feat: support background task on ctx

```js
ctx.runAtBackground(function* (ctx) {
  // async actions here
});
```

app/extend/context.js

@@ -1,15 +1,10 @@
-/**
- * 对 koa context 的所有扩展，都放在此文件统一维护。
- *
- * - koa context: https://github.com/koajs/koa/blob/master/lib/context.js
- */
-
 'use strict';
 
 const delegate = require('delegates');
 const jsonpBody = require('jsonp-body');
 const ContextLogger = require('egg-logger').EggContextLogger;
 const Cookies = require('egg-cookies');
+const co = require('co');
 const util = require('../../lib/core/util');
 
 const LOGGER = Symbol('LOGGER');
@@ -345,6 +340,31 @@ const proto = module.exports = {
   set state(val) {
     this.locals = val;
   },
+
+  /**
+   * Run generator function in the background
+   * @param  {Generator} scope - generator function, the first args is ctx
+   * ```js
+   * this.body = 'hi';
+   *
+   * this.runInBackground(function* saveUserInfo(ctx) {
+   *   yield ctx.mysql.query(sql);
+   *   yield ctx.curl(url);
+   * });
+   * ```
+   */
+  runInBackground(scope) {
+    const ctx = this;
+    const start = Date.now();
+    const taskName = scope.name || '-';
+    co(function* () {
+      yield scope(ctx);
+      ctx.coreLogger.info('[egg:background] task:%s success (%dms)', taskName, Date.now() - start);
+    }).catch(err => {
+      ctx.coreLogger.info('[egg:background] task:%s fail (%dms)', taskName, Date.now() - start);
+      ctx.coreLogger.error(err);
+    });
+  },
 };
 
 /**

test/app/extend/context.test.js

@@ -305,4 +305,48 @@ describe('test/app/extend/context.test.js', () => {
       context.state.should.equal(context.locals);
     });
   });
+
+  describe('ctx.runInBackground(scope)', () => {
+    let app;
+    before(() => {
+      app = utils.app('apps/ctx-background');
+    });
+    after(() => app.close());
+
+    it('should run background task success', done => {
+      request(app.callback())
+        .get('/')
+        .expect(200)
+        .expect('hello')
+        .end(err => {
+          if (err) return done(err);
+          setTimeout(() => {
+            const logdir = app.config.logger.dir;
+            const log = fs.readFileSync(path.join(logdir, 'ctx-background-web.log'), 'utf8');
+            log.should.match(/background run result status: 200/);
+            fs.readFileSync(path.join(logdir, 'egg-web.log'), 'utf8')
+              .should.match(/\[egg:background\] task:saveUserInfo success \(\d+ms\)/);
+            done();
+          }, 3000);
+        });
+    });
+
+    it('should run background task error', done => {
+      request(app.callback())
+        .get('/error')
+        .expect(200)
+        .expect('hello error')
+        .end(err => {
+          if (err) return done(err);
+          setTimeout(() => {
+            const logdir = app.config.logger.dir;
+            const log = fs.readFileSync(path.join(logdir, 'common-error.log'), 'utf8');
+            log.should.match(/getaddrinfo ENOTFOUND registry-not-exists\.npm/);
+            fs.readFileSync(path.join(logdir, 'egg-web.log'), 'utf8')
+              .should.match(/\[egg:background\] task:mockError fail \(\d+ms\)/);
+            done();
+          }, 2000);
+        });
+    });
+  });
 });

test/fixtures/apps/ctx-background/app/controller/error.js

@@ -0,0 +1,9 @@
+'use strict';
+
+module.exports = function* () {
+  this.body = 'hello error';
+  this.runInBackground(function* mockError(ctx) {
+    const r = yield ctx.curl('http://registry-not-exists.npm.taobao.org/pedding/latest', { dataType: 'json' });
+    ctx.logger.warn('background run result status: %s', r.status);
+  });
+};

test/fixtures/apps/ctx-background/app/controller/home.js

@@ -0,0 +1,10 @@
+'use strict';
+
+module.exports = function* () {
+  this.body = 'hello';
+  this.runInBackground(function* saveUserInfo(ctx) {
+    const domain = process.env.CI ? 'registry.npmjs.com' : 'registry.npm.taobao.org';
+    const r = yield ctx.curl(`http://${domain}/pedding/latest`, { dataType: 'json' });
+    ctx.logger.warn('background run result status: %s', r.status);
+  });
+};

test/fixtures/apps/ctx-background/app/router.js

@@ -0,0 +1,6 @@
+'use strict';
+
+module.exports = app => {
+  app.get('/', app.controller.home);
+  app.get('/error', app.controller.error);
+};

test/fixtures/apps/ctx-background/package.json

@@ -0,0 +1,3 @@
+{
+  "name": "ctx-background"
+}