jsonpath scanner: reentrant scanner

Use the flex %option reentrant to make the generated scanner
reentrant and thread-safe.  Note: The parser was already pure.

Simplify flex scan buffer management: Instead of constructing the
buffer from pieces and then using yy_scan_buffer(), we can just use
yy_scan_string(), which does the same thing internally.  (Actually, we
use yy_scan_bytes() here because we already have the length.)

Use flex yyextra to handle context information, instead of global
variables.  This complements the other changes to make the scanner
reentrant.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Andreas Karlsson <andreas@proxel.se>
Discussion: https://www.postgresql.org/message-id/flat/eb6faeac-2a8a-4b69-9189-c33c520e5b7b@eisentraut.org

Author: petere

src/backend/nls.mk

@@ -7,7 +7,7 @@ GETTEXT_TRIGGERS = $(BACKEND_COMMON_GETTEXT_TRIGGERS) \
                    GUC_check_errhint \
                    write_stderr \
                    yyerror \
-                   jsonpath_yyerror:3 \
+                   jsonpath_yyerror:4 \
                    parser_yyerror \
                    replication_yyerror:2 \
                    scanner_yyerror \

src/backend/utils/adt/jsonpath_gram.y

@@ -60,8 +60,10 @@ static bool makeItemLikeRegex(JsonPathParseItem *expr,
 %name-prefix="jsonpath_yy"
 %parse-param {JsonPathParseResult **result}
 %parse-param {struct Node *escontext}
+%parse-param {yyscan_t yyscanner}
 %lex-param {JsonPathParseResult **result}
 %lex-param {struct Node *escontext}
+%lex-param {yyscan_t yyscanner}
 
 %union
 {

src/backend/utils/adt/jsonpath_internal.h

@@ -22,17 +22,25 @@ typedef struct JsonPathString
 	int			total;
 } JsonPathString;
 
+#ifndef YY_TYPEDEF_YY_SCANNER_T
+#define YY_TYPEDEF_YY_SCANNER_T
+typedef void *yyscan_t;
+#endif
+
 #include "utils/jsonpath.h"
 #include "jsonpath_gram.h"
 
 #define YY_DECL extern int     jsonpath_yylex(YYSTYPE *yylval_param, \
 							  JsonPathParseResult **result, \
-							  struct Node *escontext)
+							  struct Node *escontext, \
+							  yyscan_t yyscanner)
 YY_DECL;
 extern int	jsonpath_yyparse(JsonPathParseResult **result,
-							 struct Node *escontext);
+							 struct Node *escontext,
+							 yyscan_t yyscanner);
 extern void jsonpath_yyerror(JsonPathParseResult **result,
 							 struct Node *escontext,
+							 yyscan_t yyscanner,
 							 const char *message);
 
 #endif							/* JSONPATH_INTERNAL_H */