fix(run): throw error if axe.run is called after a run has started bu…

…t not completed (#1914)

* fix(run): throw error if axe.run is called after a run has started but not completed

* remove comment

lib/core/public/run.js

@@ -107,11 +107,22 @@ axe.run = function(context, options, callback) {
 		});
 	}
 
+	if (axe._running) {
+		const err =
+			'Axe is already running. Use `await axe.run()` to wait ' +
+			'for the previous run to finish before starting a new run.';
+		callback(err);
+		reject(err);
+		return p;
+	}
+
+	axe._running = true;
 	axe._runRules(
 		context,
 		options,
 		function(rawResults, cleanup) {
 			let respond = function(results) {
+				axe._running = false;
 				cleanup();
 				try {
 					callback(null, results);
@@ -131,12 +142,14 @@ axe.run = function(context, options, callback) {
 					respond(results);
 				}
 			} catch (err) {
+				axe._running = false;
 				cleanup();
 				callback(err);
 				reject(err);
 			}
 		},
 		function(err) {
+			axe._running = false;
 			callback(err);
 			reject(err);
 		}

test/core/public/run.js

@@ -35,6 +35,7 @@ describe('axe.run', function() {
 		fixture.innerHTML = '';
 		axe._audit = null;
 		axe._runRules = origRunRules;
+		axe._running = false;
 	});
 
 	it('takes context, options and callback as parameters', function(done) {
@@ -115,6 +116,14 @@ describe('axe.run', function() {
 		axe.run({ someOption: true }, noop);
 	});
 
+	it('should error if axe is already running', function(done) {
+		axe.run(noop);
+		axe.run(function(err) {
+			assert.isTrue(err.indexOf('Axe is already running') !== -1);
+			done();
+		});
+	});
+
 	describe('callback', function() {
 		it('gives errors to the first argument on the callback', function(done) {
 			axe._runRules = function(ctxt, opt, resolve, reject) {