EXPLAIN FORMAT=JSON: further development

Writing JSON:
- Fix a bug in Single_line_formatting_helper
- Add Json_writer_nesting_guard - safety class

EXPLAIN JSON support
- Add basic subquery support
- Add tests for UNION/UNION ALL.

sql/my_json_writer.h

@@ -34,9 +34,30 @@ class Single_line_formatting_helper
     DISABLED
   };
 
+  /*
+    This works like a finite automaton. 
+
+    state=DISABLED means the helper is disabled - all on_XXX functions will
+    return false (which means "not handled") and do nothing.
+
+                                      +->-+
+                                      |   v
+       INACTIVE ---> ADD_MEMBER ---> IN_ARRAY--->-+
+          ^                                       |
+          +------------------<--------------------+
+                              
+    For other states: 
+    INACTIVE    - initial state, we have nothing.
+    ADD_MEMBER  - add_member() was called, the buffer has "member_name\0".
+    IN_ARRAY    - start_array() was called.
+
+
+  */
   enum enum_state state;
   enum { MAX_LINE_LEN= 80 };
   char buffer[80];
+
+  /* The data in the buffer is located between buffer[0] and buf_ptr */
   char *buf_ptr;
   uint line_len;
 
@@ -99,8 +120,9 @@ private:
   // TODO: a stack of (name, bool is_object_or_array) elements.
   int indent_level;
   enum { INDENT_SIZE = 2 };
- 
+
   friend class Single_line_formatting_helper;
+  friend class Json_writer_nesting_guard;
   bool document_start;
   bool element_started;
   bool first_child;
@@ -116,3 +138,40 @@ public:
   String output;
 };
 
+
+/*
+  RAII-based helper class to detect incorrect use of Json_writer.
+
+  The idea is that a function typically must leave Json_writer at the same
+  identation level as it was when it was invoked. Leaving it at a different 
+  level typically means we forgot to close an object or an array
+
+  So, here is a way to guard
+  void foo(Json_writer *writer)
+  {
+    Json_writer_nesting_guard(writer);
+    .. do something with writer
+
+    // at the end of the function, ~Json_writer_nesting_guard() is called
+    // and it makes sure that the nesting is the same as when the function was
+    // entered.
+  }
+*/
+
+class Json_writer_nesting_guard
+{
+  Json_writer* writer;
+  int indent_level;
+public:
+  Json_writer_nesting_guard(Json_writer *writer_arg) : 
+    writer(writer_arg),
+    indent_level(writer->indent_level)
+  {}
+
+  ~Json_writer_nesting_guard()
+  {
+    DBUG_ASSERT(indent_level == writer->indent_level);
+  }
+};
+
+