Using libvirt with the Custom executor

Tier: Free, Premium, Ultimate Offering: GitLab.com, Self-managed

Using libvirt, the Custom executor driver will create a new disk and VM for every job it executes, after which the disk and VM will be deleted.

This example is inspired by a Community Contribution !464 to add libvirt as a GitLab Runner executor.

This document does not try to explain how to set up libvirt, since it’s out of scope. However, this driver was tested using GCP Nested Virtualization, which also has details on how to set up libvirt with bridge networking. This example will use the default network that comes with when installing libvirt so make sure it’s running.

This driver requires bridge networking since each VM needs to have it’s own dedicated IP address so GitLab Runner can SSH inside of it to run commands. An SSH key can be generated using the following commands.

A base disk VM image is created so that dependencies are not downloaded every build. In the following example, virt-builder is used to create a disk VM image. 

virt-builder debian-11 \
    --size 8G \
    --output /var/lib/libvirt/images/gitlab-runner-base.qcow2 \
    --format qcow2 \
    --hostname gitlab-runner-bullseye \
    --network \
    --install curl \
    --run-command 'curl -L "https://packages.gitlab.com/install/repositories/runner/gitlab-runner/script.deb.sh" | bash' \
    --run-command 'curl -s "https://packagecloud.io/install/repositories/github/git-lfs/script.deb.sh" | bash' \
    --run-command 'useradd -m -p "" gitlab-runner -s /bin/bash' \
    --install gitlab-runner,git,git-lfs,openssh-server \
    --run-command "git lfs install --skip-repo" \
    --ssh-inject gitlab-runner:file:/root/.ssh/id_rsa.pub \
    --run-command "echo 'gitlab-runner ALL=(ALL) NOPASSWD: ALL' >> /etc/sudoers" \
    --run-command "sed -E 's/GRUB_CMDLINE_LINUX=\"\"/GRUB_CMDLINE_LINUX=\"net.ifnames=0 biosdevname=0\"/' -i /etc/default/grub" \
    --run-command "grub-mkconfig -o /boot/grub/grub.cfg" \
    --run-command "echo 'auto eth0' >> /etc/network/interfaces" \
    --run-command "echo 'allow-hotplug eth0' >> /etc/network/interfaces" \
    --run-command "echo 'iface eth0 inet dhcp' >> /etc/network/interfaces"

The command above will install all the prerequisites specified earlier.

virt-builder will set a root password automatically which is printed at the end. If you want to specify a password yourself, pass --root-password password:$SOME_PASSWORD.

Configuration

The following is an example of a GitLab Runner configuration for libvirt: 

concurrent = 1
check_interval = 0

[session_server]
  session_timeout = 1800

[[runners]]
  name = "libvirt-driver"
  url = "https://gitlab.com/"
  token = "xxxxx"
  executor = "custom"
  builds_dir = "/home/gitlab-runner/builds"
  cache_dir = "/home/gitlab-runner/cache"
  [runners.custom_build_dir]
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]
  [runners.custom]
    prepare_exec = "/opt/libvirt-driver/prepare.sh" # Path to a bash script to create VM.
    run_exec = "/opt/libvirt-driver/run.sh" # Path to a bash script to run script inside of VM over ssh.
    cleanup_exec = "/opt/libvirt-driver/cleanup.sh" # Path to a bash script to delete VM and disks.

Base

Each stage (prepare, run, and cleanup) will use the base script below to generate variables that are used throughout other scripts.

It’s important that this script is located in the same directory as the other scripts, in this case /opt/libivirt-driver/. 

#!/usr/bin/env bash

# /opt/libvirt-driver/base.sh

VM_IMAGES_PATH="/var/lib/libvirt/images"
BASE_VM_IMAGE="$VM_IMAGES_PATH/gitlab-runner-base.qcow2"
VM_ID="runner-$CUSTOM_ENV_CI_RUNNER_ID-project-$CUSTOM_ENV_CI_PROJECT_ID-concurrent-$CUSTOM_ENV_CI_CONCURRENT_PROJECT_ID-job-$CUSTOM_ENV_CI_JOB_ID"
VM_IMAGE="$VM_IMAGES_PATH/$VM_ID.qcow2"

_get_vm_ip() {
    virsh -q domifaddr "$VM_ID" | awk '{print $4}' | sed -E 's|/([0-9]+)?$||'
}

Prepare

The prepare script:

  • Copies the disk to a new path.
  • Installs a new VM from the copied disk.
  • Waits for the VM to get an IP.
  • Waits for SSH to respond on the VM. 
#!/usr/bin/env bash

# /opt/libvirt-driver/prepare.sh

currentDir="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
source ${currentDir}/base.sh # Get variables from base script.

set -eo pipefail

# trap any error, and mark it as a system failure.
trap "exit $SYSTEM_FAILURE_EXIT_CODE" ERR

# Copy base disk to use for Job.
qemu-img create -f qcow2 -b "$BASE_VM_IMAGE" "$VM_IMAGE" -F qcow2

# Install the VM
virt-install \
    --name "$VM_ID" \
    --os-variant debian11 \
    --disk "$VM_IMAGE" \
    --import \
    --vcpus=2 \
    --ram=2048 \
    --network default \
    --graphics none \
    --noautoconsole

# Wait for VM to get IP
echo 'Waiting for VM to get IP'
for i in $(seq 1 30); do
    VM_IP=$(_get_vm_ip)

    if [ -n "$VM_IP" ]; then
        echo "VM got IP: $VM_IP"
        break
    fi

    if [ "$i" == "30" ]; then
        echo 'Waited 30 seconds for VM to start, exiting...'
        # Inform GitLab Runner that this is a system failure, so it
        # should be retried.
        exit "$SYSTEM_FAILURE_EXIT_CODE"
    fi

    sleep 1s
done

# Wait for ssh to become available
echo "Waiting for sshd to be available"
for i in $(seq 1 30); do
    if ssh -i /root/.ssh/id_rsa -o StrictHostKeyChecking=no gitlab-runner@$VM_IP >/dev/null 2>/dev/null; then
        break
    fi

    if [ "$i" == "30" ]; then
        echo 'Waited 30 seconds for sshd to start, exiting...'
        # Inform GitLab Runner that this is a system failure, so it
        # should be retried.
        exit "$SYSTEM_FAILURE_EXIT_CODE"
    fi

    sleep 1s
done

Run

This will run the script generated by GitLab Runner by sending the content of the script to the VM via STDIN through SSH. 

#!/usr/bin/env bash

# /opt/libvirt-driver/run.sh

currentDir="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
source ${currentDir}/base.sh # Get variables from base script.

VM_IP=$(_get_vm_ip)

ssh -i /root/.ssh/id_rsa -o StrictHostKeyChecking=no gitlab-runner@$VM_IP /bin/bash < "${1}"
if [ $? -ne 0 ]; then
    # Exit using the variable, to make the build as failure in GitLab
    # CI.
    exit "$BUILD_FAILURE_EXIT_CODE"
fi

Cleanup

This script removes the VM and deletes the disk. 

#!/usr/bin/env bash

# /opt/libvirt-driver/cleanup.sh

currentDir="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
source ${currentDir}/base.sh # Get variables from base script.

set -eo pipefail

# Destroy VM.
virsh shutdown "$VM_ID"

# Undefine VM.
virsh undefine "$VM_ID"

# Delete VM disk.
if [ -f "$VM_IMAGE" ]; then
    rm "$VM_IMAGE"
fi