Require superuser privilege to create a binary-compatible cast, per

discussion some weeks ago.  Also, add a check that two types to be
binary-equivalenced match as to typlen, typbyval, and typalign; if
they don't then it's surely a mistake to equivalence them.

doc/src/sgml/ref/create_cast.sgml

-<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ref/create_cast.sgml,v 1.5 2002/09/18 21:35:20 tgl Exp $ -->
+<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ref/create_cast.sgml,v 1.6 2002/10/04 22:08:44 tgl Exp $ -->
 
 <refentry id="SQL-CREATECAST">
  <refmeta>
@@ -81,10 +81,9 @@ SELECT 'The time is ' || now();
 </programlisting>
    will be allowed only if the cast from type <type>timestamp</> to
    <type>text</type> is marked <literal>AS IMPLICIT</>.  Otherwise it
-   will be necessary to write one of
+   will be necessary to write the cast explicitly, for example
 <programlisting>
 SELECT 'The time is ' || CAST(now() AS text);
-SELECT 'The time is ' || now()::text;
 </programlisting>
    (We generally use the term <firstterm>implicit
    cast</firstterm> to describe this kind of cast.)
@@ -107,7 +106,9 @@ SELECT 'The time is ' || now()::text;
 
   <para>
    To be able to create a cast, you must own the source or the target
-   data type.
+   data type.  To create a binary-compatible cast, you must be superuser
+   (this restriction is made because an erroneous binary-compatible cast
+   conversion can easily crash the server).
   </para>
 
    <variablelist>

src/backend/commands/functioncmds.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/commands/functioncmds.c,v 1.22 2002/09/21 18:39:25 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/commands/functioncmds.c,v 1.23 2002/10/04 22:08:44 tgl Exp $
  *
  * DESCRIPTION
  *	  These routines take the parse tree and pick out the
@@ -756,8 +756,35 @@ CreateCast(CreateCastStmt *stmt)
 	}
 	else
 	{
+		int16	typ1len;
+		int16	typ2len;
+		bool	typ1byval;
+		bool	typ2byval;
+		char	typ1align;
+		char	typ2align;
+
 		/* indicates binary coercibility */
 		funcid = InvalidOid;
+
+		/*
+		 * Must be superuser to create binary-compatible casts, since
+		 * erroneous casts can easily crash the backend.
+		 */
+		if (!superuser())
+			elog(ERROR, "Must be superuser to create a cast WITHOUT FUNCTION");
+
+		/*
+		 * Also, insist that the types match as to size, alignment, and
+		 * pass-by-value attributes; this provides at least a crude check
+		 * that they have similar representations.  A pair of types that
+		 * fail this test should certainly not be equated.
+		 */
+		get_typlenbyvalalign(sourcetypeid, &typ1len, &typ1byval, &typ1align);
+		get_typlenbyvalalign(targettypeid, &typ2len, &typ2byval, &typ2align);
+		if (typ1len != typ2len ||
+			typ1byval != typ2byval ||
+			typ1align != typ2align)
+			elog(ERROR, "source and target datatypes are not physically compatible");
 	}
 
 	/* convert CoercionContext enum to char value for castcontext */