Race condition when running multiple kameleon builds on the same machine

[jonglezb]

When running multiple builds in parallel, one of them often fails when setting up port forwarding in qemu:

Step 24 : bootstrap/start_qemu/start_vm
--> Running the step...
[local] Qemu VM shutdown: in setup section cleaning
[local] Starting qemu...
[local] VNC port: 3
[local] qemu-system-ppc64: Could not set up host forwarding rule 'tcp::50002-:22'

This is caused by a race condition involving two steps:

• the step bootstrap/prepare_ssh_to_out_context finds a free TCP port and stores it in $SSH_FWD_PORT
• the step bootstrap/start_qemu.yaml then later uses this variable $SSH_FWD_PORT and causes qemu to bind to this port

Between these two steps, there is a race window of 5-10 seconds during which two kameleon instances can choose the same port:

--> Running the step...
[local] SSH forwarding port: 50001
Step 20 : bootstrap/prepare_ssh_to_out_context/prepare_ssh_config
Step prepare_ssh_to_out_context took: 0 secs
Step 21 : bootstrap/prepare_appliance/generate_ssh_keys
--> Running the step...
...
Step prepare_appliance took: **7 secs**
Step 24 : bootstrap/start_qemu/start_vm
--> Running the step...
[local] Qemu VM shutdown: in setup section cleaning
[local] Starting qemu...
[local] VNC port: 2
[local] qemu-system-ppc64: Could not set up host forwarding rule 'tcp::50001-:22'

A quick & dirty solution would be to randomize the port chosen by __find_free_port. I have no better idea right now.