Make the backup singleton reside in statically allocated memory.

Fix concurrent BACKUP_PHASE_FINISH issue.

Author: mariadb-andrzejjarzabek

storage/maria/ma_backup.cc

@@ -26,6 +26,7 @@
 #include <vector>
 #include <algorithm>
 #include <functional>
+#include <optional>
 #include <variant>
 
 /*
@@ -596,7 +597,7 @@ namespace
 #endif
   };
 
-  std::unique_ptr<Aria_backup> aria_backup;
+  std::optional<Aria_backup> aria_backup;
 }
 
 /**
@@ -614,7 +615,8 @@ int aria_backup_start(THD *thd, const backup_target &target,
   switch (phase)
   {
   case BACKUP_PHASE_START:
-    aria_backup= std::make_unique<Aria_backup>(thd, target);
+    assert(!aria_backup);
+    aria_backup.emplace(thd, target);
     return !aria_backup->is_initialized();
   case BACKUP_PHASE_NO_DML_NON_TRANS:
     /* FIXME: Would be better to selectively purge only the tables we need. */
@@ -663,7 +665,16 @@ int aria_backup_step(THD *thd, const backup_target &target,
 int aria_backup_end(THD *thd, const backup_target &target,
                     backup_phase phase) noexcept
 {
-  if (phase == BACKUP_PHASE_FINISH || phase == BACKUP_PHASE_ABORT)
+  /* Note - aria_backup is currently a singleton object, and it is
+  reset before BACKUP_PHASE_FINISH to allow for a new backup to run
+  concurrently. With this design BACKUP_PHASE_FINISH must be a no-op.
+  If BACKUP_PHASE_FINISH is implemented, the code should be adjusted
+  to allow for multiple backups to be run in parallel from different
+  THD, given that only one backup may run in a phase other than
+  BACKUP_PHASE_FINISH, but any number may run in BACKUP_PHASE_FINISH.
+  A possible solution would be to have s eparate Ariab_backup object
+  per THD.  */
+  if (phase == BACKUP_PHASE_NO_COMMIT || phase == BACKUP_PHASE_ABORT)
     aria_backup.reset();
   return 0;
 }