feat: perform selection before cursor operation execution if needed

This is the counterpart to the previous commit which is temporarily
required because cursors do not use `executeOperation`. This fix
should no longer be required in a very short time.

Author: mbroadst

lib/core/cursor.js

@@ -7,6 +7,7 @@ const MongoNetworkError = require('./error').MongoNetworkError;
 const mongoErrorContextSymbol = require('./error').mongoErrorContextSymbol;
 const f = require('util').format;
 const collationNotSupported = require('./utils').collationNotSupported;
+const ReadPreference = require('./topologies/read_preference');
 const BSON = retrieveBSON();
 const Long = BSON.Long;
 
@@ -584,6 +585,20 @@ var nextFunction = function(self, callback) {
 Cursor.prototype._initializeCursor = function(callback) {
   const cursor = this;
 
+  // NOTE: this goes away once cursors use `executeOperation`
+  if (cursor.topology.shouldCheckForSessionSupport()) {
+    cursor.topology.selectServer(ReadPreference.primaryPreferred, err => {
+      if (err) {
+        callback(err);
+        return;
+      }
+
+      cursor.next(callback);
+    });
+
+    return;
+  }
+
   // Very explicitly choose what is passed to selectServer
   const serverSelectOptions = {};
   if (cursor.cursorState.session) {

lib/core/sdam/topology.js

@@ -391,6 +391,17 @@ class Topology extends EventEmitter {
   }
 
   // Sessions related methods
+
+  /**
+   * @return Whether the topology should initiate selection to determine session support
+   */
+  shouldCheckForSessionSupport() {
+    return (
+      (this.description.type === TopologyType.Single && !this.description.hasKnownServers) ||
+      !this.description.hasDataBearingServers
+    );
+  }
+
   /**
    * @return Whether sessions are supported on the current topology
    */

lib/operations/execute_operation.js

@@ -6,7 +6,6 @@ const OperationBase = require('./operation').OperationBase;
 const ReadPreference = require('../core').ReadPreference;
 const isRetryableError = require('../core/error').isRetryableError;
 const maxWireVersion = require('../core/utils').maxWireVersion;
-const TopologyType = require('../core/sdam/topology_description').TopologyType;
 
 /**
  * Executes the given operation with provided arguments.
@@ -33,7 +32,7 @@ function executeOperation(topology, operation, callback) {
   if (
     topology.description != null &&
     !operation.hasAspect(Aspect.SKIP_SESSION) &&
-    shouldCheckForSessionSupport(topology)
+    topology.shouldCheckForSessionSupport()
   ) {
     // TODO: this is only supported for unified topology, the first part of this check
     //       should go away when we drop legacy topology types.
@@ -162,11 +161,4 @@ function executeWithServerSelection(topology, operation, callback) {
   });
 }
 
-function shouldCheckForSessionSupport(topology) {
-  return (
-    (topology.description.type === TopologyType.Single && !topology.description.hasKnownServers) ||
-    !topology.description.hasDataBearingServers
-  );
-}
-
 module.exports = executeOperation;