feat: support operations passed to a Cursor or subclass

This is piecemeal change towards the future we're looking for in
the cursor: no longer accepting `ns` and `cmd`, but rather just an
operation. This allows us to disambiguate options management for
the cursor, as well as reusing `executeOperation` for features
such as retryability and sessions management.

lib/core/cursor.js

@@ -9,6 +9,7 @@ const f = require('util').format;
 const collationNotSupported = require('./utils').collationNotSupported;
 const ReadPreference = require('./topologies/read_preference');
 const isUnifiedTopology = require('./utils').isUnifiedTopology;
+const executeOperation = require('../operations/execute_operation');
 
 const BSON = retrieveBSON();
 const Long = BSON.Long;
@@ -48,6 +49,13 @@ const Long = BSON.Long;
 var Cursor = function(topology, ns, cmd, options) {
   options = options || {};
 
+  if (typeof ns !== 'string') {
+    this.operation = ns;
+    ns = this.operation.ns.toString();
+    options = this.operation.options;
+    cmd = {};
+  }
+
   // Cursor pool
   this.pool = null;
   // Cursor server
@@ -689,12 +697,36 @@ Cursor.prototype._initializeCursor = function(callback) {
     done(null, result);
   };
 
+  if (cursor.operation) {
+    executeOperation(cursor.topology, cursor.operation, (err, result, server) => {
+      if (err) {
+        done(err);
+        return;
+      }
+
+      cursor.server = server;
+      cursor.cursorState.init = true;
+
+      // NOTE: this is a special internal method for cloning a cursor, consider removing
+      if (cursor.cursorState.cursorId != null) {
+        return done();
+      }
+
+      queryCallback(err, result);
+    });
+
+    return;
+  }
+
   // Very explicitly choose what is passed to selectServer
   const serverSelectOptions = {};
   if (cursor.cursorState.session) {
     serverSelectOptions.session = cursor.cursorState.session;
   }
-  if (cursor.options.readPreference) {
+
+  if (cursor.operation) {
+    serverSelectOptions.readPreference = cursor.operation.readPreference;
+  } else if (cursor.options.readPreference) {
     serverSelectOptions.readPreference = cursor.options.readPreference;
   }
 

lib/cursor.js

@@ -113,6 +113,13 @@ function Cursor(topology, ns, cmd, options) {
   const bson = topology.s.bson;
   const topologyOptions = topology.s.options;
 
+  if (typeof ns !== 'string') {
+    this.operation = ns;
+    ns = this.operation.ns.toString();
+    options = this.operation.options;
+    cmd = {};
+  }
+
   // Tailable cursor options
   const numberOfRetries = options.numberOfRetries || 5;
   const tailableRetryInterval = options.tailableRetryInterval || 500;
@@ -168,12 +175,14 @@ function Cursor(topology, ns, cmd, options) {
   this.sortValue = this.s.cmd.sort;
 
   // Get the batchSize
-  const batchSize =
-    cmd.cursor && cmd.cursor.batchSize
-      ? cmd.cursor && cmd.cursor.batchSize
-      : options.cursor && options.cursor.batchSize
-        ? options.cursor.batchSize
-        : 1000;
+  let batchSize = 1000;
+  if (cmd.cursor && cmd.cursor.batchSize) {
+    batchSize = cmd.cursor && cmd.cursor.batchSize;
+  } else if (options.cursor && options.cursor.batchSize) {
+    batchSize = options.cursor.batchSize;
+  } else if (typeof options.batchSize === 'number') {
+    batchSize = options.batchSize;
+  }
 
   // Set the batchSize
   this.setCursorBatchSize(batchSize);
@@ -224,10 +233,19 @@ for (let name in CoreCursor.prototype) {
 }
 
 Cursor.prototype._initializeCursor = function(callback) {
-  // implicitly create a session if one has not been provided
-  if (!this.s.explicitlyIgnoreSession && !this.s.session && this.s.topology.hasSessionSupport()) {
-    this.s.session = this.s.topology.startSession({ owner: this });
-    this.cursorState.session = this.s.session;
+  if (this.operation && this.operation.session != null) {
+    this.s.session = this.operation.session;
+    this.cursorState.session = this.operation.session;
+  } else {
+    // implicitly create a session if one has not been provided
+    if (!this.s.explicitlyIgnoreSession && !this.s.session && this.s.topology.hasSessionSupport()) {
+      this.s.session = this.s.topology.startSession({ owner: this });
+      this.cursorState.session = this.s.session;
+
+      if (this.operation) {
+        this.operation.session = this.s.session;
+      }
+    }
   }
 
   CoreCursor.prototype._initializeCursor.apply(this, [callback]);
@@ -1000,6 +1018,12 @@ Cursor.prototype.transformStream = function(options) {
  * @return {Promise} returns Promise if no callback passed
  */
 Cursor.prototype.explain = function(callback) {
+  if (this.operation) {
+    this.operation.explain = true;
+    executeOperation(this.s.topology, this.operation, callback);
+    return;
+  }
+
   this.s.cmd.explain = true;
 
   // Do we have a readConcern

lib/operations/common_functions.js

@@ -224,7 +224,8 @@ function nextObject(cursor, callback) {
       MongoError.create({ message: 'Cursor is closed', driver: true })
     );
   }
-  if (cursor.s.state === Cursor.INIT && cursor.s.cmd.sort) {
+
+  if (cursor.s.state === Cursor.INIT && cursor.s.cmd && cursor.s.cmd.sort) {
     try {
       cursor.s.cmd.sort = formattedOrderClause(cursor.s.cmd.sort);
     } catch (err) {