fix explain on old sharded cluster servers

Author: p

spec/integration/crud_spec.rb

@@ -147,7 +147,7 @@
       it 'passes the session' do
         client.start_session do |session|
           event = Utils.get_command_event(client, 'explain') do |client|
-            client['foo'].find({}, session: session).explain.should have_key('queryPlanner')
+            client['foo'].find({}, session: session).explain.should be_explain_output
           end
           event.command.fetch('lsid').should == session.session_id
         end
@@ -159,7 +159,7 @@
 
       it 'passes the read preference' do
         event = Utils.get_command_event(client, 'explain') do |client|
-          client['foo'].find({}, read: {mode: :secondary_preferred}).explain.should have_key('queryPlanner')
+          client['foo'].find({}, read: {mode: :secondary_preferred}).explain.should be_explain_output
         end
         event.command.fetch('$readPreference').should == {'mode' => 'secondaryPreferred'}
       end
@@ -170,7 +170,7 @@
 
       it 'passes the read preference' do
         event = Utils.get_command_event(client, 'explain') do |client|
-          client['foo', read: {mode: :secondary_preferred}].find.explain.should have_key('queryPlanner')
+          client['foo', read: {mode: :secondary_preferred}].find.explain.should be_explain_output
         end
         event.command.fetch('$readPreference').should == {'mode' => 'secondaryPreferred'}
       end
@@ -183,7 +183,7 @@
 
       it 'passes the read preference' do
         event = Utils.get_command_event(client, 'explain') do |client|
-          client['foo'].find.explain.should have_key('queryPlanner')
+          client['foo'].find.explain.should be_explain_output
         end
         event.command.fetch('$readPreference').should == {'mode' => 'secondaryPreferred'}
       end

spec/support/matchers.rb

@@ -73,3 +73,16 @@
     (Time.now - start_time).should < min_expected_time
   end
 end
+
+RSpec::Matchers.define :be_explain_output do
+  match do |actual|
+    Hash === actual && (
+      actual.key?('queryPlanner') ||
+      actual.key?('allPlans')
+    )
+  end
+
+  failure_message do |actual|
+    "expected that #{actual} is explain output: is a hash with either allPlans or queryPlanner keys present"
+  end
+end