fix: catch exceptions in fallback functions

lib/circuit.js

@@ -661,17 +661,21 @@ function handleError (error, circuit, timeout, args, latency, resolve, reject) {
 
 function fallback (circuit, err, args) {
   if (circuit[FALLBACK_FUNCTION]) {
-    const result =
+    try {
+      const result =
       circuit[FALLBACK_FUNCTION]
         .apply(circuit[FALLBACK_FUNCTION], [...args, err]);
-    /**
-     * Emitted when the circuit breaker executes a fallback function
-     * @event CircuitBreaker#fallback
-     * @type {any} the return value of the fallback function
-     */
-    circuit.emit('fallback', result, err);
-    if (result instanceof Promise) return result;
-    return Promise.resolve(result);
+      /**
+       * Emitted when the circuit breaker executes a fallback function
+       * @event CircuitBreaker#fallback
+       * @type {any} the return value of the fallback function
+       */
+      circuit.emit('fallback', result, err);
+      if (result instanceof Promise) return result;
+      return Promise.resolve(result);
+    } catch (e) {
+      return Promise.reject(e);
+    }
   }
 }
 

test/test.js

@@ -776,6 +776,22 @@ test('CircuitBreaker fallback as a CircuitBreaker', t => {
     .then(t.end);
 });
 
+test('CircuitBreaker fallback that throws returns a rejected Promise', t => {
+  t.plan(1);
+  const options = {
+    errorThresholdPercentage: 1,
+    resetTimeout: 100
+  };
+  const breaker = new CircuitBreaker(passFail, options);
+  breaker.fallback(_ => { throw new Error('Fallback failed'); });
+
+  breaker.fire(-1)
+    .then(_ => t.fail('CircuitBreaker fallback should return rejected promise'))
+    .catch(e => t.equals(e.message, 'Fallback failed'))
+    .then(_ => breaker.shutdown())
+    .then(t.end);
+});
+
 test('options.maxFailures should be deprecated', t => {
   const options = { maxFailures: 1 };
   const originalLog = console.error;