feat: set err.headerSent before app error event emit (#923)

cherry-pick from https://github.com/koajs/koa/pull/919

lib/context.js

@@ -104,14 +104,18 @@ var proto = module.exports = {
 
     if (!(err instanceof Error)) err = new Error('non-error thrown: ' + err);
 
+    var headerSent = false;
+    if (this.headerSent || !this.writable) {
+      headerSent = err.headerSent = true;
+    }
+
     // delegate
     this.app.emit('error', err, this);
 
     // nothing we can do here other
     // than delegate to the app-level
     // handler and log.
-    if (this.headerSent || !this.writable) {
-      err.headerSent = true;
+    if (headerSent) {
       return;
     }
 

test/context/onerror.js

@@ -88,6 +88,29 @@ describe('ctx.onerror(err)', function(){
     })
   })
 
+  it('should ignore error after headerSent', function(done){
+    var app = koa();
+
+    app.on('error', function(err){
+      err.message.should.equal('mock error');
+      err.headerSent.should.equal(true);
+      done();
+    });
+
+    app.use(function*() {
+      this.status = 200;
+      this.set('X-Foo', 'Bar');
+      this.res.flushHeaders();
+      yield Promise.reject(new Error('mock error'));
+      this.body = 'response';
+    });
+
+    request(app.listen())
+    .get('/')
+    .expect('X-Foo', 'Bar')
+    .expect(200, function() {});
+  })
+
   describe('when invalid err.status', function(){
     describe('not number', function(){
       it('should respond 500', function(done){