MDEV-19826 10.4 seems to crash with "pool-of-threads" (#1370)

MariaDB 10.4 was crashing when thread-handling was set to
pool-of-threads and wsrep was enabled.

There were two apparent reasons for the crash:
- Connection handling in threadpool_common.cc was missing calls to
  control wsrep client state.
- Thread specific storage which contains thread variables (THR_KEY_mysys)
  was not handled appropriately by wsrep patch when pool-of-threads
  was configured.

This patch addresses the above issues in the following way:
- Wsrep client state open/close was moved in thd_prepare_connection() and
  end_connection() to have common handling for one-thread-per-connection
  and pool-of-threads.
- Thread local storage handling in wsrep patch was reworked by introducing
  set of wsrep_xxx_threadvars() calls which replace calls to
  THD store_globals()/reset_globals() and deal with thread handling
  specifics internally.

Wsrep-lib was updated to version which relaxes internal concurrency
related sanity checks.

Rollback code from wsrep_rollback_process() was extracted to separate calls
for better readability.

Post rollback thread was removed as it was completely unused.

sql/wsrep_server_service.cc

@@ -26,6 +26,7 @@
 #include "wsrep_mysqld.h"
 #include "wsrep_schema.h"
 #include "wsrep_utils.h"
+#include "wsrep_thd.h"
 
 #include "log.h" /* sql_print_xxx() */
 #include "sql_class.h" /* system variables */
@@ -50,6 +51,10 @@ wsrep::storage_service* Wsrep_server_service::storage_service(
   init_service_thd(thd, cs.m_thd->thread_stack);
   WSREP_DEBUG("Created storage service with thread id %llu",
               thd->thread_id);
+  /* Use variables from the current thd attached to client_service.
+     This is because we need to be able to BF abort storage access
+     operations. */
+  wsrep_assign_from_threadvars(thd);
   return new Wsrep_storage_service(thd);
 }
 
@@ -62,6 +67,7 @@ wsrep::storage_service* Wsrep_server_service::storage_service(
   init_service_thd(thd, hps.m_thd->thread_stack);
   WSREP_DEBUG("Created high priority storage service with thread id %llu",
               thd->thread_id);
+  wsrep_assign_from_threadvars(thd);
   return new Wsrep_storage_service(thd);
 }
 
@@ -71,21 +77,48 @@ void Wsrep_server_service::release_storage_service(
   Wsrep_storage_service* ss=
     static_cast<Wsrep_storage_service*>(storage_service);
   THD* thd= ss->m_thd;
+  wsrep_reset_threadvars(thd);
   delete ss;
   delete thd;
 }
 
+Wsrep_applier_service*
+wsrep_create_streaming_applier(THD *orig_thd, const char *ctx)
+{
+  /* Reset variables to allow creating new variables in thread local
+     storage for new THD if needed. Note that reset must be done for
+     current_thd, as orig_thd may not be in effect. This may be the case when
+     streaming transaction is BF aborted and streaming applier
+     is created from BF aborter context. */
+  Wsrep_threadvars saved_threadvars(wsrep_save_threadvars());
+  wsrep_reset_threadvars(saved_threadvars.cur_thd);
+  THD *thd= 0;
+  Wsrep_applier_service *ret= 0;
+  if (!wsrep_create_threadvars() &&
+      (thd= new THD(next_thread_id(), true)))
+  {
+    init_service_thd(thd, orig_thd->thread_stack);
+    wsrep_assign_from_threadvars(thd);
+    WSREP_DEBUG("Created streaming applier service in %s context with "
+                "thread id %llu", ctx, thd->thread_id);
+    if (!(ret= new (std::nothrow) Wsrep_applier_service(thd)))
+    {
+      delete thd;
+    }
+  }
+  /* Restore original thread local storage state before returning. */
+  wsrep_restore_threadvars(saved_threadvars);
+  wsrep_store_threadvars(saved_threadvars.cur_thd);
+  return ret;
+}
+
 wsrep::high_priority_service*
 Wsrep_server_service::streaming_applier_service(
   wsrep::client_service& orig_client_service)
 {
   Wsrep_client_service& orig_cs=
     static_cast<Wsrep_client_service&>(orig_client_service);
-  THD* thd= new THD(next_thread_id(), true);
-  init_service_thd(thd, orig_cs.m_thd->thread_stack);
-  WSREP_DEBUG("Created streaming applier service in local context with "
-              "thread id %llu", thd->thread_id);
-  return new Wsrep_applier_service(thd);
+  return wsrep_create_streaming_applier(orig_cs.m_thd, "local");
 }
 
 wsrep::high_priority_service*
@@ -94,11 +127,7 @@ Wsrep_server_service::streaming_applier_service(
 {
   Wsrep_high_priority_service&
     orig_hps(static_cast<Wsrep_high_priority_service&>(orig_high_priority_service));
-  THD* thd= new THD(next_thread_id(), true);
-  init_service_thd(thd, orig_hps.m_thd->thread_stack);
-  WSREP_DEBUG("Created streaming applier service in high priority "
-              "context with thread id %llu", thd->thread_id);
-  return new Wsrep_applier_service(thd);
+  return wsrep_create_streaming_applier(orig_hps.m_thd, "high priority");
 }
 
 void Wsrep_server_service::release_high_priority_service(wsrep::high_priority_service* high_priority_service)
@@ -107,7 +136,9 @@ void Wsrep_server_service::release_high_priority_service(wsrep::high_priority_se
     static_cast<Wsrep_high_priority_service*>(high_priority_service);
   THD* thd= hps->m_thd;
   delete hps;
+  wsrep_store_threadvars(thd);
   delete thd;
+  wsrep_delete_threadvars();
 }
 
 void Wsrep_server_service::background_rollback(wsrep::client_state& client_state)