Merge pull request rails#54052 from SjoerdL/check-pinned-connection-in-checkout

Fix race condition in ConnectionPool#checkout on @pinned_connection

Author: byroot

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -545,20 +545,25 @@ def clear_reloadable_connections!
       # Raises:
       # - ActiveRecord::ConnectionTimeoutError no connection can be obtained from the pool.
       def checkout(checkout_timeout = @checkout_timeout)
-        if @pinned_connection
-          @pinned_connection.lock.synchronize do
-            synchronize do
+        return checkout_and_verify(acquire_connection(checkout_timeout)) unless @pinned_connection
+
+        @pinned_connection.lock.synchronize do
+          synchronize do
+            # The pinned connection may have been cleaned up before we synchronized, so check if it is still present
+            if @pinned_connection
               @pinned_connection.verify!
+
               # Any leased connection must be in @connections otherwise
               # some methods like #connected? won't behave correctly
               unless @connections.include?(@pinned_connection)
                 @connections << @pinned_connection
               end
+
+              @pinned_connection
+            else
+              checkout_and_verify(acquire_connection(checkout_timeout))
             end
           end
-          @pinned_connection
-        else
-          checkout_and_verify(acquire_connection(checkout_timeout))
         end
       end