Json_writer allows one to create duplicate object members

Json_writer code allows one to write multiple members with the same name in a JSON object.

This is likely to prevent proper processing of the created document.

The most striking example is EXPLAIN FORMAT=JSON:

{

  "query_block": {

    "select_id": 1,

    "table": {

      "table_name": "table",

      ...

    },

     "table": {

      ...

The solution has two parts:

Part #1: Follow the MDEV-23766 approach and make debug build generate assertion failures.

Part #2: Fix EXPLAIN FORMAT=JSON code to not produce such invalid (or valid but not processable) JSON output.

As for which version to fix this in: The problem with EXPLAIN FORMAT=JSON output is present from the very early versions (10.1). However, the fix will cause a lot of changes in EXPLAIN FORMAT=JSON output (the changes will be dummy but they will be there). This may not be desirable for older versions. Because of that, for now I'm setting fixVersion to 10.8.

A question to be resolved: how to modify EXPLAIN/ANALYZE to make them well-formed JSON? Should we wrap them into an array, like MySQL does or do something else?)