MDEV-25975 innodb_disallow_writes causes shutdown to hang

We will remove the parameter innodb_disallow_writes because it is badly
designed and implemented. The parameter was never allowed at startup.
It was only internally used by Galera snapshot transfer.
If a user executed
SET GLOBAL innodb_disallow_writes=ON;
the server could hang even on subsequent read operations.

During Galera snapshot transfer, we will block writes
to implement an rsync friendly snapshot, as follows:

sst_flush_tables() will acquire a global lock by executing
FLUSH TABLES WITH READ LOCK, which will block any writes
at the high level.

sst_disable_innodb_writes(), invoked via ha_disable_internal_writes(true),
will suspend or disable InnoDB background tasks or threads that could
initiate writes. As part of this, log_make_checkpoint() will be invoked
to ensure that anything in the InnoDB buf_pool.flush_list will be written
to the data files. This has the nice side effect that the Galera joiner
will avoid crash recovery.

The changes to sql/wsrep.cc and to the tests are based on a prototype
that was developed by Jan Lindström.

Reviewed by: Jan Lindström

1 files changed, 16 insertions, 0 deletions

diff --git a/sql/handler.cc b/sql/handler.cc
index cad9019c969..e4c377451c6 100644
--- a/sql/handler.cc
+++ b/sql/handler.cc
@@ -849,6 +849,22 @@ void ha_kill_query(THD* thd, enum thd_kill_levels level)
 }
 
 
+static my_bool plugin_disable_internal_writes(THD *, plugin_ref plugin,
+                                              void *disable)
+{
+  if (void(*diw)(bool)= plugin_hton(plugin)->disable_internal_writes)
+    diw(*static_cast<bool*>(disable));
+  return FALSE;
+}
+
+
+void ha_disable_internal_writes(bool disable)
+{
+  plugin_foreach(NULL, plugin_disable_internal_writes,
+                 MYSQL_STORAGE_ENGINE_PLUGIN, &disable);
+}
+
+
 /* ========================================================================
  ======================= TRANSACTIONS ===================================*/