fix(utils): fixes executeOperation to clean up sessions

Makes sure that, if an error is thrown before the async part
of executeOperation, the implicit session is cleaned up

Fixes NODE-1335

Author: daprahamian

lib/utils.js

@@ -390,23 +390,37 @@ const executeOperation = (topology, operation, args, options) => {
     }
   }
 
-  // Execute using callback
-  if (typeof callback === 'function') {
-    callback = args.pop();
-    args.push((err, result) => {
+  const makeExecuteCallback = (resolve, reject) =>
+    function executeCallback(err, result) {
       if (session && !options.returnsCursor) {
         session.endSession(() => {
           delete opOptions.session;
-          if (err) return callback(err, null);
-          return resultMutator ? callback(null, resultMutator(result)) : callback(null, result);
+          if (err) return reject(err);
+          if (resultMutator) return resolve(resultMutator(result));
+          resolve(result);
         });
       } else {
-        if (err) return callback(err, null);
-        return resultMutator ? callback(null, resultMutator(result)) : callback(null, result);
+        if (err) return reject(err);
+        if (resultMutator) return resolve(resultMutator(result));
+        resolve(result);
       }
-    });
+    };
 
-    return operation.apply(null, args);
+  // Execute using callback
+  if (typeof callback === 'function') {
+    callback = args.pop();
+    const handler = makeExecuteCallback(
+      result => callback(null, result),
+      err => callback(err, null)
+    );
+    args.push(handler);
+
+    try {
+      return operation.apply(null, args);
+    } catch (e) {
+      handler(e);
+      throw e;
+    }
   }
 
   // Return a Promise
@@ -415,22 +429,15 @@ const executeOperation = (topology, operation, args, options) => {
   }
 
   return new Promise(function(resolve, reject) {
-    args[args.length - 1] = (err, r) => {
-      if (session && !options.returnsCursor) {
-        session.endSession(() => {
-          delete opOptions.session;
-          if (err) return reject(err);
-          if (resultMutator) return resolve(resultMutator(r));
-          resolve(r);
-        });
-      } else {
-        if (err) return reject(err);
-        if (resultMutator) return resolve(resultMutator(r));
-        resolve(r);
-      }
-    };
+    const handler = makeExecuteCallback(resolve, reject);
+    args[args.length - 1] = handler;
 
-    operation.apply(null, args);
+    try {
+      return operation.apply(null, args);
+    } catch (e) {
+      handler(e);
+      throw e;
+    }
   });
 };