fix(collection): countDocuments throws error when query doesn't match docs

The countDocuments method was not properly checking that documents were
returned from the query. Added tests on top of astanciu's bug fix.

Fixes NODE-1543

Author: rweinberger

lib/operations/collection_ops.js

@@ -234,7 +234,7 @@ function countDocuments(coll, query, options, callback) {
     if (err) return handleCallback(callback, err);
     result.toArray((err, docs) => {
       if (err) handleCallback(err);
-      handleCallback(callback, null, docs[0].n);
+      handleCallback(callback, null, docs.length ? docs[0].n : 0);
     });
   });
 }

test/functional/collection_tests.js

@@ -2,6 +2,7 @@
 const test = require('./shared').assert;
 const setupDatabase = require('./shared').setupDatabase;
 const expect = require('chai').expect;
+const MongoClient = require('../..').MongoClient;
 
 describe('Collection', function() {
   before(function() {
@@ -1581,6 +1582,40 @@ describe('Collection', function() {
     }
   });
 
+  it('should correctly perform estimatedDocumentCount on non-matching query', function(done) {
+    const configuration = this.configuration;
+    const client = new MongoClient(configuration.url(), { w: 1 });
+
+    client.connect(function(err, client) {
+      const db = client.db(configuration.db);
+      const collection = db.collection('nonexistent_coll_1');
+      const close = e => client.close(() => done(e));
+
+      Promise.resolve()
+        .then(() => collection.estimatedDocumentCount({ a: 'b' }))
+        .then(count => expect(count).to.equal(0))
+        .then(() => close())
+        .catch(e => close(e));
+    });
+  });
+
+  it('should correctly perform countDocuments on non-matching query', function(done) {
+    const configuration = this.configuration;
+    const client = new MongoClient(configuration.url(), { w: 1 });
+
+    client.connect(function(err, client) {
+      const db = client.db(configuration.db);
+      const collection = db.collection('nonexistent_coll_2');
+      const close = e => client.close(() => done(e));
+
+      Promise.resolve()
+        .then(() => collection.countDocuments({ a: 'b' }))
+        .then(count => expect(count).to.equal(0))
+        .then(() => close())
+        .catch(e => close(e));
+    });
+  });
+
   describe('Retryable Writes on bulk ops', function() {
     const MongoClient = require('../../lib/mongo_client');