Add option :allow_trailing_comma to JSON#parse

Fix: ruby#401

Since we already accept comments without even a flag to turn it
off, it makes sense to go just one small bit farther and optionally
allow trailing comma so we have decent support for what is generally
defined as "JSONC" and frequently used for configuration files.

Co-Authored-By: Jan-Joost Spanjers <oss@hiberis.nl>

Author: byroot

ext/json/ext/parser/parser.c

@@ -8,7 +8,7 @@ static ID i_json_creatable_p, i_json_create, i_create_id,
           i_chr, i_deep_const_get, i_match, i_aset, i_aref,
           i_leftshift, i_new, i_try_convert, i_uminus, i_encode;
 
-static VALUE sym_max_nesting, sym_allow_nan, sym_symbolize_names, sym_freeze,
+static VALUE sym_max_nesting, sym_allow_nan, sym_allow_trailing_comma, sym_symbolize_names, sym_freeze,
              sym_create_additions, sym_create_id, sym_object_class, sym_array_class,
              sym_decimal_class, sym_match_string;
 
@@ -383,6 +383,7 @@ typedef struct JSON_ParserStruct {
     FBuffer fbuffer;
     int max_nesting;
     bool allow_nan;
+    bool allow_trailing_comma;
     bool parsing_name;
     bool symbolize_names;
     bool freeze;
@@ -477,6 +478,8 @@ static void raise_parse_error(const char *format, const char *start)
         }
     }
 
+    action allow_trailing_comma { json->allow_trailing_comma }
+
     action parse_name {
         char *np;
         json->parsing_name = true;
@@ -495,7 +498,7 @@ static void raise_parse_error(const char *format, const char *start)
 
     main := (
       begin_object
-      (pair (next_pair)*)? ignore*
+      (pair (next_pair)*((ignore* value_separator) when allow_trailing_comma)?)? ignore*
       end_object
     ) @exit;
 }%%
@@ -788,13 +791,15 @@ static char *JSON_parse_float(JSON_Parser *json, char *p, char *pe, VALUE *resul
         }
     }
 
+    action allow_trailing_comma { json->allow_trailing_comma }
+
     action exit { fhold; fbreak; }
 
     next_element  = value_separator ignore* begin_value >parse_value;
 
     main := begin_array ignore*
           ((begin_value >parse_value ignore*)
-           (ignore* next_element ignore*)*)?
+          (ignore* next_element ignore*)*((value_separator ignore*) when allow_trailing_comma)?)?
           end_array @exit;
 }%%
 
@@ -1073,16 +1078,17 @@ static int configure_parser_i(VALUE key, VALUE val, VALUE data)
 {
     JSON_Parser *json = (JSON_Parser *)data;
 
-         if (key == sym_max_nesting)       { json->max_nesting = RTEST(val) ? FIX2INT(val) : 0; }
-    else if (key == sym_allow_nan)         { json->allow_nan = RTEST(val); }
-    else if (key == sym_symbolize_names)   { json->symbolize_names = RTEST(val); }
-    else if (key == sym_freeze)            { json->freeze = RTEST(val); }
-    else if (key == sym_create_id)         { json->create_id = RTEST(val) ? val : Qfalse; }
-    else if (key == sym_object_class)      { json->object_class = RTEST(val) ? val : Qfalse; }
-    else if (key == sym_array_class)       { json->array_class = RTEST(val) ? val : Qfalse; }
-    else if (key == sym_decimal_class)     { json->decimal_class = RTEST(val) ? val : Qfalse; }
-    else if (key == sym_match_string)      { json->match_string = RTEST(val) ? val : Qfalse; }
-    else if (key == sym_create_additions)  {
+         if (key == sym_max_nesting)          { json->max_nesting = RTEST(val) ? FIX2INT(val) : 0; }
+    else if (key == sym_allow_nan)            { json->allow_nan = RTEST(val); }
+    else if (key == sym_allow_trailing_comma) { json->allow_trailing_comma = RTEST(val); }
+    else if (key == sym_symbolize_names)      { json->symbolize_names = RTEST(val); }
+    else if (key == sym_freeze)               { json->freeze = RTEST(val); }
+    else if (key == sym_create_id)            { json->create_id = RTEST(val) ? val : Qfalse; }
+    else if (key == sym_object_class)         { json->object_class = RTEST(val) ? val : Qfalse; }
+    else if (key == sym_array_class)          { json->array_class = RTEST(val) ? val : Qfalse; }
+    else if (key == sym_decimal_class)        { json->decimal_class = RTEST(val) ? val : Qfalse; }
+    else if (key == sym_match_string)         { json->match_string = RTEST(val) ? val : Qfalse; }
+    else if (key == sym_create_additions)     {
         if (NIL_P(val)) {
             json->create_additions = true;
             json->deprecated_create_additions = true;
@@ -1358,6 +1364,7 @@ void Init_parser(void)
 
     sym_max_nesting = ID2SYM(rb_intern("max_nesting"));
     sym_allow_nan = ID2SYM(rb_intern("allow_nan"));
+    sym_allow_trailing_comma = ID2SYM(rb_intern("allow_trailing_comma"));
     sym_symbolize_names = ID2SYM(rb_intern("symbolize_names"));
     sym_freeze = ID2SYM(rb_intern("freeze"));
     sym_create_additions = ID2SYM(rb_intern("create_additions"));