Github does not let you use the same SSH key as a deploy key for several projects. Knowing this, you’ve got 2 choices: edit the configuration of your 1st project and say that this SSH key is not longer a deploy key or find another solution.

Deleting the deploy key of the existing project

To know what is the project associated with your deploy key, you can run the command ssh -T -ai ~/.ssh/id_rsa [email protected] (adjust the path to your SSH key if necessary). Github will then great you with something like:

Hi AntoineAugusti/foo-project! You've successfully authenticated, but GitHub does not provide shell access.

From this point, solving your problem is just a matter of going to the settings of this repository and removing the deploy key.

The alternative: generating other SSH keys

We are going to generate a SSH key for each repository, you’ll see it’s not too much trouble.

• First, generate a new SSH key with a comprehensive name with the command ssh-keygen -t rsa -f ~/.ssh/id_vendor_foo-project -C https://github.com/vendor/foo-project (replace vendor and foo-project).
• Edit your ~/.ssh/config file to map a fake subdomain to the appropriate SSH key. You will need to add the following content:

  Host vendor_foo-project.github.com
      Hostname github.com
      IdentityFile ~/.ssh/id_vendor_foo-project
  

  This code maps a fake Github’s subdomain to the root domain and say that when connecting to the fake subdomain, we should automatically use the previously created SSH key.

• Add the newly created SSH public key as a deploy key to the repository of your choice
• Clone your Git repository with the fake subdomain: instead of using the URL given by GitHub (git clone [email protected]:vendor/foo-project.git) you will use git clone git@vendor_foo-project.github.com:vendor/foo-project.git
• From now on, running git pull will connect to GitHub with the appropriate SSH key and GitHub will not complain 🙂

If you’ve already cloned the Git repository before, you can always change the remote URL to the Git server by editing the file .git/config of your project.

Happy deploys!

Recently, I was working on package that was doing network requests inside goroutines and I encountered an issue: the program was really fast to finish, but the results were awful. This was because the number of goroutines running at the same time was too high. As a result, the network was congested, too many sockets were opened on my laptop and the final performance was degraded: requests were slow or failing.

In order to keep the network healthy while maintaining some concurrency, I wanted to limit the number of goroutines making requests at the same time. Here is a sample main file to illustrate how you can control the maximum number of goroutines that are allowed to run concurrently.

package main

import (
	"flag"
	"fmt"
	"time"
)

// Fake a long and difficult work.
func DoWork() {
	time.Sleep(500 * time.Millisecond)
}

func main() {
	maxNbConcurrentGoroutines := flag.Int("maxNbConcurrentGoroutines", 5, "the number of goroutines that are allowed to run concurrently")
	nbJobs := flag.Int("nbJobs", 100, "the number of jobs that we need to do")
	flag.Parse()

	// Dummy channel to coordinate the number of concurrent goroutines.
	// This channel should be buffered otherwise we will be immediately blocked
	// when trying to fill it.
	concurrentGoroutines := make(chan struct{}, *maxNbConcurrentGoroutines)
	// Fill the dummy channel with maxNbConcurrentGoroutines empty struct.
	for i := 0; i < *maxNbConcurrentGoroutines; i++ {
		concurrentGoroutines <- struct{}{}
	}

	// The done channel indicates when a single goroutine has
	// finished its job.
	done := make(chan bool)
	// The waitForAllJobs channel allows the main program
	// to wait until we have indeed done all the jobs.
	waitForAllJobs := make(chan bool)

	// Collect all the jobs, and since the job is finished, we can
	// release another spot for a goroutine.
	go func() {
		for i := 0; i < *nbJobs; i++ {
			<-done
			// Say that another goroutine can now start.
			concurrentGoroutines <- struct{}{}
		}
		// We have collected all the jobs, the program
		// can now terminate
		waitForAllJobs <- true
	}()

	// Try to start nbJobs jobs
	for i := 1; i <= *nbJobs; i++ {
		fmt.Printf("ID: %v: waiting to launch!\n", i)
		// Try to receive from the concurrentGoroutines channel. When we have something,
		// it means we can start a new goroutine because another one finished.
		// Otherwise, it will block the execution until an execution
		// spot is available.
		<-concurrentGoroutines
		fmt.Printf("ID: %v: it's my turn!\n", i)
		go func(id int) {
			DoWork()
			fmt.Printf("ID: %v: all done!\n", id)
			done <- true
		}(i)
	}

	// Wait for all jobs to finish
	<-waitForAllJobs
}

This file is available as a gist on GitHub if you find it more convenient.

Sample runs

For the command time go run concurrent.go -nbJobs 25 -maxNbConcurrentGoroutines 10:

ID: 1: waiting to launch!
ID: 1: it's my turn!
ID: 2: waiting to launch!
ID: 2: it's my turn!
ID: 3: waiting to launch!
ID: 3: it's my turn!
ID: 4: waiting to launch!
ID: 4: it's my turn!
ID: 5: waiting to launch!
ID: 5: it's my turn!
ID: 6: waiting to launch!
ID: 6: it's my turn!
ID: 7: waiting to launch!
ID: 7: it's my turn!
ID: 8: waiting to launch!
ID: 8: it's my turn!
ID: 9: waiting to launch!
ID: 9: it's my turn!
ID: 10: waiting to launch!
ID: 10: it's my turn!
ID: 11: waiting to launch!
ID: 1: all done!
ID: 9: all done!
ID: 11: it's my turn!
ID: 12: waiting to launch!
ID: 12: it's my turn!
ID: 7: all done!
ID: 13: waiting to launch!
ID: 5: all done!
ID: 13: it's my turn!
ID: 14: waiting to launch!
ID: 4: all done!
ID: 14: it's my turn!
ID: 8: all done!
ID: 15: waiting to launch!
ID: 15: it's my turn!
ID: 16: waiting to launch!
ID: 16: it's my turn!
ID: 10: all done!
ID: 17: waiting to launch!
ID: 2: all done!
ID: 17: it's my turn!
ID: 18: waiting to launch!
ID: 18: it's my turn!
ID: 3: all done!
ID: 19: waiting to launch!
ID: 6: all done!
ID: 19: it's my turn!
ID: 20: waiting to launch!
ID: 20: it's my turn!
ID: 21: waiting to launch!
ID: 20: all done!
ID: 16: all done!
ID: 17: all done!
ID: 12: all done!
ID: 21: it's my turn!
ID: 19: all done!
ID: 11: all done!
ID: 14: all done!
ID: 18: all done!
ID: 15: all done!
ID: 13: all done!
ID: 22: waiting to launch!
ID: 22: it's my turn!
ID: 23: waiting to launch!
ID: 23: it's my turn!
ID: 24: waiting to launch!
ID: 24: it's my turn!
ID: 25: waiting to launch!
ID: 25: it's my turn!
ID: 24: all done!
ID: 21: all done!
ID: 22: all done!
ID: 25: all done!
ID: 23: all done!
0,28s user 0,05s system 18% cpu 1,762 total

For the command time go run concurrent.go -nbJobs 10 -maxNbConcurrentGoroutines 1:

ID: 1: waiting to launch!
ID: 1: it's my turn!
ID: 2: waiting to launch!
ID: 1: all done!
ID: 2: it's my turn!
ID: 3: waiting to launch!
ID: 2: all done!
ID: 3: it's my turn!
ID: 4: waiting to launch!
ID: 3: all done!
ID: 4: it's my turn!
ID: 5: waiting to launch!
ID: 4: all done!
ID: 5: it's my turn!
ID: 6: waiting to launch!
ID: 5: all done!
ID: 6: it's my turn!
ID: 7: waiting to launch!
ID: 6: all done!
ID: 7: it's my turn!
ID: 8: waiting to launch!
ID: 7: all done!
ID: 8: it's my turn!
ID: 9: waiting to launch!
ID: 8: all done!
ID: 9: it's my turn!
ID: 10: waiting to launch!
ID: 9: all done!
ID: 10: it's my turn!
ID: 10: all done!
0,32s user 0,03s system 6% cpu 5,274 total

Questions? Feedback? Hit me on Twitter @AntoineAugusti