Re: [Tarantool-patches] [PATCH 2/2] json: print context in error mesages

[roman <roman.habibov@tarantool.org>]

Hi! Thanks for the review.

On 12.10.2020 12:20, Leonid Vasiliev wrote:
> Hi! Thank you for the patch.
> I have two small questions:
>
> On 09.10.2020 00:59, Roman Khabibov wrote:
>> Context is just a string with a few characters before and after
>> wrong token, wrong token itself and a symbolic arrow pointing to
>> this token.
>>
>> Closes #4339Hi
>> ---
>>   test/app-tap/json.test.lua        | 20 ++++++++++-
>>   third_party/lua-cjson/lua_cjson.c | 59 ++++++++++++++++++++++++++++---
>>   2 files changed, 73 insertions(+), 6 deletions(-)
>>
>> diff --git a/test/app-tap/json.test.lua b/test/app-tap/json.test.lua
>> index 6d511e686..70e9f6cf7 100755
>> --- a/test/app-tap/json.test.lua
>> +++ b/test/app-tap/json.test.lua
>> @@ -22,7 +22,7 @@ end
>>     tap.test("json", function(test)
>>       local serializer = require('json')
>> -    test:plan(51)
>> +    test:plan(57)
>>         test:test("unsigned", common.test_unsigned, serializer)
>>       test:test("signed", common.test_signed, serializer)
>> @@ -184,4 +184,22 @@ tap.test("json", function(test)
>>       test:ok(string.find(err_msg, 'comma') ~= nil, 'comma instead of 
>> T_COMMA')
>>       _, err_msg = pcall(serializer.decode, '{')
>>       test:ok(string.find(err_msg, 'end') ~= nil, 'end instead of 
>> T_END')
>> +
>> +    --
>> +    -- gh-4339: Make sure that context is printed.
>> +    --
>> +    _, err_msg = pcall(serializer.decode, '{{: "world"}')
>> +    test:ok(string.find(err_msg, '{ >> {: "worl') ~= nil, 'context #1')
>> +    _, err_msg = pcall(serializer.decode, '{"a": "world"}}')
>> +    test:ok(string.find(err_msg, '"world"} >> }') ~= nil, 'context #2')
>> +    _, err_msg = pcall(serializer.decode, '{1: "world"}')
>> +    test:ok(string.find(err_msg, '{ >> 1: "worl') ~= nil, 'context #3')
>> +    _, err_msg = pcall(serializer.decode, '{')
>> +    test:ok(string.find(err_msg, '{ >> ') ~= nil, 'context #4')
>> +    _, err_msg = pcall(serializer.decode, '}')
>> +    test:ok(string.find(err_msg, ' >> }') ~= nil, 'context #5')
>> +    serializer.cfg{decode_max_depth = 1}
>> +    _, err_msg = pcall(serializer.decode, '{"a": {a = {}}}')
>> +    test:ok(string.find(err_msg, '{"a":  >> {a = {}}') ~= nil, 
>> 'context #6')
>> +
>>   end)
>> diff --git a/third_party/lua-cjson/lua_cjson.c 
>> b/third_party/lua-cjson/lua_cjson.c
>> index 33cf30577..f9e06172d 100644
>> --- a/third_party/lua-cjson/lua_cjson.c
>> +++ b/third_party/lua-cjson/lua_cjson.c
>> @@ -831,6 +831,48 @@ static void json_next_token(json_parse_t *json, 
>> json_token_t *token)
>>       json_set_token_error(token, json, "invalid token");
>>   }
>>   +enum context_length {
>> +    CONTEXT_ARROW_LENGTH = 4,
>> +    CONTEXT_MAX_LENGTH_BEFORE = 8,
>> +    CONTEXT_MAX_LENGTH_AFTER = 8,
>> +    CONTEXT_MAX_LENGTH = CONTEXT_MAX_LENGTH_BEFORE + 
>> CONTEXT_MAX_LENGTH_AFTER +
>> +    CONTEXT_ARROW_LENGTH,
>> +};
>> +
>> +/**
>> + * Copy characters near wrong token with the position @a
>> + * column_index to a static string buffer @a context and lay out
>> + * arrow " >> " before this token.
>> + *
>> + * @param context      String static buffer to fill.
>> + * @param json         Structure with pointers to parsing string.
>> + * @param column_index Position of wrong token in the current
>> + *                     line.
>> + */
>> +static void fill_context(char *context, json_parse_t *json, int 
>> column_index)
>> +{
>> +    assert(column_index >= 0);
>> +    int length_before = column_index < CONTEXT_MAX_LENGTH_BEFORE ?
>> +                        column_index : CONTEXT_MAX_LENGTH_BEFORE;
>> +    const char *src = json->cur_line_ptr + column_index - 
>> length_before;
>> +    /* Fill context before the arrow. */
>> +    memcpy(context, src, length_before);
>> +    context += length_before;
>> +    src += length_before;
>> +
>> +    /* Make the arrow. */
>> +    *(context++) = ' ';
>> +    memset(context, '>', CONTEXT_ARROW_LENGTH - 2);
>> +    context += CONTEXT_ARROW_LENGTH - 2;
>> +    *(context++) = ' ';
>> +
>> +    /* Fill context after the arrow. */
>> +    const char *end = context + CONTEXT_MAX_LENGTH_AFTER;
>> +    for (; context < end && *src != '\0' && *src != '\n'; ++src, 
>> ++context)
>> +        *context = *src;
>> +    *context = '\0';
>> +}
>> +
>>   /* This function does not return.
>>    * DO NOT CALL WITH DYNAMIC MEMORY ALLOCATED.
>>    * The only supported exception is the temporary parser string
>> @@ -849,9 +891,13 @@ static void json_throw_parse_error(lua_State *l, 
>> json_parse_t *json,
>>       else
>>           found = json_token_type_name[token->type];
>>   +    int column_index = token->column_index;
>> +    char context[CONTEXT_MAX_LENGTH + 1];
>> +    fill_context(context, json, column_index);
>
> What for you using the additional variable "column_index"? Maybe just
> "fill_context(context, json, token->column_index)".
Yes. Fixed.
>> +
>>       /* Note: token->column_index is 0 based, display starting from 
>> 1 */
>> -    luaL_error(l, "Expected %s but found %s on line %d at character 
>> %d", exp,
>> -               found, json->line_count, token->column_index + 1);
>> +    luaL_error(l, "Expected %s but found %s on line %d at character 
>> %d here "
>> +               "'%s'", exp, found, json->line_count, column_index + 
>> 1, context);
>>   }
>>     static inline void json_decode_ascend(json_parse_t *json)
>> @@ -868,10 +914,13 @@ static void json_decode_descend(lua_State *l, 
>> json_parse_t *json, int slots)
>>           return;
>>       }
>>   +    char context[CONTEXT_MAX_LENGTH + 1];
>> +    fill_context(context, json, json->ptr - json->cur_line_ptr - 1);
>
> Typically, the code uses "token-> column_index = json-> ptr - json-> 
> cur_line_ptr;".
> Why do you use "json->ptr - json->cur_line_ptr - 1"?

Because, in this case json->ptr point to the character after the 
character we need to

point arrow. I mean (json->ptr - 1) - json->cur_line_ptr

>> +
>>       strbuf_free(json->tmp);
>> -    luaL_error(l, "Found too many nested data structures (%d) on 
>> line %d at "
>> -               "character %d", json->current_depth, json->line_count,
>> -               json->ptr - json->cur_line_ptr);
>> +    luaL_error(l, "Found too many nested data structures (%d) on 
>> line %d at cha"
>> +               "racter %d here '%s'", json->current_depth, 
>> json->line_count,
>> +               json->ptr - json->cur_line_ptr, context);
>>   }
>>     static void json_parse_object_context(lua_State *l, json_parse_t 
>> *json)
>>
I decided to rename context to err_context to avoid confusion

with json context (see functions in lua_cjson.c below).

commit 6c5cc1e6f067f1c9358a8bee03501f5df23f0191
Author: Roman Khabibov <roman.habibov@tarantool.org>
Date:   Thu Dec 12 16:55:33 2019 +0300

     json: print context in error mesages

     Context is just a string with a few characters before and after
     wrong token, wrong token itself and a symbolic arrow pointing to
     this token.

     Closes #4339

diff --git a/test/app-tap/json.test.lua b/test/app-tap/json.test.lua
index 6d511e6..70e9f6c 100755
--- a/test/app-tap/json.test.lua
+++ b/test/app-tap/json.test.lua
@@ -22,7 +22,7 @@ end

  tap.test("json", function(test)
      local serializer = require('json')
-    test:plan(51)
+    test:plan(57)

      test:test("unsigned", common.test_unsigned, serializer)
      test:test("signed", common.test_signed, serializer)
@@ -184,4 +184,22 @@ tap.test("json", function(test)
      test:ok(string.find(err_msg, 'comma') ~= nil, 'comma instead of 
T_COMMA')
      _, err_msg = pcall(serializer.decode, '{')
      test:ok(string.find(err_msg, 'end') ~= nil, 'end instead of T_END')
+
+    --
+    -- gh-4339: Make sure that context is printed.
+    --
+    _, err_msg = pcall(serializer.decode, '{{: "world"}')
+    test:ok(string.find(err_msg, '{ >> {: "worl') ~= nil, 'context #1')
+    _, err_msg = pcall(serializer.decode, '{"a": "world"}}')
+    test:ok(string.find(err_msg, '"world"} >> }') ~= nil, 'context #2')
+    _, err_msg = pcall(serializer.decode, '{1: "world"}')
+    test:ok(string.find(err_msg, '{ >> 1: "worl') ~= nil, 'context #3')
+    _, err_msg = pcall(serializer.decode, '{')
+    test:ok(string.find(err_msg, '{ >> ') ~= nil, 'context #4')
+    _, err_msg = pcall(serializer.decode, '}')
+    test:ok(string.find(err_msg, ' >> }') ~= nil, 'context #5')
+    serializer.cfg{decode_max_depth = 1}
+    _, err_msg = pcall(serializer.decode, '{"a": {a = {}}}')
+    test:ok(string.find(err_msg, '{"a":  >> {a = {}}') ~= nil, 'context 
#6')
+
  end)
diff --git a/third_party/lua-cjson/lua_cjson.c 
b/third_party/lua-cjson/lua_cjson.c
index 33cf305..38e9998 100644
--- a/third_party/lua-cjson/lua_cjson.c
+++ b/third_party/lua-cjson/lua_cjson.c
@@ -831,6 +831,50 @@ static void json_next_token(json_parse_t *json, 
json_token_t *token)
      json_set_token_error(token, json, "invalid token");
  }

+enum err_context_length {
+    ERR_CONTEXT_ARROW_LENGTH = 4,
+    ERR_CONTEXT_MAX_LENGTH_BEFORE = 8,
+    ERR_CONTEXT_MAX_LENGTH_AFTER = 8,
+    ERR_CONTEXT_MAX_LENGTH = ERR_CONTEXT_MAX_LENGTH_BEFORE +
+    ERR_CONTEXT_MAX_LENGTH_AFTER + ERR_CONTEXT_ARROW_LENGTH,
+};
+
+/**
+ * Copy characters near wrong token with the position @a
+ * column_index to a static string buffer @a err_context and lay
+ * out arrow " >> " before this token.
+ *
+ * @param context      String static buffer to fill.
+ * @param json         Structure with pointers to parsing string.
+ * @param column_index Position of wrong token in the current
+ *                     line.
+ */
+static void fill_err_context(char *err_context, json_parse_t *json,
+                             int column_index)
+{
+    assert(column_index >= 0);
+    int length_before = column_index < ERR_CONTEXT_MAX_LENGTH_BEFORE ?
+                        column_index : ERR_CONTEXT_MAX_LENGTH_BEFORE;
+    const char *src = json->cur_line_ptr + column_index - length_before;
+    /* Fill error context before the arrow. */
+    memcpy(err_context, src, length_before);
+    err_context += length_before;
+    src += length_before;
+
+    /* Make the arrow. */
+    *(err_context++) = ' ';
+    memset(err_context, '>', ERR_CONTEXT_ARROW_LENGTH - 2);
+    err_context += ERR_CONTEXT_ARROW_LENGTH - 2;
+    *(err_context++) = ' ';
+
+    /* Fill error context after the arrow. */
+    const char *end = err_context + ERR_CONTEXT_MAX_LENGTH_AFTER;
+    for (; err_context < end && *src != '\0' && *src != '\n'; ++src,
+         ++err_context)
+        *err_context = *src;
+    *err_context = '\0';
+}
+
  /* This function does not return.
   * DO NOT CALL WITH DYNAMIC MEMORY ALLOCATED.
   * The only supported exception is the temporary parser string
@@ -849,9 +893,13 @@ static void json_throw_parse_error(lua_State *l, 
json_parse_t *json,
      else
          found = json_token_type_name[token->type];

+    char err_context[ERR_CONTEXT_MAX_LENGTH + 1];
+    fill_err_context(err_context, json, token->column_index);
+
      /* Note: token->column_index is 0 based, display starting from 1 */
-    luaL_error(l, "Expected %s but found %s on line %d at character 
%d", exp,
-               found, json->line_count, token->column_index + 1);
+    luaL_error(l, "Expected %s but found %s on line %d at character %d 
here "
+               "'%s'", exp, found, json->line_count, 
token->column_index + 1,
+               err_context);
  }

  static inline void json_decode_ascend(json_parse_t *json)
@@ -868,10 +916,13 @@ static void json_decode_descend(lua_State *l, 
json_parse_t *json, int slots)
          return;
      }

+    char err_context[ERR_CONTEXT_MAX_LENGTH + 1];
+    fill_err_context(err_context, json, json->ptr - json->cur_line_ptr 
- 1);
+
      strbuf_free(json->tmp);
-    luaL_error(l, "Found too many nested data structures (%d) on line 
%d at "
-               "character %d", json->current_depth, json->line_count,
-               json->ptr - json->cur_line_ptr);
+    luaL_error(l, "Found too many nested data structures (%d) on line 
%d at cha"
+               "racter %d here '%s'", json->current_depth, 
json->line_count,
+               json->ptr - json->cur_line_ptr, err_context);
  }

  static void json_parse_object_context(lua_State *l, json_parse_t *json)