fix: don't modify passed options object

lib/connection.js

@@ -825,7 +825,6 @@ async function _createMongoClient(conn, uri, options) {
   options = processConnectionOptions(uri, options);
 
   if (options) {
-    options = clone(options);
 
     const autoIndex = options.config && options.config.autoIndex != null ?
       options.config.autoIndex :

lib/helpers/processConnectionOptions.js

@@ -9,11 +9,12 @@ function processConnectionOptions(uri, options) {
     ? opts.readPreference
     : getUriReadPreference(uri);
 
+  const clonedOpts = clone(opts);
   const resolvedOpts = (readPreference && readPreference !== 'primary' && readPreference !== 'primaryPreferred')
-    ? resolveOptsConflicts(readPreference, opts)
-    : opts;
+    ? resolveOptsConflicts(readPreference, clonedOpts)
+    : clonedOpts;
 
-  return clone(resolvedOpts);
+  return resolvedOpts;
 }
 
 function resolveOptsConflicts(pref, opts) {

test/connection.test.js

@@ -1547,4 +1547,16 @@ describe('connections:', function() {
       // currently cannot write test for continueOnError or errors in general.
     });
   });
+  describe('processConnectionOptions', function() {
+    let m = null;
+    after(async() => {
+      await m.disconnect();
+    });
+    it('should not throw an error when attempting to mutate unmutable options object gh-13335', async function() {
+      m = new mongoose.Mongoose();
+      const opts = Object.preventExtensions({ readPreference: 'secondaryPreferred' });
+      const conn = await m.connect(start.uri, opts);
+      assert.ok(conn);
+    });
+  });
 });