diff --git a/.gitignore b/.gitignore
index 00157d50d69..485b2d186cf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,8 +10,11 @@
 .ninja_*
 *.mri
 *.mri.tpl
+/.cproject
+/.project
 .gdb_history
 .vs/
+/.settings/
 errmsg.sys
 typescript
 _CPack_Packages
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 00007f92f57..75fcb311ddd 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -173,6 +173,10 @@ IF(DISABLE_SHARED)
   SET(WITHOUT_DYNAMIC_PLUGINS 1)
 ENDIF()
 OPTION(ENABLED_PROFILING "Enable profiling" ON)
+OPTION(ENABLED_JSON_WRITER_CONSISTENCY_CHECKS "Enable Json_writer_object / Json_writer_array checking to produce consistent JSON output" OFF)
+IF(ENABLED_JSON_WRITER_CONSISTENCY_CHECKS)
+  ADD_DEFINITIONS(-DENABLED_JSON_WRITER_CONSISTENCY_CHECKS)
+ENDIF()
 OPTION(WITHOUT_SERVER "Build only the client library and clients" OFF)
 IF(UNIX)
   OPTION(WITH_VALGRIND "Valgrind instrumentation" OFF)
diff --git a/cmake/pcre.cmake b/cmake/pcre.cmake
index 3d4f163fab9..5ea81f53828 100644
--- a/cmake/pcre.cmake
+++ b/cmake/pcre.cmake
@@ -11,21 +11,22 @@ MACRO(BUNDLE_PCRE2)
   FOREACH(lib pcre2-posix pcre2-8)
     ADD_LIBRARY(${lib} STATIC IMPORTED GLOBAL)
     ADD_DEPENDENCIES(${lib} pcre2)
+
+    GET_PROPERTY(MULTICONFIG GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
+    IF(MULTICONFIG)
+      SET(intdir "${CMAKE_CFG_INTDIR}/")
+    ELSE()
+      SET(intdir)
+    ENDIF()
+
+    SET(file ${dir}/src/pcre2-build/${intdir}${CMAKE_STATIC_LIBRARY_PREFIX}${lib}${CMAKE_STATIC_LIBRARY_SUFFIX})
+
     IF(WIN32)
       # Debug libary name.
       # Same condition as in pcre2 CMakeLists.txt that adds "d"
-      GET_PROPERTY(MULTICONFIG GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
-      IF(MULTICONFIG)
-        SET(intdir "${CMAKE_CFG_INTDIR}/")
-      ELSE()
-        SET(intdir)
-      ENDIF()
-
-      SET(file ${dir}/src/pcre2-build/${intdir}${CMAKE_STATIC_LIBRARY_PREFIX}${lib}${CMAKE_STATIC_LIBRARY_SUFFIX})
       SET(file_d ${dir}/src/pcre2-build/${intdir}${CMAKE_STATIC_LIBRARY_PREFIX}${lib}d${CMAKE_STATIC_LIBRARY_SUFFIX})
       SET_TARGET_PROPERTIES(${lib} PROPERTIES IMPORTED_LOCATION_DEBUG ${file_d})
     ELSE()
-      SET(file ${dir}/src/pcre2-build/${CMAKE_STATIC_LIBRARY_PREFIX}${lib}${CMAKE_STATIC_LIBRARY_SUFFIX})
       SET(file_d)
     ENDIF()
     SET(byproducts ${byproducts} BUILD_BYPRODUCTS ${file} ${file_d})
diff --git a/mysql-test/mariadb-test-run.pl b/mysql-test/mariadb-test-run.pl
index 6dcd25a9c86..330c54c8e62 100755
--- a/mysql-test/mariadb-test-run.pl
+++ b/mysql-test/mariadb-test-run.pl
@@ -1271,6 +1271,7 @@ sub command_line_setup {
   {
     $path_client_bindir= mtr_path_exists("$bindir/client_release",
 					 "$bindir/client_debug",
+					 "$bindir/client/debug",
 					 "$bindir/client$opt_vs_config",
 					 "$bindir/client",
 					 "$bindir/bin");
diff --git a/mysql-test/suite/galera_3nodes_sr/r/MDEV-26707.result b/mysql-test/suite/galera_3nodes_sr/r/MDEV-26707.result
index a041274162f..8de724c1576 100644
--- a/mysql-test/suite/galera_3nodes_sr/r/MDEV-26707.result
+++ b/mysql-test/suite/galera_3nodes_sr/r/MDEV-26707.result
@@ -71,35 +71,16 @@ connection node_3a;
 SET SESSION wsrep_sync_wait = 0;
 SET SESSION wsrep_sync_wait = DEFAULT;
 connection node_1a;
-Timeout in wait_condition.inc for SELECT COUNT(*) = 0 FROM mysql.wsrep_streaming_log
-Id	User	Host	db	Command	Time	State	Info	Progress
-1	system user		NULL	Sleep	66	wsrep aborter idle	NULL	0.000
-2	system user		NULL	Sleep	66	closing tables	NULL	0.000
-10	root	localhost	test	Sleep	58		NULL	0.000
-11	root	localhost:52722	test	Sleep	56		NULL	0.000
-12	root	localhost:52724	test	Query	0	starting	show full processlist	0.000
 SELECT COUNT(*) AS EXPECT_0 FROM  mysql.wsrep_streaming_log;
 EXPECT_0
-1
+0
 connection node_2a;
-Timeout in wait_condition.inc for SELECT COUNT(*) = 0 FROM mysql.wsrep_streaming_log
-Id	User	Host	db	Command	Time	State	Info	Progress
-1	system user		NULL	Sleep	96	wsrep aborter idle	NULL	0.000
-2	system user		NULL	Sleep	87	closing tables	NULL	0.000
-10	root	localhost:37222	test	Sleep	64		NULL	0.000
-11	root	localhost:37228	test	Query	0	starting	show full processlist	0.000
 SELECT COUNT(*) AS EXPECT_0 FROM  mysql.wsrep_streaming_log;
 EXPECT_0
-1
+0
 connection node_3a;
-Timeout in wait_condition.inc for SELECT COUNT(*) = 0 FROM mysql.wsrep_streaming_log
-Id	User	Host	db	Command	Time	State	Info	Progress
-1	system user		NULL	Sleep	122	wsrep aborter idle	NULL	0.000
-2	system user		NULL	Sleep	117	closing tables	NULL	0.000
-10	root	localhost:60992	test	Sleep	117		NULL	0.000
-11	root	localhost:60994	test	Query	0	starting	show full processlist	0.000
 SELECT COUNT(*) AS EXPECT_0 FROM  mysql.wsrep_streaming_log;
 EXPECT_0
-1
+0
 connection node_1;
 DROP TABLE t1;
diff --git a/mysql-test/suite/rpl/r/rpl_xa.result b/mysql-test/suite/rpl/r/rpl_xa.result
index a90e6e0b996..061c7b360d0 100644
--- a/mysql-test/suite/rpl/r/rpl_xa.result
+++ b/mysql-test/suite/rpl/r/rpl_xa.result
@@ -219,4 +219,65 @@ include/sync_with_master_gtid.inc
 connection master;
 drop database test_ign;
 drop table t1, t2, t3, tm;
+#
+# MDEV-26682 slave lock timeout with XA and gap locks
+#
+create table t1 (a int primary key, b int unique) engine=innodb;
+insert t1 values (1,1),(3,3),(5,5);
+connection slave;
+set session tx_isolation='repeatable-read';
+start transaction;
+select * from t1;
+a	b
+1	1
+3	3
+5	5
+connect m2, localhost, root;
+delete from t1 where a=3;
+xa start 'x1';
+update t1 set b=3 where a=5;
+xa end 'x1';
+xa prepare 'x1';
+connect m3, localhost, root;
+insert t1 values (2, 2);
+-->slave
+connection slave;
+commit;
+select * from t1;
+a	b
+1	1
+2	2
+5	5
+connection m2;
+xa rollback 'x1';
+disconnect m2;
+disconnect m3;
+connection master;
+drop table t1;
+create table t1 (id int auto_increment primary key, c1 int not null unique)
+engine=innodb;
+create table t2 (id int auto_increment primary key, c1 int not null,
+foreign key(c1) references t1(c1), unique key(c1)) engine=innodb;
+insert t1 values (869,1), (871,3), (873,4), (872,5), (870,6), (877,7);
+insert t2 values (795,6), (800,7);
+xa start '1';
+update t2 set id = 9, c1 = 5 where c1 in (null, null, null, null, null, 7, 3);
+connect con1, localhost,root;
+xa start '2';
+delete from t1 where c1 like '3%';
+xa end '2';
+xa prepare '2';
+connection master;
+xa end '1';
+xa prepare '1';
+->slave
+connection slave;
+connection slave;
+include/sync_with_master_gtid.inc
+connection con1;
+xa commit '2';
+disconnect con1;
+connection master;
+xa commit '1';
+drop table t2, t1;
 include/rpl_end.inc
diff --git a/mysql-test/suite/rpl/r/rpl_xa_gtid_pos_auto_engine.result b/mysql-test/suite/rpl/r/rpl_xa_gtid_pos_auto_engine.result
index ffd0426ab0d..35625cc7026 100644
--- a/mysql-test/suite/rpl/r/rpl_xa_gtid_pos_auto_engine.result
+++ b/mysql-test/suite/rpl/r/rpl_xa_gtid_pos_auto_engine.result
@@ -228,6 +228,67 @@ include/sync_with_master_gtid.inc
 connection master;
 drop database test_ign;
 drop table t1, t2, t3, tm;
+#
+# MDEV-26682 slave lock timeout with XA and gap locks
+#
+create table t1 (a int primary key, b int unique) engine=innodb;
+insert t1 values (1,1),(3,3),(5,5);
+connection slave;
+set session tx_isolation='repeatable-read';
+start transaction;
+select * from t1;
+a	b
+1	1
+3	3
+5	5
+connect m2, localhost, root;
+delete from t1 where a=3;
+xa start 'x1';
+update t1 set b=3 where a=5;
+xa end 'x1';
+xa prepare 'x1';
+connect m3, localhost, root;
+insert t1 values (2, 2);
+-->slave
+connection slave;
+commit;
+select * from t1;
+a	b
+1	1
+2	2
+5	5
+connection m2;
+xa rollback 'x1';
+disconnect m2;
+disconnect m3;
+connection master;
+drop table t1;
+create table t1 (id int auto_increment primary key, c1 int not null unique)
+engine=innodb;
+create table t2 (id int auto_increment primary key, c1 int not null,
+foreign key(c1) references t1(c1), unique key(c1)) engine=innodb;
+insert t1 values (869,1), (871,3), (873,4), (872,5), (870,6), (877,7);
+insert t2 values (795,6), (800,7);
+xa start '1';
+update t2 set id = 9, c1 = 5 where c1 in (null, null, null, null, null, 7, 3);
+connect con1, localhost,root;
+xa start '2';
+delete from t1 where c1 like '3%';
+xa end '2';
+xa prepare '2';
+connection master;
+xa end '1';
+xa prepare '1';
+->slave
+connection slave;
+connection slave;
+include/sync_with_master_gtid.inc
+connection con1;
+xa commit '2';
+disconnect con1;
+connection master;
+xa commit '1';
+drop table t2, t1;
 connection slave;
 include/stop_slave.inc
 SET @@global.gtid_pos_auto_engines="";
diff --git a/mysql-test/suite/rpl/t/rpl_xa.inc b/mysql-test/suite/rpl/t/rpl_xa.inc
index 38344da5e66..d22d2d2ef3d 100644
--- a/mysql-test/suite/rpl/t/rpl_xa.inc
+++ b/mysql-test/suite/rpl/t/rpl_xa.inc
@@ -1,6 +1,6 @@
 #
 # This "body" file checks general properties of XA transaction replication
-# as of MDEV-7974.
+# as of MDEV-742.
 # Parameters:
 # --let rpl_xa_check= SELECT ...
 #
@@ -353,3 +353,81 @@ source include/sync_with_master_gtid.inc;
 connection master;
 --eval drop database test_ign
 drop table t1, t2, t3, tm;
+
+--echo #
+--echo # MDEV-26682 slave lock timeout with XA and gap locks
+--echo #
+create table t1 (a int primary key, b int unique) engine=innodb;
+insert t1 values (1,1),(3,3),(5,5);
+sync_slave_with_master;
+
+# set a strong isolation level to keep the read view below.
+# alternatively a long-running select can do that too even in read-committed
+set session tx_isolation='repeatable-read';
+start transaction;
+# opens a read view to disable purge on the slave
+select * from t1;
+
+connect m2, localhost, root;
+# now, delete a value, purge it on the master, but not on the slave
+delete from t1 where a=3;
+xa start 'x1';
+# this sets a gap lock on <3>, when it exists (so, on the slave)
+update t1 set b=3 where a=5;
+xa end 'x1';
+xa prepare 'x1';
+
+connect m3, localhost, root;
+# and this tries to insert straight into the locked gap
+insert t1 values (2, 2);
+
+echo -->slave;
+sync_slave_with_master;
+commit;
+select * from t1;
+
+connection m2;
+xa rollback 'x1';
+disconnect m2;
+disconnect m3;
+
+connection master;
+
+drop table t1;
+
+create table t1 (id int auto_increment primary key, c1 int not null unique)
+engine=innodb;
+
+create table t2 (id int auto_increment primary key, c1 int not null,
+foreign key(c1) references t1(c1), unique key(c1)) engine=innodb;
+
+insert t1 values (869,1), (871,3), (873,4), (872,5), (870,6), (877,7);
+insert t2 values (795,6), (800,7);
+
+xa start '1';
+update t2 set id = 9, c1 = 5 where c1 in (null, null, null, null, null, 7, 3);
+
+connect con1, localhost,root;
+xa start '2';
+delete from t1 where c1 like '3%';
+xa end '2';
+xa prepare '2';
+
+connection master;
+xa end '1';
+xa prepare '1';
+
+echo ->slave;
+
+sync_slave_with_master;
+
+connection slave;
+source include/sync_with_master_gtid.inc;
+
+connection con1;
+xa commit '2';
+disconnect con1;
+
+connection master;
+xa commit '1';
+drop table t2, t1;
diff --git a/plugin/type_inet/mysql-test/type_inet/type_inet6_innodb.result b/plugin/type_inet/mysql-test/type_inet/type_inet6_innodb.result
index 5f7063b8f4b..a6911751747 100644
--- a/plugin/type_inet/mysql-test/type_inet/type_inet6_innodb.result
+++ b/plugin/type_inet/mysql-test/type_inet/type_inet6_innodb.result
@@ -88,5 +88,32 @@ Warnings:
 Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where `test`.`t1`.`a` = INET6'::ff'
 DROP TABLE t1;
 #
+# MDEV-26742 Assertion `field->type_handler() == this' failed in FixedBinTypeBundle<NATIVE_LEN, MAX_CHAR_LEN>::Type_handler_fbt::stored_field_cmp_to_item
+#
+CREATE TABLE t1 (pk inet6, c text) engine=myisam;
+INSERT INTO t1 VALUES ('::',1);
+CREATE TABLE t2 (d text, KEY (d)) engine=innodb ;
+Warnings:
+Note	1071	Specified key was too long; max key length is 3072 bytes
+INSERT INTO t2 VALUES (2);
+SELECT * FROM t2 JOIN t1 ON ( t1.pk > t2.d);
+d	pk	c
+Warnings:
+Warning	1292	Incorrect inet6 value: '2'
+UPDATE t2 JOIN t1 ON ( t1.pk > t2.d) SET t1.c = 1;
+ERROR 22007: Incorrect inet6 value: '2'
+SET sql_mode='';
+UPDATE t2 JOIN t1 ON ( t1.pk > t2.d) SET t1.c = 1;
+Warnings:
+Warning	1292	Incorrect inet6 value: '2'
+SET sql_mode=DEFAULT;
+SELECT * FROM t1;
+pk	c
+::	1
+SELECT * FROM t2;
+d
+2
+DROP TABLE t1, t2;
+#
 # End of 10.5 tests
 #
diff --git a/plugin/type_inet/mysql-test/type_inet/type_inet6_innodb.test b/plugin/type_inet/mysql-test/type_inet/type_inet6_innodb.test
index dd6049abbf3..55826cc3e3f 100644
--- a/plugin/type_inet/mysql-test/type_inet/type_inet6_innodb.test
+++ b/plugin/type_inet/mysql-test/type_inet/type_inet6_innodb.test
@@ -12,6 +12,24 @@
 SET default_storage_engine=InnoDB;
 --source type_inet6_engines.inc
 
+--echo #
+--echo # MDEV-26742 Assertion `field->type_handler() == this' failed in FixedBinTypeBundle<NATIVE_LEN, MAX_CHAR_LEN>::Type_handler_fbt::stored_field_cmp_to_item
+--echo #
+
+CREATE TABLE t1 (pk inet6, c text) engine=myisam;
+INSERT INTO t1 VALUES ('::',1);
+CREATE TABLE t2 (d text, KEY (d)) engine=innodb ;
+INSERT INTO t2 VALUES (2);
+SELECT * FROM t2 JOIN t1 ON ( t1.pk > t2.d);
+--error ER_TRUNCATED_WRONG_VALUE
+UPDATE t2 JOIN t1 ON ( t1.pk > t2.d) SET t1.c = 1;
+SET sql_mode='';
+UPDATE t2 JOIN t1 ON ( t1.pk > t2.d) SET t1.c = 1;
+SET sql_mode=DEFAULT;
+SELECT * FROM t1;
+SELECT * FROM t2;
+DROP TABLE t1, t2;
+
 
 --echo #
 --echo # End of 10.5 tests
diff --git a/plugin/type_inet/mysql-test/type_inet/type_inet6_myisam.result b/plugin/type_inet/mysql-test/type_inet/type_inet6_myisam.result
index c8dba6ff959..ba65d61cb08 100644
--- a/plugin/type_inet/mysql-test/type_inet/type_inet6_myisam.result
+++ b/plugin/type_inet/mysql-test/type_inet/type_inet6_myisam.result
@@ -88,5 +88,24 @@ Warnings:
 Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where `test`.`t1`.`a` = INET6'::ff'
 DROP TABLE t1;
 #
+# MDEV-26742 Assertion `field->type_handler() == this' failed in FixedBinTypeBundle<NATIVE_LEN, MAX_CHAR_LEN>::Type_handler_fbt::stored_field_cmp_to_item
+#
+CREATE TABLE t1 (c varchar(64), key(c)) engine=myisam;
+INSERT INTO t1 VALUES ('0::1'),('::1'),('::2');
+SELECT * FROM t1 WHERE c>CAST('::1' AS INET6);
+c
+::2
+EXPLAIN SELECT * FROM t1 WHERE c>CAST('::1' AS INET6);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	c	c	67	NULL	3	Using where; Using index
+SELECT * FROM t1 WHERE c=CAST('::1' AS INET6);
+c
+0::1
+::1
+EXPLAIN SELECT * FROM t1 WHERE c=CAST('::1' AS INET6);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	c	c	67	NULL	3	Using where; Using index
+DROP TABLE t1;
+#
 # End of 10.5 tests
 #
diff --git a/plugin/type_inet/mysql-test/type_inet/type_inet6_myisam.test b/plugin/type_inet/mysql-test/type_inet/type_inet6_myisam.test
index c5183f01cf0..0ba8369ac95 100644
--- a/plugin/type_inet/mysql-test/type_inet/type_inet6_myisam.test
+++ b/plugin/type_inet/mysql-test/type_inet/type_inet6_myisam.test
@@ -10,6 +10,18 @@
 SET default_storage_engine=MyISAM;
 --source type_inet6_engines.inc
 
+--echo #
+--echo # MDEV-26742 Assertion `field->type_handler() == this' failed in FixedBinTypeBundle<NATIVE_LEN, MAX_CHAR_LEN>::Type_handler_fbt::stored_field_cmp_to_item
+--echo #
+
+CREATE TABLE t1 (c varchar(64), key(c)) engine=myisam;
+INSERT INTO t1 VALUES ('0::1'),('::1'),('::2');
+SELECT * FROM t1 WHERE c>CAST('::1' AS INET6);
+EXPLAIN SELECT * FROM t1 WHERE c>CAST('::1' AS INET6);
+SELECT * FROM t1 WHERE c=CAST('::1' AS INET6);
+EXPLAIN SELECT * FROM t1 WHERE c=CAST('::1' AS INET6);
+DROP TABLE t1;
+
 
 --echo #
 --echo # End of 10.5 tests
diff --git a/sql/field.cc b/sql/field.cc
index a5cd595f05b..63d39c77d95 100644
--- a/sql/field.cc
+++ b/sql/field.cc
@@ -1276,6 +1276,21 @@ bool Field::can_be_substituted_to_equal_item(const Context &ctx,
 }
 
 
+bool Field::cmp_is_done_using_type_handler_of_this(const Item_bool_func *cond,
+                                                   const Item *item) const
+{
+  /*
+    We could eventually take comparison_type_handler() from cond,
+    instead of calculating it again. But only some descendants of
+    Item_bool_func has this method. So this needs some hierarchy changes.
+    Another option is to pass "class Context" to this method.
+  */
+  Type_handler_hybrid_field_type cmp(type_handler_for_comparison());
+  return !cmp.aggregate_for_comparison(item->type_handler_for_comparison()) &&
+         cmp.type_handler() == type_handler_for_comparison();
+}
+
+
 /*
   This handles all numeric and BIT data types.
 */ 
@@ -7368,7 +7383,7 @@ bool
 Field_longstr::cmp_to_string_with_same_collation(const Item_bool_func *cond,
                                                  const Item *item) const
 {
-  return item->cmp_type() == STRING_RESULT &&
+  return cmp_is_done_using_type_handler_of_this(cond, item) &&
          charset() == cond->compare_collation();
 }
 
@@ -7377,7 +7392,7 @@ bool
 Field_longstr::cmp_to_string_with_stricter_collation(const Item_bool_func *cond,
                                                      const Item *item) const
 {
-  return item->cmp_type() == STRING_RESULT &&
+  return cmp_is_done_using_type_handler_of_this(cond, item) &&
          (charset() == cond->compare_collation() ||
           cond->compare_collation()->state & MY_CS_BINSORT);
 }
diff --git a/sql/field.h b/sql/field.h
index 0408ae90b3f..0b8f317b5b8 100644
--- a/sql/field.h
+++ b/sql/field.h
@@ -1651,6 +1651,8 @@ protected:
   }
   int warn_if_overflow(int op_result);
   Copy_func *get_identical_copy_func() const;
+  bool cmp_is_done_using_type_handler_of_this(const Item_bool_func *cond,
+                                              const Item *item) const;
   bool can_optimize_scalar_range(const RANGE_OPT_PARAM *param,
                                  const KEY_PART *key_part,
                                  const Item_bool_func *cond,
diff --git a/sql/my_json_writer.cc b/sql/my_json_writer.cc
index a70ae164f18..55e71bc1b52 100644
--- a/sql/my_json_writer.cc
+++ b/sql/my_json_writer.cc
@@ -260,6 +260,10 @@ void Json_writer::add_str(const String &str)
   add_str(str.ptr(), str.length());
 }
 
+#ifdef ENABLED_JSON_WRITER_CONSISTENCY_CHECKS
+thread_local std::vector<bool> Json_writer_struct::named_items_expectation;
+#endif
+
 Json_writer_temp_disable::Json_writer_temp_disable(THD *thd_arg)
 {
   thd= thd_arg;
diff --git a/sql/my_json_writer.h b/sql/my_json_writer.h
index 27aec74d08d..d82313f996f 100644
--- a/sql/my_json_writer.h
+++ b/sql/my_json_writer.h
@@ -15,8 +15,12 @@
 
 #ifndef JSON_WRITER_INCLUDED
 #define JSON_WRITER_INCLUDED
+
 #include "my_base.h"
 #include "sql_select.h"
+
+#include <vector>
+
 class Opt_trace_stmt;
 class Opt_trace_context;
 class Json_writer;
@@ -308,6 +312,9 @@ public:
 /* A common base for Json_writer_object and Json_writer_array */
 class Json_writer_struct
 {
+#ifdef ENABLED_JSON_WRITER_CONSISTENCY_CHECKS
+  static thread_local std::vector<bool> named_items_expectation;
+#endif
 protected:
   Json_writer* my_writer;
   Json_value_helper context;
@@ -317,16 +324,35 @@ protected:
   bool closed;
 
 public:
-  explicit Json_writer_struct(THD *thd)
+  explicit Json_writer_struct(THD *thd, bool expect_named_children)
   {
     my_writer= thd->opt_trace.get_current_json();
     context.init(my_writer);
     closed= false;
+#ifdef ENABLED_JSON_WRITER_CONSISTENCY_CHECKS
+    named_items_expectation.push_back(expect_named_children);
+#endif
   }
-  bool trace_started()
+
+  virtual ~Json_writer_struct()
+  {
+#ifdef ENABLED_JSON_WRITER_CONSISTENCY_CHECKS
+    named_items_expectation.pop_back();
+#endif
+  }
+
+  bool trace_started() const
   {
     return my_writer != 0;
   }
+
+#ifdef ENABLED_JSON_WRITER_CONSISTENCY_CHECKS
+  bool named_item_expected() const
+  {
+    return named_items_expectation.size() > 1
+        && *(named_items_expectation.rbegin() + 1);
+  }
+#endif
 };
 
 
@@ -347,15 +373,21 @@ private:
   }
 public:
   explicit Json_writer_object(THD *thd)
-  : Json_writer_struct(thd)
+  : Json_writer_struct(thd, true)
   {
+#ifdef ENABLED_JSON_WRITER_CONSISTENCY_CHECKS
+    DBUG_ASSERT(!named_item_expected());
+#endif
     if (unlikely(my_writer))
       my_writer->start_object();
   }
 
   explicit Json_writer_object(THD* thd, const char *str)
-    : Json_writer_struct(thd)
+  : Json_writer_struct(thd, true)
   {
+#ifdef ENABLED_JSON_WRITER_CONSISTENCY_CHECKS
+    DBUG_ASSERT(named_item_expected());
+#endif
     if (unlikely(my_writer))
       my_writer->add_member(str).start_object();
   }
@@ -519,14 +551,22 @@ public:
 class Json_writer_array : public Json_writer_struct
 {
 public:
-  Json_writer_array(THD *thd): Json_writer_struct(thd)
+  Json_writer_array(THD *thd)
+  : Json_writer_struct(thd, false)
   {
+#ifdef ENABLED_JSON_WRITER_CONSISTENCY_CHECKS
+    DBUG_ASSERT(!named_item_expected());
+#endif
     if (unlikely(my_writer))
       my_writer->start_array();
   }
 
-  Json_writer_array(THD *thd, const char *str) : Json_writer_struct(thd)
+  Json_writer_array(THD *thd, const char *str)
+  : Json_writer_struct(thd, false)
   {
+#ifdef ENABLED_JSON_WRITER_CONSISTENCY_CHECKS
+    DBUG_ASSERT(named_item_expected());
+#endif
     if (unlikely(my_writer))
       my_writer->add_member(str).start_array();
   }
diff --git a/storage/innobase/include/lock0lock.h b/storage/innobase/include/lock0lock.h
index c2fef3baaac..5f051b8ffbe 100644
--- a/storage/innobase/include/lock0lock.h
+++ b/storage/innobase/include/lock0lock.h
@@ -427,6 +427,10 @@ lock_rec_unlock(
 and release possible other transactions waiting because of these locks. */
 void lock_release(trx_t* trx);
 
+/** Release non-exclusive locks on XA PREPARE,
+and release possible other transactions waiting because of these locks. */
+void lock_release_on_prepare(trx_t *trx);
+
 /** Release locks on a table whose creation is being rolled back */
 ATTRIBUTE_COLD void lock_release_on_rollback(trx_t *trx, dict_table_t *table);
 
diff --git a/storage/innobase/lock/lock0lock.cc b/storage/innobase/lock/lock0lock.cc
index 47e30e075a0..dcef069f049 100644
--- a/storage/innobase/lock/lock0lock.cc
+++ b/storage/innobase/lock/lock0lock.cc
@@ -3883,6 +3883,124 @@ released:
 #endif
 }
 
+/** Release non-exclusive locks on XA PREPARE,
+and wake up possible other transactions waiting because of these locks.
+@param trx   transaction in XA PREPARE state
+@return whether all locks were released */
+static bool lock_release_on_prepare_try(trx_t *trx)
+{
+  /* At this point, trx->lock.trx_locks can still be modified by other
+  threads to convert implicit exclusive locks into explicit ones.
+
+  The function lock_table_create() should never be invoked on behalf
+  of a transaction that is running in another thread. Also there, we
+  will assert that the current transaction be active. */
+  DBUG_ASSERT(trx->state == TRX_STATE_PREPARED);
+
+  bool all_released= true;
+  lock_sys.rd_lock(SRW_LOCK_CALL);
+  trx->mutex_lock();
+
+  /* Note: Normally, trx->mutex is not held while acquiring
+  a lock table latch, but here we are following the opposite order.
+  To avoid deadlocks, we only try to acquire the lock table latches
+  but not keep waiting for them. */
+
+  for (lock_t *prev, *lock= UT_LIST_GET_LAST(trx->lock.trx_locks); lock;
+       lock= prev)
+  {
+    ut_ad(lock->trx == trx);
+    prev= UT_LIST_GET_PREV(trx_locks, lock);
+    if (!lock->is_table())
+    {
+      ut_ad(!lock->index->table->is_temporary());
+      if (lock->mode() == LOCK_X && !lock->is_gap())
+        continue;
+      auto &lock_hash= lock_sys.hash_get(lock->type_mode);
+      auto cell= lock_hash.cell_get(lock->un_member.rec_lock.page_id.fold());
+      auto latch= lock_sys_t::hash_table::latch(cell);
+      if (latch->try_acquire())
+      {
+        lock_rec_dequeue_from_page(lock, false);
+        latch->release();
+      }
+      else
+        all_released= false;
+    }
+    else
+    {
+      dict_table_t *table= lock->un_member.tab_lock.table;
+      ut_ad(!table->is_temporary());
+      switch (lock->mode()) {
+      case LOCK_IS:
+      case LOCK_S:
+        if (table->lock_mutex_trylock())
+        {
+          lock_table_dequeue(lock, false);
+          table->lock_mutex_unlock();
+        }
+        else
+          all_released= false;
+        break;
+      case LOCK_IX:
+      case LOCK_X:
+        ut_ad(table->id >= DICT_HDR_FIRST_ID || trx->dict_operation);
+        /* fall through */
+      default:
+        break;
+      }
+    }
+  }
+
+  lock_sys.rd_unlock();
+  trx->mutex_unlock();
+  return all_released;
+}
+
+/** Release non-exclusive locks on XA PREPARE,
+and release possible other transactions waiting because of these locks. */
+void lock_release_on_prepare(trx_t *trx)
+{
+  for (ulint count= 5; count--; )
+    if (lock_release_on_prepare_try(trx))
+      return;
+
+  LockMutexGuard g{SRW_LOCK_CALL};
+  trx->mutex_lock();
+
+  for (lock_t *prev, *lock= UT_LIST_GET_LAST(trx->lock.trx_locks); lock;
+       lock= prev)
+  {
+    ut_ad(lock->trx == trx);
+    prev= UT_LIST_GET_PREV(trx_locks, lock);
+    if (!lock->is_table())
+    {
+      ut_ad(!lock->index->table->is_temporary());
+      if (lock->mode() != LOCK_X || lock->is_gap())
+        lock_rec_dequeue_from_page(lock, false);
+    }
+    else
+    {
+      dict_table_t *table= lock->un_member.tab_lock.table;
+      ut_ad(!table->is_temporary());
+      switch (lock->mode()) {
+      case LOCK_IS:
+      case LOCK_S:
+        lock_table_dequeue(lock, false);
+        break;
+      case LOCK_IX:
+      case LOCK_X:
+        ut_ad(table->id >= DICT_HDR_FIRST_ID || trx->dict_operation);
+        /* fall through */
+      default:
+        break;
+      }
+    }
+  }
+
+  trx->mutex_unlock();
+}
+
 /** Release locks on a table whose creation is being rolled back */
 ATTRIBUTE_COLD void lock_release_on_rollback(trx_t *trx, dict_table_t *table)
 {
diff --git a/storage/innobase/trx/trx0trx.cc b/storage/innobase/trx/trx0trx.cc
index ad49d3e9c8e..43ef05f4f2b 100644
--- a/storage/innobase/trx/trx0trx.cc
+++ b/storage/innobase/trx/trx0trx.cc
@@ -1868,6 +1868,20 @@ trx_prepare(
 		We must not be holding any mutexes or latches here. */
 
 		trx_flush_log_if_needed(lsn, trx);
+
+		if (!UT_LIST_GET_LEN(trx->lock.trx_locks)
+		    || trx->isolation_level == TRX_ISO_SERIALIZABLE) {
+			/* Do not release any locks at the
+			SERIALIZABLE isolation level. */
+		} else if (!trx->mysql_thd
+			   || thd_sql_command(trx->mysql_thd)
+			   != SQLCOM_XA_PREPARE) {
+			/* Do not release locks for XA COMMIT ONE PHASE
+			or for internal distributed transactions
+			(XID::get_my_xid() would be nonzero). */
+		} else {
+			lock_release_on_prepare(trx);
+		}
 	}
 }
 
diff --git a/storage/spider/mysql-test/spider/bugfix/r/mdev_26539.result b/storage/spider/mysql-test/spider/bugfix/r/mdev_26539.result
new file mode 100644
index 00000000000..4e195fddfad
--- /dev/null
+++ b/storage/spider/mysql-test/spider/bugfix/r/mdev_26539.result
@@ -0,0 +1,36 @@
+for master_1
+for child2
+child2_1
+child2_2
+child2_3
+for child3
+#
+# MDEV-26539 SIGSEGV in spider_check_and_set_trx_isolation and I_P_List_iterator from THD::drop_temporary_table (10.5.3 opt only) on ALTER
+#
+connection child2_1;
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+CREATE TABLE tbl_a (
+c INT
+) ENGINE=InnoDB DEFAULT CHARSET=utf8;
+connection master_1;
+CREATE DATABASE auto_test_local;
+USE auto_test_local;
+CREATE TABLE tbl_a (
+c INT
+) ENGINE=Spider DEFAULT CHARSET=utf8 COMMENT='table "tbl_a"' PARTITION BY LIST COLUMNS (c) (
+PARTITION pt1 DEFAULT COMMENT = 'srv "s_2_1"'
+);
+INSERT INTO tbl_a VALUES (1);
+ALTER TABLE tbl_a CHECK PARTITION ALL;
+Table	Op	Msg_type	Msg_text
+auto_test_local.tbl_a	check	status	OK
+DROP DATABASE auto_test_local;
+connection child2_1;
+DROP DATABASE auto_test_remote;
+for master_1
+for child2
+child2_1
+child2_2
+child2_3
+for child3
diff --git a/storage/spider/mysql-test/spider/bugfix/t/mdev_26539.cnf b/storage/spider/mysql-test/spider/bugfix/t/mdev_26539.cnf
new file mode 100644
index 00000000000..05dfd8a0bce
--- /dev/null
+++ b/storage/spider/mysql-test/spider/bugfix/t/mdev_26539.cnf
@@ -0,0 +1,3 @@
+!include include/default_mysqld.cnf
+!include ../my_1_1.cnf
+!include ../my_2_1.cnf
diff --git a/storage/spider/mysql-test/spider/bugfix/t/mdev_26539.test b/storage/spider/mysql-test/spider/bugfix/t/mdev_26539.test
new file mode 100644
index 00000000000..f2561f8c9a5
--- /dev/null
+++ b/storage/spider/mysql-test/spider/bugfix/t/mdev_26539.test
@@ -0,0 +1,40 @@
+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+
+--echo #
+--echo # MDEV-26539 SIGSEGV in spider_check_and_set_trx_isolation and I_P_List_iterator from THD::drop_temporary_table (10.5.3 opt only) on ALTER
+--echo #
+
+--connection child2_1
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+eval CREATE TABLE tbl_a (
+    c INT
+) $CHILD2_1_ENGINE $CHILD2_1_CHARSET;
+
+--connection master_1
+CREATE DATABASE auto_test_local;
+USE auto_test_local;
+
+eval CREATE TABLE tbl_a (
+    c INT
+) $MASTER_1_ENGINE $MASTER_1_CHARSET COMMENT='table "tbl_a"' PARTITION BY LIST COLUMNS (c) (
+    PARTITION pt1 DEFAULT COMMENT = 'srv "s_2_1"'
+);
+
+INSERT INTO tbl_a VALUES (1);
+ALTER TABLE tbl_a CHECK PARTITION ALL;
+
+DROP DATABASE auto_test_local;
+
+--connection child2_1
+DROP DATABASE auto_test_remote;
+
+--disable_query_log
+--disable_result_log
+--source ../../t/test_deinit.inc
+--enable_result_log
+--enable_query_log
diff --git a/storage/spider/spd_trx.cc b/storage/spider/spd_trx.cc
index defa9be3f8c..c5095e7ff55 100644
--- a/storage/spider/spd_trx.cc
+++ b/storage/spider/spd_trx.cc
@@ -3802,10 +3802,8 @@ int spider_check_trx_and_get_conn(
   }
   spider->wide_handler->trx = trx;
   spider->set_error_mode();
-  if (
-    spider->wide_handler->sql_command != SQLCOM_DROP_TABLE &&
-    spider->wide_handler->sql_command != SQLCOM_ALTER_TABLE
-  ) {
+  if (spider->wide_handler->sql_command != SQLCOM_DROP_TABLE)
+  {
     SPIDER_TRX_HA *trx_ha = spider_check_trx_ha(trx, spider);
     if (!trx_ha || trx_ha->wait_for_reusing)
       spider_trx_set_link_idx_for_all(spider);