RUBY-3463 Fix cursor behaviour on load balanced (#2893)

Author: comandeo-mongo

gemfiles/standard.rb

@@ -65,6 +65,7 @@ def standard_dependencies
     gem 'tilt'
     # solargraph depends on rbs, which won't build on jruby for some reason
     gem 'solargraph', platforms: :mri
+    gem 'ruby-lsp', platforms: :mri
   end
 
   gem 'libmongocrypt-helper', '~> 1.8.0' if ENV['FLE'] == 'helper'

lib/mongo/cluster/reapers/cursor_reaper.rb

@@ -194,7 +194,12 @@ def kill_cursors
             server_api: server.options[:server_api],
             connection_global_id: kill_spec.connection_global_id,
           }
-          op.execute(server, context: Operation::Context.new(options: options))
+          if connection = kill_spec.connection
+            op.execute_with_connection(connection, context: Operation::Context.new(options: options))
+            connection.connection_pool.check_in(connection)
+          else
+            op.execute(server, context: Operation::Context.new(options: options))
+          end
 
           if session = kill_spec.session
             if session.implicit?

lib/mongo/collection/view/aggregation.rb

@@ -118,14 +118,26 @@ def effective_read_preference(connection)
         end
 
         def send_initial_query(server, context)
-          server.with_connection do |connection|
+          if server.load_balancer?
+            # Connection will be checked in when cursor is drained.
+            connection = server.pool.check_out(context: context)
             initial_query_op(
               context.session,
               effective_read_preference(connection)
             ).execute_with_connection(
               connection,
               context: context
             )
+          else
+            server.with_connection do |connection|
+              initial_query_op(
+                context.session,
+                effective_read_preference(connection)
+              ).execute_with_connection(
+                connection,
+                context: context
+              )
+            end
           end
         end
       end

lib/mongo/collection/view/iterable.rb

@@ -168,7 +168,14 @@ def initial_query_op(session)
         end
 
         def send_initial_query(server, context)
-          initial_query_op(context.session).execute(server, context: context)
+          operation = initial_query_op(context.session)
+          if server.load_balancer?
+            # Connection will be checked in when cursor is drained.
+            connection = server.pool.check_out(context: context)
+            operation.execute_with_connection(connection, context: context)
+          else
+            operation.execute(server, context: context)
+          end
         end
 
         def use_query_cache?

lib/mongo/collection/view/map_reduce.rb

@@ -73,8 +73,15 @@ def each
           session = client.get_session(@options)
           server = cluster.next_primary(nil, session)
           context = Operation::Context.new(client: client, session: session, operation_timeouts: view.operation_timeouts)
-          result = send_initial_query(server, context)
-          result = send_fetch_query(server, session) unless inline?
+          if server.load_balancer?
+            # Connection will be checked in when cursor is drained.
+            connection = server.pool.check_out(context: context)
+            result = send_initial_query_with_connection(connection, context.session, context: context)
+            result = send_fetch_query_with_connection(connection, session) unless inline?
+          else
+            result = send_initial_query(server, context)
+            result = send_fetch_query(server, session) unless inline?
+          end
           @cursor = Cursor.new(view, result, server, session: session)
           if block_given?
             @cursor.each do |doc|
@@ -306,7 +313,7 @@ def find_command_spec(session)
           Builder::MapReduce.new(map_function, reduce_function, view, options.merge(session: session)).command_specification
         end
 
-        def fetch_query_op(server, session)
+        def fetch_query_op(session)
           spec = {
             coll_name: out_collection_name,
             db_name: out_database_name,
@@ -320,7 +327,16 @@ def fetch_query_op(server, session)
         end
 
         def send_fetch_query(server, session)
-          fetch_query_op(server, session).execute(server, context: Operation::Context.new(client: client, session: session))
+          fetch_query_op(session).execute(server, context: Operation::Context.new(client: client, session: session))
+        end
+
+        def send_fetch_query_with_connection(connection, session)
+          fetch_query_op(
+            session
+          ).execute_with_connection(
+            connection,
+            context: Operation::Context.new(client: client, session: session)
+          )
         end
       end
     end

lib/mongo/collection/view/readable.rb

@@ -736,7 +736,13 @@ def parallel_scan(cursor_count, options = {})
               session: session,
               connection_global_id: result.connection_global_id,
             )
-            result = op.execute(server, context: context)
+            result = if server.load_balancer?
+                       # Connection will be checked in when cursor is drained.
+                       connection = server.pool.check_out(context: context)
+                       op.execute_with_connection(connection, context: context)
+                     else
+                       op.execute(server, context: context)
+                     end
             Cursor.new(self, result, server, session: session)
           end
         end

lib/mongo/cursor.rb

@@ -91,10 +91,19 @@ def initialize(view, result, server, options = {})
       @context = @options[:context]&.with(connection_global_id: connection_global_id_for_context) || fresh_context
       @explicitly_closed = false
       @lock = Mutex.new
-      unless closed?
+      if server.load_balancer?
+        # We need the connection in the cursor only in load balanced topology;
+        # we do not need an additional reference to it otherwise.
+        @connection = @initial_result.connection
+      end
+      if closed?
+        check_in_connection
+      else
         register
-        ObjectSpace.define_finalizer(self, self.class.finalize(kill_spec(@connection_global_id),
-          cluster))
+        ObjectSpace.define_finalizer(
+          self,
+          self.class.finalize(kill_spec(@connection_global_id), cluster)
+        )
       end
     end
 
@@ -104,6 +113,9 @@ def initialize(view, result, server, options = {})
     # @api private
     attr_reader :initial_result
 
+    # @api private
+    attr_reader :connection
+
     # Finalize the cursor for garbage collection. Schedules this cursor to be included
     # in a killCursors operation executed by the Cluster's CursorReaper.
     #
@@ -315,6 +327,7 @@ def close(opts = {})
       @lock.synchronize do
         @explicitly_closed = true
       end
+      check_in_connection
     end
 
     # Get the parsed collection name.
@@ -395,6 +408,7 @@ def kill_spec(connection_global_id)
         connection_global_id: connection_global_id,
         server_address: server.address,
         session: @session,
+        connection: @connection
       )
     end
 
@@ -464,7 +478,10 @@ def process(result)
       # the @cursor_id may be zero (all results fit in the first batch).
       # Thus we need to check both @cursor_id and the cursor_id of the result
       # prior to calling unregister here.
-      unregister if !closed? && result.cursor_id == 0
+      if !closed? && result.cursor_id == 0
+        unregister
+        check_in_connection
+      end
       @cursor_id = set_cursor_id(result)
 
       if result.respond_to?(:post_batch_resume_token)
@@ -496,7 +513,12 @@ def unregister
     end
 
     def execute_operation(op, context: nil)
-      op.execute(@server, context: context || possibly_refreshed_context)
+      op_context = context || possibly_refreshed_context
+      if @connection.nil?
+        op.execute(@server, context: op_context)
+      else
+        op.execute_with_connection(@connection, context: op_context)
+      end
     end
 
     # Considers the timeout mode and will either return the cursor's
@@ -545,6 +567,22 @@ def connection_global_id_for_context
         @connection_global_id
       end
     end
+
+    # Returns the connection that was used to create the cursor back to the
+    # corresponding connection pool.
+    #
+    # In a load balanced topology cursors must use the same connection for the
+    # initial and all subsequent operations. Therefore, the connection is not
+    # checked into the pool after the initial operation is completed, but
+    # only when the cursor is drained.
+    def check_in_connection
+      # Connection nil means the connection has been already checked in.
+      return if @connection.nil?
+      return unless @connection.server.load_balancer?
+
+      @connection.connection_pool.check_in(@connection)
+      @connection = nil
+    end
   end
 end
 

lib/mongo/cursor/kill_spec.rb

@@ -31,22 +31,25 @@ def initialize(
         db_name:,
         connection_global_id:,
         server_address:,
-        session:
+        session:,
+        connection: nil
       )
         @cursor_id = cursor_id
         @coll_name = coll_name
         @db_name = db_name
         @connection_global_id = connection_global_id
         @server_address = server_address
         @session = session
+        @connection = connection
       end
 
       attr_reader :cursor_id,
       :coll_name,
       :db_name,
       :connection_global_id,
       :server_address,
-      :session
+      :session,
+      :connection
 
       def ==(other)
         cursor_id == other.cursor_id &&

lib/mongo/database/view.rb

@@ -288,11 +288,20 @@ def send_initial_query(server, session, context, options = {})
         if opts.key?(:deserialize_as_bson)
           execution_opts[:deserialize_as_bson] = opts.delete(:deserialize_as_bson)
         end
-        initial_query_op(session, opts).execute(
-          server,
-          context: context,
-          options: execution_opts
-        )
+        if server.load_balancer?
+          connection = server.pool.check_out(context: context)
+          initial_query_op(session, opts).execute_with_connection(
+            connection,
+            context: context,
+            options: execution_opts
+          )
+        else
+          initial_query_op(session, opts).execute(
+            server,
+            context: context,
+            options: execution_opts
+          )
+        end
       end
     end
   end

lib/mongo/index/view.rb

@@ -404,7 +404,12 @@ def normalize_models(models, server)
       end
 
       def send_initial_query(server, session, context)
-        initial_query_op(session).execute(server, context: context)
+        if server.load_balancer?
+          connection = server.pool.check_out(context: context)
+          initial_query_op(session).execute_with_connection(connection, context: context)
+        else
+          initial_query_op(session).execute(server, context: context)
+        end
       end
     end
   end