feat(sessions): adding implicit cursor session support

Adds ability to add owner to session (which allows you to note
who is responsible for closing the session), and adds part of
implicit session support to cursors.

Part of NODE-1326

lib/cursor.js

@@ -171,6 +171,27 @@ Cursor.prototype.cursorSkip = function() {
   return this.cursorState.skip;
 };
 
+Cursor.prototype._endSession = function(options, callback) {
+  if (typeof options === 'function') {
+    callback = options;
+    options = {};
+  }
+  options = options || {};
+
+  const session = this.cursorState.session;
+
+  if (session && (options.force || session.owner === this)) {
+    this.cursorState.session = undefined;
+    session.endSession(callback);
+    return true;
+  }
+
+  if (callback) {
+    callback();
+  }
+  return false;
+};
+
 //
 // Handle callback (including any exceptions thrown)
 var handleCallback = function(callback, err, result) {
@@ -475,15 +496,11 @@ Cursor.prototype.rewind = function() {
  */
 var isConnectionDead = function(self, callback) {
   if (self.pool && self.pool.isDestroyed()) {
-    self.cursorState.notified = true;
     self.cursorState.killed = true;
-    self.cursorState.documents = [];
-    self.cursorState.cursorIndex = 0;
-    callback(
-      new MongoNetworkError(
-        f('connection to host %s:%s was destroyed', self.pool.host, self.pool.port)
-      )
+    const err = new MongoNetworkError(
+      f('connection to host %s:%s was destroyed', self.pool.host, self.pool.port)
     );
+    _setCursorNotifiedImpl(self, () => callback(err));
     return true;
   }
 
@@ -496,11 +513,8 @@ var isConnectionDead = function(self, callback) {
 var isCursorDeadButNotkilled = function(self, callback) {
   // Cursor is dead but not marked killed, return null
   if (self.cursorState.dead && !self.cursorState.killed) {
-    self.cursorState.notified = true;
     self.cursorState.killed = true;
-    self.cursorState.documents = [];
-    self.cursorState.cursorIndex = 0;
-    handleCallback(callback, null, null);
+    setCursorNotified(self, callback);
     return true;
   }
 
@@ -524,10 +538,7 @@ var isCursorDeadAndKilled = function(self, callback) {
  */
 var isCursorKilled = function(self, callback) {
   if (self.cursorState.killed) {
-    self.cursorState.notified = true;
-    self.cursorState.documents = [];
-    self.cursorState.cursorIndex = 0;
-    handleCallback(callback, null, null);
+    setCursorNotified(self, callback);
     return true;
   }
 
@@ -539,20 +550,24 @@ var isCursorKilled = function(self, callback) {
  */
 var setCursorDeadAndNotified = function(self, callback) {
   self.cursorState.dead = true;
-  self.cursorState.notified = true;
-  self.cursorState.documents = [];
-  self.cursorState.cursorIndex = 0;
-  handleCallback(callback, null, null);
+  setCursorNotified(self, callback);
 };
 
 /**
  * Mark cursor as being notified
  */
 var setCursorNotified = function(self, callback) {
+  _setCursorNotifiedImpl(self, () => handleCallback(callback, null, null));
+};
+
+var _setCursorNotifiedImpl = function(self, callback) {
   self.cursorState.notified = true;
   self.cursorState.documents = [];
   self.cursorState.cursorIndex = 0;
-  handleCallback(callback, null, null);
+  if (self._endSession) {
+    return self._endSession(undefined, () => callback());
+  }
+  return callback();
 };
 
 var nextFunction = function(self, callback) {
@@ -654,6 +669,10 @@ var nextFunction = function(self, callback) {
     self._find(function(err) {
       if (err) return handleCallback(callback, err, null);
 
+      if (self.cursorState.cursorId && self.cursorState.cursorId.isZero() && self._endSession) {
+        self._endSession();
+      }
+
       if (
         self.cursorState.documents.length === 0 &&
         self.cursorState.cursorId &&
@@ -696,6 +715,10 @@ var nextFunction = function(self, callback) {
     self._getmore(function(err, doc, connection) {
       if (err) return handleCallback(callback, err);
 
+      if (self.cursorState.cursorId && self.cursorState.cursorId.isZero() && self._endSession) {
+        self._endSession();
+      }
+
       // Save the returned connection to ensure all getMore's fire over the same connection
       self.connection = connection;
 

lib/sessions.js

@@ -39,6 +39,9 @@ class ClientSession extends EventEmitter {
     }
 
     this.operationTime = null;
+
+    this.explicit = !!options.explicit;
+    this.owner = options.owner;
   }
 
   /**