refactor: store full ismaster on connection until StreamDescription

Storing a local `ServerDescription` is not adequate for the needs
of wire protocol determination, in particular when a monitor
connection is initially connected. Until then, we will store the
full `ismaster` directly on the connection, like we did with the
previous connection type.

Author: mbroadst

lib/cmap/connection.js

@@ -6,6 +6,7 @@ const MongoError = require('../core/error').MongoError;
 const MongoNetworkError = require('../core/error').MongoNetworkError;
 const MongoWriteConcernError = require('../core/error').MongoWriteConcernError;
 const CommandResult = require('../core/connection/command_result');
+const ServerDescription = require('../core/sdam/server_description').ServerDescription;
 const wp = require('../core/wireprotocol');
 const apm = require('../core/connection/apm');
 const updateSessionFromResponse = require('../core/sessions').updateSessionFromResponse;
@@ -18,6 +19,7 @@ const kGeneration = Symbol('generation');
 const kLastUseTime = Symbol('lastUseTime');
 const kClusterTime = Symbol('clusterTime');
 const kDescription = Symbol('description');
+const kIsMaster = Symbol('ismaster');
 
 class Connection extends EventEmitter {
   constructor(stream, options) {
@@ -79,6 +81,10 @@ class Connection extends EventEmitter {
     return this[kDescription];
   }
 
+  get ismaster() {
+    return this[kIsMaster];
+  }
+
   // the `connect` method stores the result of the handshake ismaster on the connection
   set ismaster(response) {
     if (response.compression) {
@@ -93,7 +99,11 @@ class Connection extends EventEmitter {
       }
     }
 
-    this[kDescription] = response;
+    // TODO: This should be using a `StreamDescription`
+    this[kDescription] = new ServerDescription(this.address, response);
+
+    // TODO: remove this, and only use the `StreamDescription` in the future
+    this[kIsMaster] = response;
   }
 
   get generation() {

lib/core/sdam/monitor.js

@@ -167,7 +167,7 @@ function checkServer(monitor, callback) {
     }
 
     monitor[kConnection] = conn;
-    callback(undefined, conn.description);
+    callback(undefined, conn.ismaster);
   });
 }