2 files changed, 37 insertions, 10 deletions
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
index 53dbbd8c21..61bf5477aa 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
@@ -506,14 +506,16 @@ module ActiveRecord
       # +conn+: an AbstractAdapter object, which was obtained by earlier by
       # calling #checkout on this pool.
       def checkin(conn)
-        synchronize do
-          remove_connection_from_thread_cache conn
+        conn.lock.synchronize do
+          synchronize do
+            remove_connection_from_thread_cache conn
 
-          conn._run_checkin_callbacks do
-            conn.expire
-          end
+            conn._run_checkin_callbacks do
+              conn.expire
+            end
 
-          @available.add conn
+            @available.add conn
+          end
         end
       end
 
@@ -857,9 +859,9 @@ module ActiveRecord
     # All Active Record models use this handler to determine the connection pool that they
     # should use.
     #
-    # The ConnectionHandler class is not coupled with the Active models, as it has no knowlodge
+    # The ConnectionHandler class is not coupled with the Active models, as it has no knowledge
     # about the model. The model needs to pass a specification name to the handler,
-    # in order to lookup the correct connection pool.
+    # in order to look up the correct connection pool.
     class ConnectionHandler
       def initialize
         # These caches are keyed by spec.name (ConnectionSpecification#name).
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
index 4682afc188..46d7f84efd 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
@@ -2,8 +2,33 @@ module ActiveRecord
   module ConnectionAdapters #:nodoc:
     # Abstract representation of an index definition on a table. Instances of
     # this type are typically created and returned by methods in database
-    # adapters. e.g. ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter#indexes
-    IndexDefinition = Struct.new(:table, :name, :unique, :columns, :lengths, :orders, :where, :type, :using, :comment) #:nodoc:
+    # adapters. e.g. ActiveRecord::ConnectionAdapters::MySQL::SchemaStatements#indexes
+    class IndexDefinition # :nodoc:
+      attr_reader :table, :name, :unique, :columns, :lengths, :orders, :where, :type, :using, :comment
+
+      def initialize(
+        table, name,
+        unique = false,
+        columns = [],
+        lengths: {},
+        orders: {},
+        where: nil,
+        type: nil,
+        using: nil,
+        comment: nil
+      )
+        @table = table
+        @name = name
+        @unique = unique
+        @columns = columns
+        @lengths = lengths
+        @orders = orders
+        @where = where
+        @type = type
+        @using = using
+        @comment = comment
+      end
+    end
 
     # Abstract representation of a column definition. Instances of this type
     # are typically created by methods in TableDefinition, and added to the