Fix Assert failure in XMLTABLE parser

In an XMLTABLE expression, columns can be marked NOT NULL, and the
parser internally fabricates an option named "is_not_null" to
represent this.  However, the parser also allows users to specify
arbitrary option names.  This creates a conflict: a user can
explicitly use "is_not_null" as an option name and assign it a
non-Boolean value, which violates internal assumptions and triggers an
assertion failure.

To fix, this patch checks whether a user-supplied name collides with
the internally reserved option name and raises an error if so.
Additionally, the internal name is renamed to "__pg__is_not_null" to
further reduce the risk of collision with user-defined names.

Reported-by: Евгений Горбанев <gorbanyoves@basealt.ru>
Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Alvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/6bac9886-65bf-4cec-96bd-e304159f28db@basealt.ru
Backpatch-through: 15

src/backend/parser/gram.y

@@ -14048,7 +14048,7 @@ xmltable_column_el:
 										 parser_errposition(defel->location)));
 							fc->colexpr = defel->arg;
 						}
-						else if (strcmp(defel->defname, "is_not_null") == 0)
+						else if (strcmp(defel->defname, "__pg__is_not_null") == 0)
 						{
 							if (nullability_seen)
 								ereport(ERROR,
@@ -14091,13 +14091,20 @@ xmltable_column_option_list:
 
 xmltable_column_option_el:
 			IDENT b_expr
-				{ $$ = makeDefElem($1, $2, @1); }
+				{
+					if (strcmp($1, "__pg__is_not_null") == 0)
+						ereport(ERROR,
+								(errcode(ERRCODE_SYNTAX_ERROR),
+								 errmsg("option name \"%s\" cannot be used in XMLTABLE", $1),
+								 parser_errposition(@1)));
+					$$ = makeDefElem($1, $2, @1);
+				}
 			| DEFAULT b_expr
 				{ $$ = makeDefElem("default", $2, @1); }
 			| NOT NULL_P
-				{ $$ = makeDefElem("is_not_null", (Node *) makeBoolean(true), @1); }
+				{ $$ = makeDefElem("__pg__is_not_null", (Node *) makeBoolean(true), @1); }
 			| NULL_P
-				{ $$ = makeDefElem("is_not_null", (Node *) makeBoolean(false), @1); }
+				{ $$ = makeDefElem("__pg__is_not_null", (Node *) makeBoolean(false), @1); }
 			| PATH b_expr
 				{ $$ = makeDefElem("path", $2, @1); }
 		;

src/test/regress/expected/xml.out

@@ -1373,6 +1373,10 @@ EXPLAIN (COSTS OFF, VERBOSE) SELECT * FROM xmltableview1;
 -- errors
 SELECT * FROM XMLTABLE (ROW () PASSING null COLUMNS v1 timestamp) AS f (v1, v2);
 ERROR:  XMLTABLE function has 1 columns available but 2 columns specified
+SELECT * FROM XMLTABLE (ROW () PASSING null COLUMNS v1 timestamp __pg__is_not_null 1) AS f (v1);
+ERROR:  option name "__pg__is_not_null" cannot be used in XMLTABLE
+LINE 1: ...MLTABLE (ROW () PASSING null COLUMNS v1 timestamp __pg__is_n...
+                                                             ^
 -- XMLNAMESPACES tests
 SELECT * FROM XMLTABLE(XMLNAMESPACES('http://x.y' AS zz),
                       '/zz:rows/zz:row'

src/test/regress/expected/xml_1.out

@@ -1047,6 +1047,10 @@ EXPLAIN (COSTS OFF, VERBOSE) SELECT * FROM xmltableview1;
 -- errors
 SELECT * FROM XMLTABLE (ROW () PASSING null COLUMNS v1 timestamp) AS f (v1, v2);
 ERROR:  XMLTABLE function has 1 columns available but 2 columns specified
+SELECT * FROM XMLTABLE (ROW () PASSING null COLUMNS v1 timestamp __pg__is_not_null 1) AS f (v1);
+ERROR:  option name "__pg__is_not_null" cannot be used in XMLTABLE
+LINE 1: ...MLTABLE (ROW () PASSING null COLUMNS v1 timestamp __pg__is_n...
+                                                             ^
 -- XMLNAMESPACES tests
 SELECT * FROM XMLTABLE(XMLNAMESPACES('http://x.y' AS zz),
                       '/zz:rows/zz:row'

src/test/regress/expected/xml_2.out

@@ -1359,6 +1359,10 @@ EXPLAIN (COSTS OFF, VERBOSE) SELECT * FROM xmltableview1;
 -- errors
 SELECT * FROM XMLTABLE (ROW () PASSING null COLUMNS v1 timestamp) AS f (v1, v2);
 ERROR:  XMLTABLE function has 1 columns available but 2 columns specified
+SELECT * FROM XMLTABLE (ROW () PASSING null COLUMNS v1 timestamp __pg__is_not_null 1) AS f (v1);
+ERROR:  option name "__pg__is_not_null" cannot be used in XMLTABLE
+LINE 1: ...MLTABLE (ROW () PASSING null COLUMNS v1 timestamp __pg__is_n...
+                                                             ^
 -- XMLNAMESPACES tests
 SELECT * FROM XMLTABLE(XMLNAMESPACES('http://x.y' AS zz),
                       '/zz:rows/zz:row'

src/test/regress/sql/xml.sql

@@ -435,6 +435,8 @@ EXPLAIN (COSTS OFF, VERBOSE) SELECT * FROM xmltableview1;
 -- errors
 SELECT * FROM XMLTABLE (ROW () PASSING null COLUMNS v1 timestamp) AS f (v1, v2);
 
+SELECT * FROM XMLTABLE (ROW () PASSING null COLUMNS v1 timestamp __pg__is_not_null 1) AS f (v1);
+
 -- XMLNAMESPACES tests
 SELECT * FROM XMLTABLE(XMLNAMESPACES('http://x.y' AS zz),
                       '/zz:rows/zz:row'