Add libvirt Custom executor example

reference https://gitlab.com/gitlab-org/gitlab-runner/issues/4257

2 files changed, 235 insertions, 0 deletions

diff --git a/docs/executors/custom_examples/index.md b/docs/executors/custom_examples/index.md
index dd2aeb60..d61e30c8 100644
--- a/docs/executors/custom_examples/index.md
+++ b/docs/executors/custom_examples/index.md
@@ -4,3 +4,4 @@ The following are examples of using the [Custom executor](../custom.md)
 for environments not supported natively by GitLab Runner:
 
 - [LXD](lxd.md)
+- [libvirt](libvirt.md)
diff --git a/docs/executors/custom_examples/libvirt.md b/docs/executors/custom_examples/libvirt.md
new file mode 100644
index 00000000..55cb11ed
--- /dev/null
+++ b/docs/executors/custom_examples/libvirt.md
@@ -0,0 +1,234 @@
+# Using libvirt with the Custom executor
+
+Using [libvirt](https://libvirt.org/), the Custom executor 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](https://gitlab.com/gitlab-org/gitlab-runner/merge_requests/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 executor was tested using [GCP Nested
+Virtualization](https://cloud.google.com/compute/docs/instances/enable-nested-virtualization-vm-instances),
+which also has [details on how to setup
+libvirt](https://cloud.google.com/compute/docs/instances/enable-nested-virtualization-vm-instances#starting_a_private_bridge_between_the_host_and_nested_vms)
+with bridge networking. This example will use the `default` network that
+comes with when installing libvirt so make sure it's running.
+
+This executor 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](https://docs.gitlab.com/ee/ssh/#generating-a-new-ssh-key-pair).
+
+A base disk VM image is created so that dependencies are not downloaded
+every build. In the following example,
+[virt-builder](http://libguestfs.org/virt-builder.1.html) is used to
+create a disk VM image.
+
+```sh
+virt-builder debian-9 \
+    --size 8G \
+    --output /var/lib/libvirt/images/gitlab-runner-base.qcow2 \
+    --format qcow2 \
+    --hostname gitlab-runner-stretch \
+    --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](../custom.md#prerequisite-software-for-running-a-job) 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:$SOME_PASSWORD`](http://libguestfs.org/virt-builder.1.html#setting-the-root-password).
+
+## Configuration
+
+The following is an example of Runner configuration for libvirt:
+
+```toml
+concurrent = 1
+check_interval = 0
+
+[session_server]
+  session_timeout = 1800
+
+[[runners]]
+  name = "libvirt-executor"
+  url = "https://gitlab.com/"
+  token = "xxxxx"
+  executor = "custom"
+  builds_dir = "/home/gitlab-runner/builds"
+  cache_dir = "/home/gitlab-runner/cache"
+  [runners.custom_build_dir]
+  [runners.cache]
+    [runners.cache.s3]
+    [runners.cache.gcs]
+  [runners.custom]
+    prepare_exec = "/opt/libvirt-executor/prepare.sh" # Path to a bash script to create VM.
+    run_exec = "/opt/libvirt-executor/run.sh" # Path to a bash script to run script inside of VM over ssh.
+    cleanup_exec = "/opt/libvirt-executor/cleanup.sh" # Path to a bash script to delete VM and disks.
+```
+
+## Base
+
+Each stage ([prepare](#prepare), [run](#run), and [cleanup](#cleanup))
+will use the base script below to generate variables that are used
+throughout other scripts.
+
+It's important that this script is located in the same directory as the
+other scripts, in this case `/opt/libivirt-executor/`.
+
+```sh
+#!/usr/bin/env bash
+
+# /opt/libivrt-executor/base.sh
+
+VM_IMAGES_PATH="/var/lib/libvirt/images"
+BASE_VM_IMAGE="$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.
+
+```sh
+#!/usr/bin/env bash
+
+# /opt/libivrt-executor/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 $SYSTEM_FAILURE_EXIT_CODE" ERR
+
+# Copy base disk to use for Job.
+cp "$BASE_VM_IMAGE" "$VM_IMAGE"
+
+# Install the VM
+virt-install \
+    --name "$VM_ID" \
+    --os-variant debian9 \
+    --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.
+
+```sh
+#!/usr/bin/env bash
+
+# /opt/libivrt-executor/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.
+
+```sh
+#!/usr/bin/env bash
+
+# /opt/libivrt-executor/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
+```