MDEV-20780 Fixes for failures on galera_sr_ddl_master (#1425)

Test galera_sr_ddl_master would sometimes fail due to leftover
streaming replication fragments. Rollbacker thread would attempt to
open streaming_log table to remove the fragments, but would fail in
check_stack_overrun(). Ultimately the check_stack_overrun() failure
was caused by rollbacker missing to switch the victim's THD thread
stack to rollbacker's thread stack.

Also in this patch:
- Remove duplicate functionality in rollbacker helper functions,
  and extract rollbacker fragment removal into function
  wsrep_remove_streaming_fragments()
- Reuse open_for_write() in wsrep_schema::remove_fragments
- Partially revert changes to galera_sr_ddl_master test from
  commit 44a11a7c085f4f5a4042100df0828d54d596103d. Removed unnecessary
  wait condition and isolation level setting

1 files changed, 10 insertions, 0 deletions

diff --git a/sql/wsrep_server_service.h b/sql/wsrep_server_service.h
index 6336fe2c473..4017c9b2d58 100644
--- a/sql/wsrep_server_service.h
+++ b/sql/wsrep_server_service.h
@@ -87,4 +87,14 @@ class Wsrep_applier_service;
 Wsrep_applier_service*
 wsrep_create_streaming_applier(THD *orig_thd, const char *ctx);
 
+/**
+   Helper method to create new storage service.
+
+   @param orig_thd Original thd context to copy operation context from.
+   @param ctx Context string for debug logging.
+*/
+class Wsrep_storage_service;
+Wsrep_storage_service*
+wsrep_create_storage_service(THD *orig_thd, const char *ctx);
+
 #endif /* WSREP_SERVER_SERVICE */