Fix establishing connections for models with custom connection specification names

We have an internal engine whose models, for backwards compatibility with older Rails versions, override #connection_specification_name to connect to a shared DB rather than using Rails 6 multi-DB support.

b8fc0150 changed ActiveRecord::ConnectionAdapters::ConnectionPool#retrieve_connection_pool to pass ActiveRecord::DatabaseConfigurations::DatabaseConfig#config_hash to #establish_connection where it previously passed ActiveRecord::ConnectionAdapters::ConnectionSpecification#to_hash. But DatabaseConfig#config_hash does not include the specification name, so #establish_connection falls back to `"primary"`. The host app's primary DB connections and our engine's shared DB connections get mixed in the same pool as a result. Engine models will sometimes erroneously check out app DB connections and app models will sometimes check out engine DB connections.

To fix, we merge the connection specification name into the DB config hash before passing it into ConnectionPool#establish_connection.

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

@@ -1164,7 +1164,7 @@ def retrieve_connection_pool(spec_name)
             # A connection was established in an ancestor process that must have
             # subsequently forked. We can't reuse the connection, but we can copy
             # the specification and establish a new connection with it.
-            establish_connection(ancestor_pool.spec.db_config.configuration_hash).tap do |pool|
+            establish_connection(ancestor_pool.spec.db_config.configuration_hash.merge(name: spec_name)).tap do |pool|
               pool.schema_cache = ancestor_pool.schema_cache if ancestor_pool.schema_cache
             end
           else

activerecord/test/cases/connection_adapters/connection_handler_test.rb

@@ -353,6 +353,28 @@ def test_forked_child_recovers_from_disconnected_parent
           end
         end
 
+        def test_retrieve_connection_pool_preserves_connection_specification_name
+          rd, wr = IO.pipe
+          rd.binmode
+          wr.binmode
+
+          @handler.establish_connection ActiveRecord::Base.configurations["arunit"].merge(name: "foo")
+
+          pid = fork {
+            rd.close
+            pool = @handler.retrieve_connection_pool("foo")
+            wr.write pool.spec.name
+            wr.close
+            exit!
+          }
+
+          wr.close
+
+          Process.waitpid pid
+          assert_equal "foo", rd.read
+          rd.close
+        end
+
         def test_retrieve_connection_pool_copies_schema_cache_from_ancestor_pool
           @pool.schema_cache = @pool.connection.schema_cache
           @pool.schema_cache.add("posts")