MDEV-16462: explain format=json produces illegal json text

[bsrikanth-mariadb]

The attached_condition field of the produced json text had "String with illegal unicode symbol" instead of the proper unicode value.

The reason is that, the item field when being written to the json writer in write_item(), used a String with charset my_charset_bin, and that prevented the string from being escaped.

Solution is to change: -

1. charset to my_charset_utf8mb4_bin in sql_explain.cc. and in Item::print_value()
2. handle string type specially if the charset is my_charset_bin i.e. to append hex value.

[gemini-code-assist]

Code Review

This pull request fixes MDEV-16462, an issue where EXPLAIN FORMAT=JSON produced illegal JSON in the attached_condition field. The fix modifies sql/sql_explain.cc to use system_charset_info instead of &my_charset_bin for the item string buffer and passes the string directly to writer->add_str(). It also adds a corresponding test case. There are no review comments, so I have no feedback to provide.

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[spetrunia]

See https://jira.mariadb.org/browse/MDEV-16462?focusedCommentId=334630&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-334630 and one comment above it.

Please make JSON here use UTF8MB4.

[spetrunia]

Let's try with these changes:

diff --git a/sql/item.cc b/sql/item.cc
index 3bdd7f82da3..4d7e3c9a2fe 100644
--- a/sql/item.cc
+++ b/sql/item.cc
@@ -556,7 +556,19 @@ void Item::print_value(String *str)
     switch (cmp_type()) {
     case STRING_RESULT:
     case TIME_RESULT:
-      append_unescaped(str, ptr->ptr(), ptr->length());
+      if (ptr->charset() == &my_charset_bin)
+      {
+        //str->append('\'');
+        //TODO: but this won't escape the "'" or "\" characters!
+        //str->append_semi_hex(ptr->ptr(), ptr->length(), ptr->charset());
+        //str->append('\'');
+        StringBuffer<64> escaped_val;
+        escaped_val.set_hex(ptr->ptr(), ptr->length());
+        str->append(STRING_WITH_LEN("0x"));
+        str->append(escaped_val);
+      }
+      else
+        append_unescaped(str, ptr->ptr(), ptr->length());
       break;
     case DECIMAL_RESULT:
     case REAL_RESULT:
diff --git a/sql/sql_explain.cc b/sql/sql_explain.cc
index a212f86fe18..288dd0637c7 100644
--- a/sql/sql_explain.cc
+++ b/sql/sql_explain.cc
@@ -1784,7 +1784,7 @@ static void write_item(Json_writer *writer, Item *item)
   item->print(&str, QT_EXPLAIN);
 
   thd->variables.option_bits= save_option_bits;
-  writer->add_escaped_str(str.c_ptr_safe(), str.length());
+  writer->add_str(str.c_ptr_safe(), str.length());
 }
 
 static void append_item_to_str(String *out, Item *item)

[spetrunia]

What will happen in Item::print_value for TIME_RESULT?
I've tried this and I've got a TIME value with my_charset_numeric=latin1.
But just for code clarity purposes, please change the code structure to be :

    case STRING_RESULT:
      if (ptr->charset() == &my_charset_bin)
      {
        ... 
        break;
      }
    case TIME_RESULT:
        append_unescaped(str, ptr->ptr(), ptr->length());
      break;

[bsrikanth-mariadb]

What will happen in Item::print_value for TIME_RESULT? I've tried this and I've got a TIME value with my_charset_numeric=latin1. But just for code clarity purposes, please change the code structure to be :

    case STRING_RESULT:
      if (ptr->charset() == &my_charset_bin)
      {
        ... 
        break;
      }
    case TIME_RESULT:
        append_unescaped(str, ptr->ptr(), ptr->length());
      break;

yes, changed it