Node.js触发回调两次，为什么？

Question

In the following code, why does the createFile callback fire twice? This only happens when the server (below) is processing two or more requests at the same time, not if only one request has been made. Output at the bottom of the post. The client making the request is not a browser, but another node.js script iterating through a directory and sending a http post request with the file to the server. The request is created like this:

fs.createReadStream(fileName).pipe(httprequest(options, function(error, response, body) { }));

function myRequest(request, response) {
  function writeFile(filePath, request, callback) {
    newFilePath = "/home/pi/upload"+filePath; //filePath looks like this: /home/blah/file.txt, the code below creates this structure under another directory, so newFilePath becomes /home/pi/upload/home/blah/file.txt
    tempFileName = path.basename(filePath)+".temp";
    console.log("Processing "+filePath+"->"+newFilePath+" with tempname " +tempFileName);
    var createFile = request.pipe(fs.createWriteStream(tempFileName));
    createFile.on("finish", function(error) { //Why does it fire the callback twice?
      if(error) {
        throw error;
      } else {
        moveFile(tempFileName, newFilePath, function(error) {
          if(error) {
            throw error;
          } else {
            console.log("OK");
          }
        });
      }
    });
  }

  function moveFile(tempFileName, newFilePath, callback) {
    dirPath = path.dirname(newFilePath);
    fs.stat(dirPath, function(error, stats) { //check if dir exists
      if(error == null) {
        console.log(dirPath+" already exists");
        fs.stat(tempFileName, function(error, stats) { //check if file exists
          if(error == null) {
            console.log("OK, writing "+newFilePath);
            fs.rename(tempFileName, newFilePath, function(error) {
              if(error) { //Error on the second run, because the file has been moved in the first run, shouldn't happen?
                throw error;
              } else {
                var myCB = JSON.stringify({fMove: "OK"});
                callback(myCB);
              }
            });
          } else {
            console.log("File exists");
          }
        });
      }
    });
  }
  writeFile(fileName, request, function() {
    //Do some stuff
  });
  request.on("end", function() {
    //Do other stuff
  }
});

http.createServer(myRequest).listen(8888);

Output from my script

Processing /home/pi/app/temp/client.js->/home/pi/upload/home/pi/app/temp/client.js with tempname client.js.temp
/home/pi/upload/home/pi/app/temp already exists
/home/pi/upload/home/pi/app/temp already exists
OK, Writing /home/pi/upload/home/pi/app/temp/client.js
OK, Writing /home/pi/upload/home/pi/app/temp/client.js

/home/pi/app/server.js:67
            throw error;
                  ^
{"fMove":"OK"}

Answer 1

Incorrect error handling made the script faulty.

In the moveFile function this part was wrong:

fs.rename(tempFileName, newFilePath, function(error) {
              if(error) {
                throw error;
              } else {
                var myCB = JSON.stringify({fMove: "OK"}); 
                callback(myCB); // <-- Malformatted callback! Should be callback(null, myCB);
              }

Which made this part of writeFile trigger on error and for some reason run twice:

moveFile(tempFileName, newFilePath, function(error) { //Should be moveFile(tempFileName, newFilePath, function(error, status) {
          if(error) {
            throw error;
          } else {
            console.log("OK");
          }
        });

When I fixed my code so it handles the error correctly, it works as intended!