Merge pull request #1513 from evoskuil/master

Use BC_API, make retainer and default_arena final.

include/bitcoin/system/allocator.hpp

@@ -257,6 +257,7 @@ class allocator
     }
 
 private:
+    // This is thread safe.
     arena* arena_;
 };
 

include/bitcoin/system/arena.hpp

@@ -74,7 +74,7 @@ bool operator==(const arena& left, const arena& right) NOEXCEPT;
 /// quuxplusone.github.io/blog/2023/06/02/not-so-quick-pmr
 /// quuxplusone.github.io/blog/2023/06/03/p1144-pmr-koans
 /// ***************************************************************************
-class default_arena
+class BC_API default_arena final
   : public arena
 {
 public:

include/bitcoin/system/retainer.hpp

@@ -26,7 +26,7 @@
 namespace libbitcoin {
 
 /// Shared lock object to inform allocator that memory may be freed.
-class retainer
+class BC_API retainer final
 {
 public:
     using ptr = std::shared_ptr<retainer>;
@@ -46,6 +46,7 @@ class retainer
     BC_POP_WARNING()
 
 private:
+    // This is thread safe.
     std::shared_lock<std::shared_mutex> shared_lock_;
 };