A fix for Bug#41158 "DROP TABLE holds LOCK_open during unlink()".

Remove acquisition of LOCK_open around file system operations,
since such operations are now protected by metadata locks.
Rework table discovery algorithm to not require LOCK_open.

No new tests added since all MDL locking operations are covered
in lock.test and mdl_sync.test, and as long as these tests
pass despite the increased concurrency, consistency must be
unaffected.

mysql-test/t/disabled.def:
  Disable NDB tests due to Bug#55799.
sql/datadict.cc:
  No longer necessary to protect ha_create_table() with
  LOCK_open. Serial execution is now ensured by metadata
  locks.
sql/ha_ndbcluster.cc:
  Do not manipulate with LOCK_open in cluster code.
sql/ha_ndbcluster_binlog.cc:
  Do not manipulate with LOCK_open in cluster code.
sql/ha_ndbcluster_binlog.h:
  Update function signature.
sql/handler.cc:
  Implement ha_check_if_table_exists().
  @todo: some engines provide ha_table_exists_in_engine()
  handlerton call, for those we perhaps shouldn't
  call ha_discover(), to be more efficient.
  Since currently it's only NDB, postpone till
  integration with NDB.
sql/handler.h:
  Declare ha_check_if_table_exists() function.
sql/mdl.cc:
  Remove an obsolete comment.
sql/sql_base.cc:
  Update to a new signature of close_cached_tables():
  from now on we always call it without LOCK_open.
  Update comments.
  Remove get_table_share_with_create(), we should
  not attempt to create a table under LOCK_open.
  Introduce get_table_share_with_discover() instead,
  which would request a back off action if the table
  exists in engine.
  Remove acquisition of LOCK_open for 
  data dictionary operations, such as check_if_table_exists().
  Do not use get_table_share_with_create/discover for views,
  where it's not needed.
  Make tdc_remove_table() optionally acquire LOCK_open
  to simplify usage of this function.
  Use the right mutex in the partitioning code when
  manipulating with thd->open_tables.
sql/sql_base.h:
  Update signatures of changes functions.
sql/sql_insert.cc:
  Do not wrap quick_rm_table() with LOCK_open acquisition, 
  this is unnecessary.
sql/sql_parse.cc:
  Update to the new calling convention of tdc_remove_table().
  Update to the new signature of close_cached_tables().
  Update comments.
sql/sql_rename.cc:
  Update to the new calling convention of tdc_remove_table().
  Remove acquisition of LOCK_open around filesystem
  operations.
sql/sql_show.cc:
  Remove get_trigger_table_impl().
  Do not acquire LOCK_open for a dirty read of the trigger
  file.
sql/sql_table.cc:
  Do not acquire LOCK_open for filesystem operations.
sql/sql_trigger.cc:
  Do not require LOCK_open for trigger file I/O.
sql/sql_truncate.cc:
  Update to the new signature of tdc_remove_table().
sql/sql_view.cc:
  Do not require LOCK_open for view I/O.
  Use tdc_remove_table() to expel view share.
  Update comments.
sql/sys_vars.cc:
  Update to the new signature of close_cached_tables().

1 files changed, 28 insertions, 0 deletions

diff --git a/sql/handler.cc b/sql/handler.cc
index 9893b3cac16..567dbe6ea49 100644
--- a/sql/handler.cc
+++ b/sql/handler.cc
@@ -3670,6 +3670,34 @@ int ha_create_table_from_engine(THD* thd, const char *db, const char *name)
   DBUG_RETURN(error != 0);
 }
 
+
+/**
+  Try to find a table in a storage engine. 
+
+  @param db   Normalized table schema name
+  @param name Normalized table name.
+  @param[out] exists Only valid if the function succeeded.
+
+  @retval TRUE   An error is found
+  @retval FALSE  Success, check *exists
+*/
+
+bool
+ha_check_if_table_exists(THD* thd, const char *db, const char *name,
+                         bool *exists)
+{
+  uchar *frmblob= NULL;
+  size_t frmlen;
+  DBUG_ENTER("ha_check_if_table_exists");
+
+  *exists= ! ha_discover(thd, db, name, &frmblob, &frmlen);
+  if (*exists)
+    my_free(frmblob);
+
+  DBUG_RETURN(FALSE);
+}
+
+
 void st_ha_check_opt::init()
 {
   flags= sql_flags= 0;