EXPLAIN FORMAT=JSON : Fix MDEV-7266, bug in pretty-printer

- Single_line_formatting_helper should not accidentally exit the
  DISABLED state. No JSON construct should be able to move the
  Single_line_formatting_helper from DISABLED state.

mysql-test/r/explain_json.result

-drop table if exists t0,t1;
+drop table if exists t0,t1,t2;
 create table t0(a int);
 insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
 explain format=json select * from t0;

mysql-test/r/explain_json_innodb.result

+drop table if exists t0,t1,t2;
+#
+# MDEV-7266: Assertion `!element_started' failed in Json_writer& Json_writer::add_member
+# 
+CREATE TABLE t1 (a INT) ENGINE=InnoDB;
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (column_name_1 INT, column_name_2 VARCHAR(52)) ENGINE=InnoDB;
+INSERT INTO t2 VALUES (3,'United States');
+CREATE TABLE t3 (b INT, c VARCHAR(3), PRIMARY KEY (c,b)) ENGINE=InnoDB;
+INSERT INTO t3 VALUES (4,'USA'),(5,'CAN');
+EXPLAIN FORMAT=JSON SELECT * FROM t1 WHERE 0 < ALL ( 
+SELECT tbl_alias1.column_name_1 FROM t2 AS tbl_alias1, t3 AS tbl_alias2 
+WHERE tbl_alias2.b = tbl_alias1.column_name_1 AND tbl_alias2.c = tbl_alias1.column_name_2 
+);
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "t1",
+      "access_type": "ALL",
+      "rows": 2,
+      "filtered": 100
+    },
+    "subqueries": [
+      {
+        "query_block": {
+          "select_id": 2,
+          "table": {
+            "table_name": "tbl_alias1",
+            "access_type": "ALL",
+            "rows": 1,
+            "filtered": 100,
+            "attached_condition": "((tbl_alias1.column_name_2 is not null) and (tbl_alias1.column_name_1 is not null))"
+          },
+          "table": {
+            "table_name": "tbl_alias2",
+            "access_type": "eq_ref",
+            "possible_keys": ["PRIMARY"],
+            "key": "PRIMARY",
+            "key_length": "9",
+            "used_key_parts": ["c", "b"],
+            "ref": [
+              "test.tbl_alias1.column_name_2",
+              "test.tbl_alias1.column_name_1"
+            ],
+            "rows": 1,
+            "filtered": 100,
+            "attached_condition": "(tbl_alias2.c = tbl_alias1.column_name_2)",
+            "using_index": true
+          }
+        }
+      }
+    ]
+  }
+}
+drop table t1,t2,t3;

mysql-test/t/explain_json_innodb.test

+#
+# MariaDB's EXPLAIN FORMAT=JSON tests that require InnoDB.
+#
+--source include/have_innodb.inc
+
+--disable_warnings
+drop table if exists t0,t1,t2;
+--enable_warnings
+
+--echo #
+--echo # MDEV-7266: Assertion `!element_started' failed in Json_writer& Json_writer::add_member
+--echo # 
+
+CREATE TABLE t1 (a INT) ENGINE=InnoDB;
+INSERT INTO t1 VALUES (1),(2);
+
+CREATE TABLE t2 (column_name_1 INT, column_name_2 VARCHAR(52)) ENGINE=InnoDB;
+INSERT INTO t2 VALUES (3,'United States');
+
+CREATE TABLE t3 (b INT, c VARCHAR(3), PRIMARY KEY (c,b)) ENGINE=InnoDB;
+INSERT INTO t3 VALUES (4,'USA'),(5,'CAN');
+
+EXPLAIN FORMAT=JSON SELECT * FROM t1 WHERE 0 < ALL ( 
+  SELECT tbl_alias1.column_name_1 FROM t2 AS tbl_alias1, t3 AS tbl_alias2 
+  WHERE tbl_alias2.b = tbl_alias1.column_name_1 AND tbl_alias2.c = tbl_alias1.column_name_2 
+);
+
+drop table t1,t2,t3;

sql/my_json_writer.cc

@@ -218,6 +218,7 @@ bool Single_line_formatting_helper::on_start_array()
   }
   else
   {
+    if (state != DISABLED)
       state= INACTIVE;
     // TODO: what if we have accumulated some stuff already? shouldn't we
     // flush it?
@@ -313,6 +314,9 @@ void Single_line_formatting_helper::flush_on_one_line()
 
 void Single_line_formatting_helper::disable_and_flush()
 {
+  if (state == DISABLED)
+    return;
+
   bool start_array= (state == IN_ARRAY);
   state= DISABLED;
   // deactivate ourselves and flush all accumulated calls.