty));
+		/* disallow negative or NaN penalty */
+		if (isnan(penalty) || penalty < 0.0)
+			penalty = 0.0;
+	}
 	else if (isNullOrig && isNullAdd)
 		penalty = 0.0;
 	else
-		penalty = 1e10;			/* try to prevent to mix null and non-null
-								 * value */
+		penalty = 1e10;			/* try to prevent mixing null and non-null
+								 * values */
 
 	return penalty;
 }