Fix connection leak when a thread checks in additional connections.

The code in `ConnectionPool#release` assumed that a single thread only
ever holds a single connection, and thus that releasing a connection
only requires the owning thread_id.

There is a trivial counterexample to this assumption: code that checks
out additional connections from the pool in the same thread. For
instance:

    connection_1 = ActiveRecord::Base.connection
    connection_2 = ActiveRecord::Base.connection_pool.checkout
    ActiveRecord::Base.connection_pool.checkin(connection_2)
    connection_3 = ActiveRecord::Base.connection

At this point, connection_1 has been removed from the
`@reserved_connections` hash, causing a NEW connection to be returned as
connection_3 and the loss of any tracking info on connection_1. As long
as the thread in this example lives, connection_1 will be inaccessible
and un-reapable. If this block of code runs more times than the size of
the connection pool in a single thread, every subsequent connection
attempt will timeout, as all of the available connections have been
leaked.

Reverts parts of 9e457a8654fa89fe329719f88ae3679aefb21e56 and
essentially all of 4367d2f05cbeda855820e25a08353d4b7b3457ac

1 files changed, 6 insertions, 4 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 0fa00d03a3..3968b90341 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
@@ -363,7 +363,7 @@ module ActiveRecord
             conn.expire
           end
 
-          release owner
+          release conn, owner
 
           @available.add conn
         end
@@ -376,7 +376,7 @@ module ActiveRecord
           @connections.delete conn
           @available.delete conn
 
-          release conn.owner
+          release conn, conn.owner
 
           @available.add checkout_new_connection if @available.any_waiting?
         end
@@ -424,10 +424,12 @@ module ActiveRecord
         end
       end
 
-      def release(owner)
+      def release(conn, owner)
         thread_id = owner.object_id
 
-        @reserved_connections.delete thread_id
+        if @reserved_connections[thread_id] == conn
+          @reserved_connections.delete thread_id
+        end
       end
 
       def new_connection