fix(wire-protocol): don't allow override of slaveOk

The wire protocol refactor accidentally allowed the `slaveOk`
bit to be set in layers above the common `command` method. This
change reverts that, and always explicitly sets the value based
on the current read preference.

lib/wireprotocol/2_6_support.js

@@ -130,14 +130,16 @@ class WireProtocol {
     const commandOptions = Object.assign(
       {
         command: true,
-        slaveOk: readPreference.slaveOk(),
         numberToSkip: 0,
         numberToReturn: -1,
         checkKeys: false
       },
       options
     );
 
+    // This value is not overridable
+    commandOptions.slaveOk = readPreference.slaveOk();
+
     try {
       const query = new Query(bson, `${databaseNamespace(ns)}.$cmd`, finalCmd, commandOptions);
       pool.write(query, commandOptions, callback);

lib/wireprotocol/3_2_support.js

@@ -171,14 +171,16 @@ class WireProtocol {
     const commandOptions = Object.assign(
       {
         command: true,
-        slaveOk: readPreference.slaveOk(),
         numberToSkip: 0,
         numberToReturn: -1,
         checkKeys: false
       },
       options
     );
 
+    // This value is not overridable
+    commandOptions.slaveOk = readPreference.slaveOk();
+
     try {
       const query = new Query(bson, `${databaseNamespace(ns)}.$cmd`, finalCmd, commandOptions);
       pool.write(query, commandOptions, callback);

test/tests/unit/pool_tests.js

@@ -4,6 +4,7 @@ const expect = require('chai').expect;
 const mock = require('mongodb-mock-server');
 const Server = require('../../../lib/topologies/server');
 const MongoWriteConcernError = require('../../../lib/error').MongoWriteConcernError;
+const sinon = require('sinon');
 
 const test = {};
 describe('Pool (unit)', function() {
@@ -47,4 +48,33 @@ describe('Pool (unit)', function() {
 
     client.connect();
   });
+
+  it('should not allow overriding `slaveOk` when connected to a mongos', function(done) {
+    test.server.setMessageHandler(request => {
+      const doc = request.document;
+      if (doc.ismaster) {
+        request.reply(Object.assign({ msg: 'isdbgrid' }, mock.DEFAULT_ISMASTER));
+      } else if (doc.insert) {
+        request.reply({ ok: 1 });
+      }
+    });
+
+    const client = new Server(test.server.address());
+    client.on('error', done);
+    client.once('connect', () => {
+      const poolWrite = sinon.spy(client.s.pool, 'write');
+
+      client.insert('fake.ns', { a: 1 }, { slaveOk: true }, err => {
+        expect(err).to.not.exist;
+
+        const query = poolWrite.getCalls()[0].args[0];
+        expect(query.slaveOk).to.be.false;
+
+        client.s.pool.write.restore();
+        done();
+      });
+    });
+
+    client.connect();
+  });
 });