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diff --git a/sql/my_json_writer.h b/sql/my_json_writer.h
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+/* Copyright (C) 2014 SkySQL Ab, MariaDB Corporation Ab
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; version 2 of the License.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1335 USA */
+
+#ifndef JSON_WRITER_INCLUDED
+#define JSON_WRITER_INCLUDED
+#include "my_base.h"
+#include "sql_select.h"
+class Opt_trace_stmt;
+class Opt_trace_context;
+class Json_writer;
+struct TABLE_LIST;
+
+
+/*
+  Single_line_formatting_helper is used by Json_writer to do better formatting
+  of JSON documents. 
+
+  The idea is to catch arrays that can be printed on one line:
+
+    arrayName : [ "boo", 123, 456 ] 
+
+  and actually print them on one line. Arrrays that occupy too much space on
+  the line, or have nested members cannot be printed on one line.
+  
+  We hook into JSON printing functions and try to detect the pattern. While
+  detecting the pattern, we will accumulate "boo", 123, 456 as strings.
+
+  Then, 
+   - either the pattern is broken, and we print the elements out, 
+   - or the pattern lasts till the end of the array, and we print the 
+     array on one line.
+*/
+
+class Single_line_formatting_helper
+{
+  enum enum_state
+  {
+    INACTIVE,
+    ADD_MEMBER,
+    IN_ARRAY,
+    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;
+
+  Json_writer *owner;
+public:
+  Single_line_formatting_helper() : state(INACTIVE), buf_ptr(buffer) {}
+
+  void init(Json_writer *owner_arg) { owner= owner_arg; }
+
+  bool on_add_member(const char *name, size_t len);
+
+  bool on_start_array();
+  bool on_end_array();
+  void on_start_object();
+  // on_end_object() is not needed.
+   
+  bool on_add_str(const char *str, size_t num_bytes);
+
+  void flush_on_one_line();
+  void disable_and_flush();
+};
+
+
+/*
+  Something that looks like class String, but has an internal limit of
+  how many bytes one can append to it.
+
+  Bytes that were truncated due to the size limitation are counted.
+*/
+
+class String_with_limit
+{
+public:
+
+  String_with_limit() : size_limit(SIZE_T_MAX), truncated_len(0)
+  {
+    str.length(0);
+  }
+
+  size_t get_truncated_bytes() const { return truncated_len; }
+  size_t get_size_limit() { return size_limit; }
+
+  void set_size_limit(size_t limit_arg)
+  {
+    // Setting size limit to be shorter than length will not have the desired
+    // effect
+    DBUG_ASSERT(str.length() < size_limit);
+    size_limit= limit_arg;
+  }
+
+  void append(const char *s, size_t size)
+  {
+    if (str.length() + size <= size_limit)
+    {
+      // Whole string can be added, just do it
+      str.append(s, size);
+    }
+    else
+    {
+      // We cannot add the whole string
+      if (str.length() < size_limit)
+      {
+        // But we can still add something
+        size_t bytes_to_add = size_limit - str.length();
+        str.append(s, bytes_to_add);
+        truncated_len += size - bytes_to_add;
+      }
+      else
+        truncated_len += size;
+    }
+  }
+
+  void append(const char *s)
+  {
+    append(s, strlen(s));
+  }
+
+  void append(char c)
+  {
+    if (str.length() + 1 > size_limit)
+      truncated_len++;
+    else
+      str.append(c);
+  }
+
+  const String *get_string() { return &str; }
+  size_t length() { return str.length(); }
+private:
+  String str;
+
+  // str must not get longer than this many bytes.
+  size_t size_limit;
+
+  // How many bytes were truncated from the string
+  size_t truncated_len;
+};
+
+/*
+  A class to write well-formed JSON documents. The documents are also formatted
+  for human readability.
+*/
+
+class Json_writer
+{
+public:
+  /* Add a member. We must be in an object. */
+  Json_writer& add_member(const char *name);
+  Json_writer& add_member(const char *name, size_t len);
+  
+  /* Add atomic values */
+  void add_str(const char* val);
+  void add_str(const char* val, size_t num_bytes);
+  void add_str(const String &str);
+  void add_str(Item *item);
+  void add_table_name(const JOIN_TAB *tab);
+  void add_table_name(const TABLE* table);
+
+  void add_ll(longlong val);
+  void add_ull(ulonglong val);
+  void add_size(longlong val);
+  void add_double(double val);
+  void add_bool(bool val);
+  void add_null();
+
+private:
+  void add_unquoted_str(const char* val);
+  void add_unquoted_str(const char* val, size_t len);
+public:
+  /* Start a child object */
+  void start_object();
+  void start_array();
+
+  void end_object();
+  void end_array();
+  
+  /*
+    One can set a limit of how large a JSON document should be.
+    Writes beyond that size will be counted, but will not be collected.
+  */
+  void set_size_limit(size_t mem_size) { output.set_size_limit(mem_size); }
+
+  size_t get_truncated_bytes() { return output.get_truncated_bytes(); }
+
+  Json_writer() : 
+    indent_level(0), document_start(true), element_started(false), 
+    first_child(true)
+  {
+    fmt_helper.init(this);
+  }
+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;
+
+  Single_line_formatting_helper fmt_helper;
+
+  void append_indent();
+  void start_element();
+  void start_sub_element();
+
+public:
+  String_with_limit output;
+};
+
+/* A class to add values to Json_writer_object and Json_writer_array */
+class Json_value_helper
+{
+  Json_writer* writer;
+
+public:
+  void init(Json_writer *my_writer) { writer= my_writer; }
+  void add_str(const char* val)
+  {
+      writer->add_str(val);
+  }
+  void add_str(const char* val, size_t length)
+  {
+      writer->add_str(val, length);
+  }
+  void add_str(const String &str)
+  {
+      writer->add_str(str.ptr(), str.length());
+  }
+  void add_str(const LEX_CSTRING &str)
+  {
+      writer->add_str(str.str, str.length);
+  }
+  void add_str(Item *item)
+  {
+      writer->add_str(item);
+  }
+
+  void add_ll(longlong val)
+  {
+      writer->add_ll(val);
+  }
+  void add_size(longlong val)
+  {
+      writer->add_size(val);
+  }
+  void add_double(double val)
+  {
+      writer->add_double(val);
+  }
+  void add_bool(bool val)
+  {
+      writer->add_bool(val);
+  }
+  void add_null()
+  {
+      writer->add_null();
+  }
+  void add_table_name(const JOIN_TAB *tab)
+  {
+      writer->add_table_name(tab);
+  }
+  void add_table_name(const TABLE* table)
+  {
+      writer->add_table_name(table);
+  }
+};
+
+/* A common base for Json_writer_object and Json_writer_array */
+class Json_writer_struct
+{
+protected:
+  Json_writer* my_writer;
+  Json_value_helper context;
+  /*
+    Tells when a json_writer_struct has been closed or not
+  */
+  bool closed;
+
+public:
+  explicit Json_writer_struct(THD *thd)
+  {
+    my_writer= thd->opt_trace.get_current_json();
+    context.init(my_writer);
+    closed= false;
+  }
+  bool trace_started()
+  {
+    return my_writer != 0;
+  }
+};
+
+
+/*
+  RAII-based class to start/end writing a JSON object into the JSON document
+
+  There is "ignore mode": one can initialize Json_writer_object with a NULL
+  Json_writer argument, and then all its calls will do nothing. This is used
+  by optimizer trace which can be enabled or disabled.
+*/
+
+class Json_writer_object : public Json_writer_struct
+{
+private:
+  void add_member(const char *name)
+  {
+    my_writer->add_member(name);
+  }
+public:
+  explicit Json_writer_object(THD *thd)
+  : Json_writer_struct(thd)
+  {
+    if (unlikely(my_writer))
+      my_writer->start_object();
+  }
+
+  explicit Json_writer_object(THD* thd, const char *str)
+    : Json_writer_struct(thd)
+  {
+    if (unlikely(my_writer))
+      my_writer->add_member(str).start_object();
+  }
+
+  ~Json_writer_object()
+  {
+    if (my_writer && !closed)
+      my_writer->end_object();
+    closed= TRUE;
+  }
+
+  Json_writer_object& add(const char *name, bool value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+    {
+      add_member(name);
+      context.add_bool(value);
+    }
+    return *this;
+  }
+  Json_writer_object& add(const char *name, ulonglong value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+    {
+      add_member(name);
+      context.add_ll(static_cast<longlong>(value));
+    }
+    return *this;
+  }
+  Json_writer_object& add(const char *name, longlong value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+    {
+      add_member(name);
+      context.add_ll(value);
+    }
+    return *this;
+  }
+  Json_writer_object& add(const char *name, double value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+    {
+      add_member(name);
+      context.add_double(value);
+    }
+    return *this;
+  }
+  #ifndef _WIN64
+  Json_writer_object& add(const char *name, size_t value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+    {
+      add_member(name);
+      context.add_ll(static_cast<longlong>(value));
+    }
+    return *this;
+  }
+  #endif
+  Json_writer_object& add(const char *name, const char *value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+    {
+      add_member(name);
+      context.add_str(value);
+    }
+    return *this;
+  }
+  Json_writer_object& add(const char *name, const char *value, size_t num_bytes)
+  {
+    add_member(name);
+    context.add_str(value, num_bytes);
+    return *this;
+  }
+  Json_writer_object& add(const char *name, const LEX_CSTRING &value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+    {
+      add_member(name);
+      context.add_str(value.str, value.length);
+    }
+    return *this;
+  }
+  Json_writer_object& add(const char *name, Item *value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+    {
+      add_member(name);
+      context.add_str(value);
+    }
+    return *this;
+  }
+  Json_writer_object& add_null(const char*name)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+    {
+      add_member(name);
+      context.add_null();
+    }
+    return *this;
+  }
+  Json_writer_object& add_table_name(const JOIN_TAB *tab)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+    {
+      add_member("table");
+      context.add_table_name(tab);
+    }
+    return *this;
+  }
+  Json_writer_object& add_table_name(const TABLE *table)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+    {
+      add_member("table");
+      context.add_table_name(table);
+    }
+    return *this;
+  }
+  Json_writer_object& add_select_number(uint select_number)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+    {
+      add_member("select_id");
+      if (unlikely(select_number >= INT_MAX))
+        context.add_str("fake");
+      else
+        context.add_ll(static_cast<longlong>(select_number));
+    }
+    return *this;
+  }
+  void end()
+  {
+    DBUG_ASSERT(!closed);
+    if (unlikely(my_writer))
+      my_writer->end_object();
+    closed= TRUE;
+  }
+};
+
+
+/*
+  RAII-based class to start/end writing a JSON array into the JSON document
+
+  There is "ignore mode": one can initialize Json_writer_array with a NULL
+  Json_writer argument, and then all its calls will do nothing. This is used
+  by optimizer trace which can be enabled or disabled.
+*/
+
+class Json_writer_array : public Json_writer_struct
+{
+public:
+  Json_writer_array(THD *thd): Json_writer_struct(thd)
+  {
+    if (unlikely(my_writer))
+      my_writer->start_array();
+  }
+
+  Json_writer_array(THD *thd, const char *str) : Json_writer_struct(thd)
+  {
+    if (unlikely(my_writer))
+      my_writer->add_member(str).start_array();
+  }
+  ~Json_writer_array()
+  {
+    if (unlikely(my_writer && !closed))
+    {
+      my_writer->end_array();
+      closed= TRUE;
+    }
+  }
+  void end()
+  {
+    DBUG_ASSERT(!closed);
+    if (unlikely(my_writer))
+      my_writer->end_array();
+    closed= TRUE;
+  }
+
+  Json_writer_array& add(bool value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+      context.add_bool(value);
+    return *this;
+  }
+  Json_writer_array& add(ulonglong value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+      context.add_ll(static_cast<longlong>(value));
+    return *this;
+  }
+  Json_writer_array& add(longlong value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+      context.add_ll(value);
+    return *this;
+  }
+  Json_writer_array& add(double value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+      context.add_double(value);
+    return *this;
+  }
+  #ifndef _WIN64
+  Json_writer_array& add(size_t value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+      context.add_ll(static_cast<longlong>(value));
+    return *this;
+  }
+  #endif
+  Json_writer_array& add(const char *value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+      context.add_str(value);
+    return *this;
+  }
+  Json_writer_array& add(const char *value, size_t num_bytes)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+      context.add_str(value, num_bytes);
+    return *this;
+  }
+  Json_writer_array& add(const LEX_CSTRING &value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+      context.add_str(value.str, value.length);
+    return *this;
+  }
+  Json_writer_array& add(Item *value)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+      context.add_str(value);
+    return *this;
+  }
+  Json_writer_array& add_null()
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+      context.add_null();
+    return *this;
+  }
+  Json_writer_array& add_table_name(const JOIN_TAB *tab)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+      context.add_table_name(tab);
+    return *this;
+  }
+  Json_writer_array& add_table_name(const TABLE *table)
+  {
+    DBUG_ASSERT(!closed);
+    if (my_writer)
+      context.add_table_name(table);
+    return *this;
+  }
+};
+
+/*
+  RAII-based class to disable writing into the JSON document
+  The tracing is disabled as soon as the object is created.
+  The destuctor is called as soon as we exit the scope of the object
+  and the tracing is enabled back.
+*/
+
+class Json_writer_temp_disable
+{
+public:
+  Json_writer_temp_disable(THD *thd_arg);
+  ~Json_writer_temp_disable();
+  THD *thd;
+};
+
+/*
+  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
+{
+#ifdef DBUG_OFF
+public:
+  Json_writer_nesting_guard(Json_writer *) {}
+#else
+  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);
+  }
+#endif
+};
+
+#endif