设置标头两次和/或未正确处理回调 - 导致错误 [ERR_HTTP_HEADERS_SENT]答案

【问题标题】：Set headers twice and/or not correctly handling a callback - resulting in Error [ERR_HTTP_HEADERS_SENT]设置标头两次和/或未正确处理回调 - 导致错误 [ERR_HTTP_HEADERS_SENT]
【发布时间】：2020-02-18 15:42:22
【问题描述】：

我使用此代码获得了预期的图像，但它没有显示在我的 pug 模板中，而是显示在浏览器 devtools 控制台中（我希望这是有道理的），而且我似乎正在尝试设置标题两次。我的代码生成 [ERR_HTTP_HEADERS_SENT] 如下所示。

我已经阅读了许多有关此问题的 StackOverflow 问题，包括对问题的彻底分解：

Error: Can't set headers after they are sent to the client

我认为实际发生的情况是我没有正确处理回调，但我不确定。

我真的在这方面苦苦挣扎，可以使用一些帮助来指出我哪里出错了。

route.js：

router.get('/issue/filedisplay/:id', issue_controller.file_display);

controller.js：

// controller.js
    exports.file_display = function(req, res) {
      gfs.files.findOne(
        { _id: mongoose.Types.ObjectId(req.params.id) },
        (err, file) => {
          //res.contentType(file.contentType);
          // Check if file
          if (!file || file.length === 0) {
            return res.status(404).json({
              err: 'No file exists',
            });
          }
          const readstream = gfs.createReadStream(file._id);
          readstream.pipe(res);
          readstream.on('close', () => {
            res.render('filedisplay', {
              title: 'Display Image',
              file: file._id,
            });
          });
          //return res.json(file);
          //return res;
        }
      );
    };

帕格模板：

//-filedisplay.pug
extends layout

block content
  h1= title
    p Display Image
    div
      img.image(src="/list/issue/filedisplay/" + file alt="")

错误：

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:482:11)
    at ServerResponse.header (/home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/node_modules/express/lib/response.js:771:10)
    at ServerResponse.send (/home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/node_modules/express/lib/response.js:170:12)
    at done (/home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/node_modules/express/lib/response.js:1008:10)
    at Object.exports.renderFile (/home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/node_modules/pug/lib/index.js:421:12)
    at View.exports.__express [as engine] (/home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/node_modules/pug/lib/index.js:464:11)
    at View.render (/home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/node_modules/express/lib/view.js:135:8)
    at tryRender (/home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/node_modules/express/lib/application.js:640:10)
    at Function.render (/home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/node_modules/express/lib/application.js:592:3)
    at ServerResponse.render (/home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/node_modules/express/lib/response.js:1012:7)
    at /home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/app.js:82:7
    at Layer.handle_error (/home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/node_modules/express/lib/router/layer.js:71:5)
    at trim_prefix (/home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/node_modules/express/lib/router/index.js:315:13)
    at /home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/node_modules/express/lib/router/index.js:284:7
    at Function.process_params (/home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/node_modules/express/lib/router/index.js:335:12)
    at next (/home/don/dev/sandbox/fileupload-ex/file-upload-gridfs/node_modules/express/lib/router/index.js:275:10)

【问题讨论】：

标签： mongoose callback http-headers gridfs-stream

【解决方案1】：

此问题已关闭。它没有解决，代码仍然被破坏。在上面的控制器代码中，我真正需要的只是：

readstream.pipe(res);

之所以可行，是因为我在引用链接中添加了 target="_blank"，导致渲染图像在新的浏览器窗口中打开，这正是我真正想要的。因此，在新的 pug 模板中渲染图像是一个不必要的步骤，正是在“on”方法中渲染代码导致了问题。

【讨论】：