Support plpgsql variable names that conflict with unreserved SQL keywords.

A variable name matching a statement-introducing keyword, such as
"comment" or "update", caused parse failures if one tried to write
a statement using that keyword.  Commit bb1b8f69 already addressed
this scenario for the case of variable names matching unreserved
plpgsql keywords, but we didn't think about unreserved core-grammar
keywords.  The same heuristic (viz, it can't be a variable name
unless the next token is assignment or '[') should work fine for
that case too, and as a bonus the code gets shorter and less
duplicative.

Per bug #15555 from Feike Steenbergen.  Since this hasn't been
complained of before, and is easily worked around anyway,
I won't risk a back-patch.

Discussion: https://postgr.es/m/15555-149bbd70ddc7b4b6@postgresql.org

src/pl/plpgsql/src/pl_comp.c

@@ -1353,6 +1353,9 @@ make_datum_param(PLpgSQL_expr *expr, int dno, int location)
  * yytxt is the original token text; we need this to check for quoting,
  * so that later checks for unreserved keywords work properly.
  *
+ * We attempt to recognize the token as a variable only if lookup is true
+ * and the plpgsql_IdentifierLookup context permits it.
+ *
  * If recognized as a variable, fill in *wdatum and return true;
  * if not recognized, fill in *word and return false.
  * (Note: those two pointers actually point to members of the same union,
@@ -1360,17 +1363,17 @@ make_datum_param(PLpgSQL_expr *expr, int dno, int location)
  * ----------
  */
 bool
-plpgsql_parse_word(char *word1, const char *yytxt,
+plpgsql_parse_word(char *word1, const char *yytxt, bool lookup,
 				   PLwdatum *wdatum, PLword *word)
 {
 	PLpgSQL_nsitem *ns;
 
 	/*
-	 * We should do nothing in DECLARE sections.  In SQL expressions, there's
-	 * no need to do anything either --- lookup will happen when the
-	 * expression is compiled.
+	 * We should not lookup variables in DECLARE sections.  In SQL
+	 * expressions, there's no need to do so either --- lookup will happen
+	 * when the expression is compiled.
 	 */
-	if (plpgsql_IdentifierLookup == IDENTIFIER_LOOKUP_NORMAL)
+	if (lookup && plpgsql_IdentifierLookup == IDENTIFIER_LOOKUP_NORMAL)
 	{
 		/*
 		 * Do a lookup in the current namespace stack

src/pl/plpgsql/src/pl_scanner.c

@@ -328,6 +328,7 @@ plpgsql_yylex(void)
 				push_back_token(tok2, &aux2);
 				if (plpgsql_parse_word(aux1.lval.str,
 									   core_yy.scanbuf + aux1.lloc,
+									   true,
 									   &aux1.lval.wdatum,
 									   &aux1.lval.word))
 					tok1 = T_DATUM;
@@ -349,53 +350,40 @@ plpgsql_yylex(void)
 			push_back_token(tok2, &aux2);
 
 			/*
-			 * If we are at start of statement, prefer unreserved keywords
-			 * over variable names, unless the next token is assignment or
-			 * '[', in which case prefer variable names.  (Note we need not
-			 * consider '.' as the next token; that case was handled above,
-			 * and we always prefer variable names in that case.)  If we are
-			 * not at start of statement, always prefer variable names over
-			 * unreserved keywords.
+			 * See if it matches a variable name, except in the context where
+			 * we are at start of statement and the next token isn't
+			 * assignment or '['.  In that case, it couldn't validly be a
+			 * variable name, and skipping the lookup allows variable names to
+			 * be used that would conflict with plpgsql or core keywords that
+			 * introduce statements (e.g., "comment").  Without this special
+			 * logic, every statement-introducing keyword would effectively be
+			 * reserved in PL/pgSQL, which would be unpleasant.
+			 *
+			 * If it isn't a variable name, try to match against unreserved
+			 * plpgsql keywords.  If not one of those either, it's T_WORD.
+			 *
+			 * Note: we must call plpgsql_parse_word even if we don't want to
+			 * do variable lookup, because it sets up aux1.lval.word for the
+			 * non-variable cases.
 			 */
-			if (AT_STMT_START(plpgsql_yytoken) &&
-				!(tok2 == '=' || tok2 == COLON_EQUALS || tok2 == '['))
+			if (plpgsql_parse_word(aux1.lval.str,
+								   core_yy.scanbuf + aux1.lloc,
+								   (!AT_STMT_START(plpgsql_yytoken) ||
+									(tok2 == '=' || tok2 == COLON_EQUALS ||
+									 tok2 == '[')),
+								   &aux1.lval.wdatum,
+								   &aux1.lval.word))
+				tok1 = T_DATUM;
+			else if (!aux1.lval.word.quoted &&
+					 (kw = ScanKeywordLookup(aux1.lval.word.ident,
+											 unreserved_keywords,
+											 num_unreserved_keywords)))
 			{
-				/* try for unreserved keyword, then for variable name */
-				if (core_yy.scanbuf[aux1.lloc] != '"' &&
-					(kw = ScanKeywordLookup(aux1.lval.str,
-											unreserved_keywords,
-											num_unreserved_keywords)))
-				{
-					aux1.lval.keyword = kw->name;
-					tok1 = kw->value;
-				}
-				else if (plpgsql_parse_word(aux1.lval.str,
-											core_yy.scanbuf + aux1.lloc,
-											&aux1.lval.wdatum,
-											&aux1.lval.word))
-					tok1 = T_DATUM;
-				else
-					tok1 = T_WORD;
+				aux1.lval.keyword = kw->name;
+				tok1 = kw->value;
 			}
 			else
-			{
-				/* try for variable name, then for unreserved keyword */
-				if (plpgsql_parse_word(aux1.lval.str,
-									   core_yy.scanbuf + aux1.lloc,
-									   &aux1.lval.wdatum,
-									   &aux1.lval.word))
-					tok1 = T_DATUM;
-				else if (!aux1.lval.word.quoted &&
-						 (kw = ScanKeywordLookup(aux1.lval.word.ident,
-												 unreserved_keywords,
-												 num_unreserved_keywords)))
-				{
-					aux1.lval.keyword = kw->name;
-					tok1 = kw->value;
-				}
-				else
-					tok1 = T_WORD;
-			}
+				tok1 = T_WORD;
 		}
 	}
 	else

src/pl/plpgsql/src/plpgsql.h

@@ -1175,7 +1175,7 @@ extern PLpgSQL_function *plpgsql_compile(FunctionCallInfo fcinfo,
 extern PLpgSQL_function *plpgsql_compile_inline(char *proc_source);
 extern void plpgsql_parser_setup(struct ParseState *pstate,
 					 PLpgSQL_expr *expr);
-extern bool plpgsql_parse_word(char *word1, const char *yytxt,
+extern bool plpgsql_parse_word(char *word1, const char *yytxt, bool lookup,
 				   PLwdatum *wdatum, PLword *word);
 extern bool plpgsql_parse_dblword(char *word1, char *word2,
 					  PLwdatum *wdatum, PLcword *cword);

src/test/regress/expected/plpgsql.out

@@ -4781,6 +4781,27 @@ select unreserved_test();
               43
 (1 row)
 
+create or replace function unreserved_test() returns int as $$
+declare
+  comment int := 21;
+begin
+  comment := comment * 2;
+  comment on function unreserved_test() is 'this is a test';
+  return comment;
+end
+$$ language plpgsql;
+select unreserved_test();
+ unreserved_test 
+-----------------
+              42
+(1 row)
+
+select obj_description('unreserved_test()'::regprocedure, 'pg_proc');
+ obj_description 
+-----------------
+ this is a test
+(1 row)
+
 drop function unreserved_test();
 --
 -- Test FOREACH over arrays

src/test/regress/sql/plpgsql.sql

@@ -3892,6 +3892,20 @@ $$ language plpgsql;
 
 select unreserved_test();
 
+create or replace function unreserved_test() returns int as $$
+declare
+  comment int := 21;
+begin
+  comment := comment * 2;
+  comment on function unreserved_test() is 'this is a test';
+  return comment;
+end
+$$ language plpgsql;
+
+select unreserved_test();
+
+select obj_description('unreserved_test()'::regprocedure, 'pg_proc');
+
 drop function unreserved_test();
 
 --