AthAllocators: Fix potential deadlock.

AthAllocators: Fix potential deadlock.

In ArenaHeader::allocator(), we need to release the header lock
before creating the LockedAllocator instance.  Otherwise we can deadlock
if an algorithm creates multiple allocators.

See ATR-28749.

Control/AthAllocators/AthAllocators/ArenaHeader.icc

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2024 CERN for the benefit of the ATLAS collaboration
 */
 
 // $Id: ArenaHeader.icc 470529 2011-11-24 23:54:22Z ssnyder $
@@ -41,16 +41,19 @@ LockedAllocator ArenaHeader::allocator (size_t i)
 inline
 LockedAllocator ArenaHeader::allocator (const EventContext& ctx, size_t i)
 {
+  ArenaBase* a = nullptr;
   size_t slot = ctx.slot();
   {
     std::lock_guard<std::mutex> lock (m_mutex);
     if (slot < m_slots.size()) {
-      ArenaBase* a = m_slots[slot];
-      if (a) {
-        return a->allocator (i);
-      }
+      a = m_slots[slot];
     }
   }
+  // Need to release the header lock before creating the allocator;
+  // otherwise we can deadlock (ATR-28749).
+  if (a) {
+    return a->allocator (i);
+  }
   return allocator (i);
 }