MDEV-39412: parse error reading tabs in ranges

Note:
while reading from information_schema.optimizer_context one level of unescaping
is already done i.e. (\\t becomes \t or \\\\t becomes \\t)

w.r.t the MDEV, there are 2 problems: -

1.
When reading from the sql script file, json parser is not able to parse
the range value in json_read_value() from json_lib.c
"ranges": [
            "(b\t\t\t\t\t\t) <= (b) <= (b???????)"
          ],
mainly the \t\t stuff, and hence a warning.
It also stops loading the context into memory.
Since, a new table is created with empty data, and without context,
we get Impossible WHERE noticed after reading const tables

2.
There is unescaping call being made in read_string() from sql_json_lib.cc
while parsing of the context. With this \\t was becoming \t.
However, print_range() from opt_range.cc already does escaping of the values.
The value "b\t\t\t" was in fact produced as "\b\\t\\t\\t".
Later, we try to compare range values from the query and the context.

Here a mismatch is found because, in one case there is escaping,
and in the other case escaping got removed.

Solution
========
Since, there are 2 levels of unescaping being performed, 1. during sql
parse of the context from information_schema, and 2. during
read_string. So, we need to have 2 levels of escaping.

First is done in the dump_mrr_info_calls() - here
json_escape_to_string() is used.

Second is done at the end of store_optimizer_context(), for the entire
opt_context. Here a newly introduced function escape_json_for_sql_literal()
is used which does escaping only for backslash, and single quote.

Author: bsrikanth-mariadb

mysql-test/main/opt_context_replay_basic.result

@@ -337,4 +337,33 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	btn	NULL	NULL	NULL	4	Using where
 set optimizer_replay_context='';
 drop table t1;
+#
+# MDEV-39412: Failed to parse saved optimizer context: error reading ranges value
+#
+set optimizer_record_context=0;
+CREATE TABLE t1(
+a VARCHAR(8),
+b VARCHAR(8),
+KEY(A),
+KEY(B)
+);
+INSERT INTO t1 SELECT REPEAT('a',8), REPEAT('b',8) FROM seq_1_to_10;
+set optimizer_record_context=1;
+EXPLAIN 
+SELECT * FROM t1 FORCE INDEX(a,b) WHERE a LIKE 'a%' OR b LIKE 'b%' 
+ORDER BY a,b;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index_merge	a,b	a,b	35,35	NULL	10	Using sort_union(a,b); Using where; Using filesort
+select context into dumpfile "../../tmp/dump1.sql" 
+from information_schema.optimizer_context;
+drop table t1;
+set optimizer_replay_context='opt_context';
+# Same query as above, must have same explain:
+EXPLAIN 
+SELECT * FROM t1 FORCE INDEX(a,b) WHERE a LIKE 'a%' OR b LIKE 'b%' 
+ORDER BY a,b;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index_merge	a,b	a,b	35,35	NULL	10	Using sort_union(a,b); Using where; Using filesort
+set optimizer_replay_context='';
+drop table t1;
 drop database db1;

mysql-test/main/opt_context_replay_basic.test

@@ -200,4 +200,41 @@ set optimizer_replay_context='';
 --remove_file "$MYSQLTEST_VARDIR/tmp/dump1.sql"
 drop table t1;
 
+--echo #
+--echo # MDEV-39412: Failed to parse saved optimizer context: error reading ranges value
+--echo #
+set optimizer_record_context=0;
+
+CREATE TABLE t1(
+    a VARCHAR(8),
+    b VARCHAR(8),
+    KEY(A),
+    KEY(B)
+);
+INSERT INTO t1 SELECT REPEAT('a',8), REPEAT('b',8) FROM seq_1_to_10;
+
+set optimizer_record_context=1;
+EXPLAIN 
+SELECT * FROM t1 FORCE INDEX(a,b) WHERE a LIKE 'a%' OR b LIKE 'b%' 
+ORDER BY a,b;
+
+select context into dumpfile "../../tmp/dump1.sql" 
+from information_schema.optimizer_context;
+drop table t1;
+
+--disable_query_log
+--disable_result_log
+--source "$MYSQLTEST_VARDIR/tmp/dump1.sql"
+--enable_query_log
+--enable_result_log
+set optimizer_replay_context='opt_context';
+--echo # Same query as above, must have same explain:
+EXPLAIN 
+SELECT * FROM t1 FORCE INDEX(a,b) WHERE a LIKE 'a%' OR b LIKE 'b%' 
+ORDER BY a,b;
+
+set optimizer_replay_context='';
+--remove_file "$MYSQLTEST_VARDIR/tmp/dump1.sql"
+drop table t1;
+
 drop database db1;

sql/opt_context_store_replay.cc

@@ -256,12 +256,18 @@ void dump_mrr_info_calls(List<Multi_range_read_const_call_record> *mrr_list,
     Json_writer_object irc_wrapper(ctx_writer);
     irc_wrapper.add("index_name", irc->idx_name);
 
+    List_iterator rc_li(irc->range_list);
+    Json_writer_array ranges_wrapper(ctx_writer, "ranges");
+    StringBuffer<128> escaped_range_info;
+    while (const char *range_str= rc_li++)
     {
-      Json_writer_array ranges_wrapper(ctx_writer, "ranges");
-      List_iterator rc_li(irc->range_list);
-      while (const char *range_str= rc_li++)
-        ranges_wrapper.add(range_str, strlen(range_str));
+      const String range_info(range_str, strlen(range_str),
+                              system_charset_info);
+      json_escape_to_string(&range_info, &escaped_range_info);
+      ranges_wrapper.add(escaped_range_info.c_ptr_safe(),
+                         escaped_range_info.length());
     }
+    ranges_wrapper.end();
 
     irc_wrapper.add("num_rows", irc->rows);
     {
@@ -515,6 +521,31 @@ static bool store_db_ddl(THD *thd, HASH *db_name_hash, String &sql_script,
   return false;
 }
 
+/*
+  @brief
+    Append the json "str" to sql_script, by escaping only backslash,
+    and single quote
+*/
+static void escape_json_for_sql_literal(String &sql_script, const char *str,
+                                        size_t len)
+{
+  const char *end= str + len;
+  for (; str < end; str++)
+  {
+    switch (*str)
+    {
+    case '\\':
+      sql_script.append(STRING_WITH_LEN("\\\\"));
+      break;
+    case '\'':
+      sql_script.append(STRING_WITH_LEN("\\'"));
+      break;
+    default:
+      sql_script.append(*str);
+    }
+  }
+}
+
 /*
   @brief
     Dump definitions, basic stats of all tables and views used by the
@@ -688,9 +719,12 @@ bool store_optimizer_context(THD *thd)
     const char *SET_OPT_CONTEXT_VAR= "set @opt_context=\'\n";
     const char *SET_REPLAY_CONTEXT_VAR=
         "set optimizer_replay_context=\'opt_context\'";
-    String *s= const_cast<String *>(ctx_writer.output.get_string());
     sql_script.append(SET_OPT_CONTEXT_VAR, strlen(SET_OPT_CONTEXT_VAR));
-    sql_script.append(*s);
+    String *opt_context= const_cast<String *>(ctx_writer.output.get_string());
+    // require extra escaping of the opt_ctx so as to counter the
+    // unescaping done by sql parse
+    escape_json_for_sql_literal(sql_script, opt_context->c_ptr_safe(),
+                                opt_context->length());
     sql_script.append(STRING_WITH_LEN("\n\';#opt_context_ends\n\n"));
     sql_script.append(SET_REPLAY_CONTEXT_VAR, strlen(SET_REPLAY_CONTEXT_VAR));
     sql_script.append(STRING_WITH_LEN(";\n\n"));

unittest/sql/json_reader-t.cc

@@ -39,25 +39,29 @@ int main(int args, char **argv)
   MEM_ROOT alloc;
   json_engine_t je;
   int rc;
+  const char *esc_str_val= "a\bc";
   init_alloc_root(0, &alloc, 32768, 0, 0);
   mem_root_dynamic_array_init(&alloc, 0, &je.stack,
                               sizeof(int), NULL, JSON_DEPTH_DEFAULT,
                               JSON_DEPTH_INC, MYF(0));
   system_charset_info= &my_charset_utf8mb3_bin;
-  const char *js_doc="{ \"str_val\": \"abc\", \"double_val\": 1234.5 }";
+  const char *js_doc= "{ \"str_val\": \"abc\", \"double_val\": 1234.5, "
+                      "\"esc_str_val\": \"a\\bc\" }";
   json_scan_start(&je, &my_charset_utf8mb3_bin, (const uchar *) js_doc,
                   (const uchar *) js_doc + strlen(js_doc));
 
   char *parsed_name;
   double parsed_dbl;
+  char *parsed_esc_str;
   Read_named_member array[]= {
-      {"str_val",    Read_string(&alloc, &parsed_name), false},
+      {"str_val", Read_string(&alloc, &parsed_name), false},
       {"double_val", Read_double(&parsed_dbl), false},
-      {NULL, Read_double(NULL), false }
-  };
+      {"esc_str_val", Read_string(&alloc, &parsed_esc_str), false},
+      {NULL, Read_double(NULL), false}};
   String err_buf;
 
-  rc= json_read_object(&je, array, &err_buf);
+  rc= json_read_object(&je, array, &err_buf) ||
+      strcmp(parsed_esc_str, esc_str_val);
   ok(!rc, "Basic object read");
   free_root(&alloc, 0);
 #if 0