fix: avoid calling fallback function twice (#198)

lib/circuit.js

@@ -355,10 +355,12 @@ class CircuitBreaker extends EventEmitter {
             }
           })
             .catch(error => {
-              this.semaphore.release();
-              const latencyEndTime = Date.now() - latencyStartTime;
-              handleError(
-                error, this, timeout, args, latencyEndTime, resolve, reject);
+              if (!timeoutError) {
+                this.semaphore.release();
+                const latencyEndTime = Date.now() - latencyStartTime;
+                handleError(
+                  error, this, timeout, args, latencyEndTime, resolve, reject);
+              }
             });
         } catch (error) {
           this.semaphore.release();

test/test.js

@@ -272,6 +272,24 @@ test('Passes error as last argument to the fallback function', t => {
   breaker.fire(fails).catch(t.fail);
 });
 
+test('Fallback is not called twice for the same execution when action fails after timing out', t => {
+  t.plan(1);
+
+  const actionDuration = 200;
+  const breaker = circuit(timedFailingFunction, { timeout: actionDuration / 2 });
+
+  breaker.fallback((ms, err) => {
+    t.ok(err);
+    return Promise.reject(err);
+  });
+
+  breaker.fire(actionDuration)
+    .catch((err) => noop(err));
+
+  // keep this test alive until action finishes
+  setTimeout(() => noop, actionDuration);
+});
+
 test('Passes arguments to the fallback function', t => {
   t.plan(1);
   const fails = -1;
@@ -774,6 +792,7 @@ const identity = common.identity;
 const passFail = common.passFail;
 const slowFunction = common.slowFunction;
 const timedFunction = common.timedFunction;
+const timedFailingFunction = common.timedFailingFunction;
 const nonPromise = common.nonPromise;
 const callbackFunction = common.callbackFunction;
 const failedCallbackFunction = common.failedCallbackFunction;