Fix CREATE DATABASE so we can pg_upgrade DBs with OIDs above 2^31.

Commit aa0105141 repeated one of the oldest mistakes in our book:
thinking that OID is the same as int32.  It isn't of course, and
unsurprisingly the first person who came along with a database
OID above 2 billion broke it.  Repair.

Per bug #17677 from Sergey Pankov.  Back-patch to v15.

Discussion: https://postgr.es/m/17677-a99fa067d7ed71c9@postgresql.org

src/backend/commands/dbcommands.c

@@ -815,7 +815,7 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 		}
 		else if (strcmp(defel->defname, "oid") == 0)
 		{
-			dboid = defGetInt32(defel);
+			dboid = defGetObjectId(defel);
 
 			/*
 			 * We don't normally permit new databases to be created with

src/backend/commands/define.c

@@ -213,6 +213,39 @@ defGetInt64(DefElem *def)
 	return 0;					/* keep compiler quiet */
 }
 
+/*
+ * Extract an OID value from a DefElem.
+ */
+Oid
+defGetObjectId(DefElem *def)
+{
+	if (def->arg == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_SYNTAX_ERROR),
+				 errmsg("%s requires a numeric value",
+						def->defname)));
+	switch (nodeTag(def->arg))
+	{
+		case T_Integer:
+			return (Oid) intVal(def->arg);
+		case T_Float:
+
+			/*
+			 * Values too large for int4 will be represented as Float
+			 * constants by the lexer.  Accept these if they are valid OID
+			 * strings.
+			 */
+			return DatumGetObjectId(DirectFunctionCall1(oidin,
+														CStringGetDatum(castNode(Float, def->arg)->fval)));
+		default:
+			ereport(ERROR,
+					(errcode(ERRCODE_SYNTAX_ERROR),
+					 errmsg("%s requires a numeric value",
+							def->defname)));
+	}
+	return 0;					/* keep compiler quiet */
+}
+
 /*
  * Extract a possibly-qualified name (as a List of Strings) from a DefElem.
  */

src/backend/parser/gram.y

@@ -11067,9 +11067,9 @@ createdb_opt_items:
 		;
 
 createdb_opt_item:
-			createdb_opt_name opt_equal SignedIconst
+			createdb_opt_name opt_equal NumericOnly
 				{
-					$$ = makeDefElem($1, (Node *) makeInteger($3), @1);
+					$$ = makeDefElem($1, $3, @1);
 				}
 			| createdb_opt_name opt_equal opt_boolean_or_string
 				{

src/include/commands/defrem.h

@@ -150,6 +150,7 @@ extern double defGetNumeric(DefElem *def);
 extern bool defGetBoolean(DefElem *def);
 extern int32 defGetInt32(DefElem *def);
 extern int64 defGetInt64(DefElem *def);
+extern Oid	defGetObjectId(DefElem *def);
 extern List *defGetQualifiedName(DefElem *def);
 extern TypeName *defGetTypeName(DefElem *def);
 extern int	defGetTypeLength(DefElem *def);