Web terminal connection failure - mux handler not found

Summary

Trying to use web terminals results in a connection failure.

Steps to reproduce

• Setup HAproxy to handle SSL (websocket config based on this page)

  # relevant frontend/backend
  frontend vip-https                                               
    bind virtual_ip:443 ssl no-sslv3 crt /etc/ssl/private/ssl.pem
    mode http                                                      
    rspadd Strict-Transport-Security:\ max-age=15768000            
    acl is_request_for_git hdr(Host) -i gitlab
    acl is_websocket hdr(Upgrade) -i WebSocket                     
    use_backend git-ws if is_request_for_git is_websocket          
    use_backend git-server if is_request_for_git                   
  
  backend git-ws                                                   
    mode http                                                      
    timeout tunnel 3600s                                           
    stats enable                                                   
    reqadd X-Forwarded-Connection:\ Upgrade                        
    reqadd X-Forwarded-Upgrade:\ websocket                         
    server runner runner-ip:8093 check

• Setup gitlab v11.8.1 using omnibus leveraging HAproxy for SSL

  # important part of the gitlab config
  external_url "https://git-dev"
  nginx['listen_port'] = 80
  nginx['listen_https'] = false
  nginx['proxy_set_headers'] = {"X-Forwarded-Proto"=>"https", "X-Forwarded-Ssl"=>"no"}

• Setup gitlab runner v11.8.0 on Ubuntu 18.04 using the docker executor (config based on this doc)

  # runner config
  concurrent = 1                                
  check_interval = 0                            
                                          
  [session_server]                              
    listen_address = "runner-ip:8093"         
    session_timeout = 1800                      
                                          
  [[runners]]                                   
    name = "test web terminals"                 
    url = "https://git-dev/"
    token = "token"              
    executor = "docker"                         
    [runners.docker]                            
      tls_verify = false                        
      image = "ruby:2.5.3"                      
      privileged = false                        
      disable_entrypoint_overwrite = false      
      oom_kill_disable = false                  
      disable_cache = false                     
      volumes = ["/cache"]                      
      shm_size = 0                              
    [runners.cache]                             
      [runners.cache.s3]                        
      [runners.cache.gcs]                       

• Setup a project that uses Gitlab CI

Actual behavior

• Push a commit to trigger a Gitlab CI job

• Go to the running job's page

• Click the Debug button on the job's page to access the web terminal and see this:

  • 

• And this in Chrome's developer tools JS console:

  WebSocket connection to 'wss://mygitlab/mygroup/myproject/-/jobs/myjobid/terminal.ws' failed:
  Error during WebSocket handshake: Unexpected response code: 404

• In /var/log/syslog on the gitlab runner:

  gitlab-runner[7397]: #033[31;1mERROR: Mux handler not found #033[0;m  #033[31;1mbuilds#033[0;m=0 #033[31;1muri#033[0;m=/mygroup/myproject/-/jobs/myjobid/terminal.ws

• Be sad 😞

Expected behavior

I should be able to access the web terminal. Please?