MDEV-14648 Restore fix for MySQL BUG#39053 - UNINSTALL PLUGIN does not allow the storage engine to cleanup open connections

Also, allow the MariaDB 10.2 server to link InnoDB dynamically
against ha_innodb.so (which is what mysql-test-run.pl expects
to exist, instead of the default name ha_innobase.so).

wsrep_load_data_split(): Instead of referring to innodb_hton_ptr,
check the handlerton::db_type. This was recently broken by me in
MDEV-11415.

innodb_lock_schedule_algorithm: Define as a weak global symbol,
so that WITH_WSREP will not depend on InnoDB being linked statically.
I tested this manually. Notably, running a test that only does
	SET GLOBAL wsrep_on=1;
with a static or dynamic InnoDB and
	./mtr --mysqld=--loose-innodb-lock-schedule-algorithm=fcfs
will crash with SIGSEGV at shutdown. With the default VATS
combination the wsrep_on is properly refused for both the
static and dynamic InnoDB.

ha_close_connection(): Do invoke the method also for plugins
for which UNINSTALL PLUGIN was deferred due to open connections.
Thanks to @svoj for pointing this out.

thd_to_trx(): Return a pointer, not a reference to a pointer.

check_trx_exists(): Invoke thd_set_ha_data() for assigning a transaction.

log_write_checkpoint_info(): Remove an unused DEBUG_SYNC point
that would cause an assertion failure on shutdown after deferred
UNINSTALL PLUGIN.

This was tested as follows:

cmake -DWITH_WSREP=1 -DPLUGIN_INNOBASE:STRING=DYNAMIC \
-DWITH_MARIABACKUP:BOOL=OFF ...
make
cd mysql-test
./mtr innodb.innodb_uninstall

1 files changed, 4 insertions, 1 deletions

diff --git a/sql/wsrep_var.cc b/sql/wsrep_var.cc
index a6122a7b1bc..b9eb546bf76 100644
--- a/sql/wsrep_var.cc
+++ b/sql/wsrep_var.cc
@@ -42,7 +42,10 @@ int wsrep_init_vars()
   return 0;
 }
 
-extern ulong innodb_lock_schedule_algorithm;
+/* This is intentionally declared as a weak global symbol, so that
+linking will succeed even if the server is built with a dynamically
+linked InnoDB. */
+ulong innodb_lock_schedule_algorithm __attribute__((weak));
 
 bool wsrep_on_update (sys_var *self, THD* thd, enum_var_type var_type)
 {