Fix Database#listen in postgres adapter to ignore all notifications after UNLISTEN

Previously, there was a race condition. If not all notifications
received between the LISTEN and UNLISTEN were handled, subsequent
notifications would stick around until the next time listen (or
wait_for_notify) was called, and returned at that point, which
could be for the previous channel and not the current channel.

Use a loop to ensure that all notifications received have been
ignored after the UNLISTEN, so the next LISTEN starts with a
clean slate.

Author: jeremyevans

CHANGELOG

@@ -1,5 +1,7 @@
 === master
 
+* Fix Database#listen in postgres adapter to ignore all notifications after UNLISTEN (jeremyevans)
+
 * Make Model.any? forward to the dataset, to work better with the any_not_empty extension (jeremyevans) (#2337)
 
 * Make the rcte_tree and split_values plugins work together (jeremyevans)

lib/sequel/adapters/postgres.rb

@@ -516,6 +516,7 @@ def listen(channels, opts=OPTS, &block)
                 end
               ensure
                 conn.execute("UNLISTEN *")
+                true while conn.wait_for_notify(0)
               end
             end
           end

spec/adapters/postgres_spec.rb

@@ -3312,6 +3312,17 @@ def c.get_last_result; end
       proc{@db.listen('foo', :loop=>true)}.must_raise Sequel::Error
     end
 
+    it "should clear notifications received after UNLISTEN" do
+      DB.listen('foo', :after_listen=>proc{2.times{@db.notify('foo', :payload=>'bar')}}) do |x, _, y|
+        x.must_equal 'foo'
+        y.must_equal 'bar'
+      end.must_equal 'foo'
+      DB.listen('quux', :after_listen=>proc{@db.notify('quux', :payload=>'baz')}) do |x, _, y|
+        x.must_equal 'quux'
+        y.must_equal 'baz'
+      end.must_equal 'quux'
+    end
+
     it "should accept a :timeout option in listen" do
       @db.listen('foo2', :timeout=>0.001).must_be_nil
       called = false