MDEV-38120: Move Json_string and Json_saved_parser_state into sql_json_lib.h

Move classes Json_string, and Json_saved_parser_state from
opt_histogram_json.cc to the newly created file sql_json_lib.h
Additionally, make the function json_unescape_to_string public earlier
defined as static in opt_histogram_json.cc, by
adding the declaration in sql_json_lib.h

Needed this change so that, the classes and functions can be reused in
other pieces of the code.

Author: bsrikanth-mariadb

sql/opt_histogram_json.cc

@@ -19,7 +19,7 @@
 #include "my_json_writer.h"
 #include "sql_statistics.h"
 #include "opt_histogram_json.h"
-
+#include "sql_json_lib.h"
 
 /*
   @brief
@@ -31,7 +31,7 @@
     succeeds.
 */
 
-static bool json_unescape_to_string(const char *val, int val_len, String* out)
+bool json_unescape_to_string(const char *val, int val_len, String* out)
 {
   // Make sure 'out' has some memory allocated.
   if (!out->alloced_length() && out->alloc(128))
@@ -416,64 +416,6 @@ void Histogram_json_hb::init_for_collection(MEM_ROOT *mem_root,
   size= (size_t)size_arg;
 }
 
-
-/*
-  A syntax sugar interface to json_string_t
-*/
-class Json_string
-{
-  json_string_t str;
-public:
-  explicit Json_string(const char *name)
-  {
-    json_string_set_str(&str, (const uchar*)name,
-                        (const uchar*)name + strlen(name));
-    json_string_set_cs(&str, system_charset_info);
-  }
-  json_string_t *get() { return &str; }
-};
-
-
-/*
-  This [partially] saves the JSON parser state and then can rollback the parser
-  to it.
-
-  The goal of this is to be able to make multiple json_key_matches() calls:
-
-    Json_saved_parser_state save(je);
-    if (json_key_matches(je, KEY_NAME_1)) {
-      ...
-      return;
-    }
-    save.restore_to(je);
-    if (json_key_matches(je, KEY_NAME_2)) {
-      ...
-    }
-
-  This allows one to parse JSON objects where [optional] members come in any
-  order.
-*/
-
-class Json_saved_parser_state
-{
-  const uchar *c_str;
-  my_wc_t c_next;
-  int state;
-public:
-  explicit Json_saved_parser_state(const json_engine_t *je) :
-    c_str(je->s.c_str),
-    c_next(je->s.c_next),
-    state(je->state)
-  {}
-  void restore_to(json_engine_t *je)
-  {
-    je->s.c_str= c_str;
-    je->s.c_next= c_next;
-    je->state= state;
-  }
-};
-
-
 /*
   @brief
     Read a constant from JSON document and save it in *out.

sql/sql_json_lib.h

@@ -0,0 +1,76 @@
+/*
+   Copyright (c) 2025, MariaDB
+   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 SQL_JSON_LIB
+#define SQL_JSON_LIB
+
+/*
+  A syntax sugar interface to json_string_t
+*/
+class Json_string
+{
+  json_string_t str;
+
+public:
+  explicit Json_string(const char *name)
+  {
+    json_string_set_str(&str, (const uchar *) name,
+                        (const uchar *) name + strlen(name));
+    json_string_set_cs(&str, system_charset_info);
+  }
+  json_string_t *get() { return &str; }
+};
+
+/*
+  This [partially] saves the JSON parser state and then can rollback the parser
+  to it.
+  The goal of this is to be able to make multiple json_key_matches() calls:
+    Json_saved_parser_state save(je);
+    if (json_key_matches(je, KEY_NAME_1)) {
+      ...
+      return;
+    }
+    save.restore_to(je);
+    if (json_key_matches(je, KEY_NAME_2)) {
+      ...
+    }
+  This allows one to parse JSON objects where [optional] members come in any
+  order.
+*/
+class Json_saved_parser_state
+{
+  const uchar *c_str;
+  my_wc_t c_next;
+  int state;
+
+public:
+  explicit Json_saved_parser_state(const json_engine_t *je)
+      : c_str(je->s.c_str), c_next(je->s.c_next), state(je->state)
+  {
+  }
+  void restore_to(json_engine_t *je)
+  {
+    je->s.c_str= c_str;
+    je->s.c_next= c_next;
+    je->state= state;
+  }
+};
+
+/*
+  @brief
+    Un-escape a JSON string and save it into *out.
+*/
+bool json_unescape_to_string(const char *val, int val_len, String *out);
+
+#endif