1 files changed, 12 insertions, 5 deletions
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
index 0705c22a8c..60655b673c 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
@@ -7,7 +7,6 @@ require 'active_record/connection_adapters/schema_cache'
 require 'active_record/connection_adapters/sql_type_metadata'
 require 'active_record/connection_adapters/abstract/schema_dumper'
 require 'active_record/connection_adapters/abstract/schema_creation'
-require 'monitor'
 require 'arel/collectors/bind'
 require 'arel/collectors/sql_string'
 
@@ -70,7 +69,6 @@ module ActiveRecord
       include DatabaseLimits
       include QueryCache
       include ActiveSupport::Callbacks
-      include MonitorMixin
       include ColumnDumper
 
       SIMPLE_INT = /\A\d+\z/
@@ -141,12 +139,20 @@ module ActiveRecord
         SchemaCreation.new self
       end
 
+      # this method must only be called while holding connection pool's mutex
       def lease
-        synchronize do
-          unless in_use?
-            @owner = Thread.current
+        if in_use?
+          msg = 'Cannot lease connection, '
+          if @owner == Thread.current
+            msg << 'it is already leased by the current thread.'
+          else
+            msg << "it is already in use by a different thread: #{@owner}. " <<
+                   "Current thread: #{Thread.current}."
           end
+          raise ActiveRecordError, msg
         end
+
+        @owner = Thread.current
       end
 
       def schema_cache=(cache)
@@ -154,6 +160,7 @@ module ActiveRecord
         @schema_cache = cache
       end
 
+      # this method must only be called while holding connection pool's mutex
       def expire
         @owner = nil
       end