Cirrus: Add a readme

Signed-off-by: Chris Evich <cevich@redhat.com>

contrib/cirrus/README.md

@@ -0,0 +1,75 @@
+![PODMAN logo](../../logo/podman-logo-source.svg)
+
+# Cirrus-CI
+
+Similar to other integrated github CI/CD services, Cirrus utilizes a simple
+YAML-based configuration/description file: ``.cirrus.yml``.  Ref: https://cirrus-ci.org/
+
+## Workflow
+
+All tasks execute in parallel, unless there are conditions or dependencies
+which alter this behavior.  Within each task, each script executes in sequence,
+so long as any previous script exited successfully.  The overall state of each
+task (pass or fail) is set based on the exit status of the last script to execute.
+
+### ``full_vm_testing`` Task
+
+1. Unconditionally, spin up one VM per ``matrix: image_name`` item defined
+   in ``.cirrus.yml``.  Once accessible, ``ssh`` into each VM and run the following
+   scripts.
+
+2. ``setup_environment.sh``: Configure root's ``.bash_profile``
+   for all subsequent scripts (each run in a new shell).  Any
+   distribution-specific environment variables are also defined
+   here.  For example, setting tags/flags to use compiling.
+
+3. ``verify_source.sh``: Perform per-distribution source
+   verification, lint-checking, etc.  This acts as a minimal
+   gate, blocking extended use of VMs when a PR's code or commits
+   would otherwise not be accepted.  Should run for less than a minute.
+
+4. ``unit_test.sh``: Execute unit-testing, as defined by the ``Makefile``.
+   This should execute within 10-minutes, but often much faster.
+
+5. ``integration_test.sh``: Execute integration-testing.  This is
+   much more involved, and relies on access to external
+   resources like container images and code from other repositories.
+   Total execution time is capped at 2-hours (includes all the above)
+   but this script normally completes in less than an hour.
+
+### ``build_vm_images`` Task
+
+1. When a PR is merged (``$CIRRUS_BRANCH`` == ``master``), run another
+   round of the ``full_vm_testing`` task (above).
+
+2. After confirming the tests all pass post-merge, spin up a special VM
+   capable of communicating with the GCE API.  Once accessible, ``ssh`` into
+   the special VM and run the following scripts.
+
+3. ``setup_environment.sh``: Configure root's ``.bash_profile``
+   for all subsequent scripts (each run in a new shell).  Any
+   distribution-specific environment variables are also defined
+   here.  For example, setting tags/flags to use compiling.
+
+4. ``build_vm_images.sh``: Examine the merged PR's description on github.
+   If it contains the magic string ``***CIRRUS: REBUILD IMAGES***``, then
+   continue.  Otherwise display a message, take no further action, and
+   exit successfully.  This prevents production of new VM images unless
+   they are called for, thereby saving the cost of needlessly storing them.
+
+5. If the magic string was found, utilize [the packer tool](http://packer.io/docs/)
+   to produce new VM images.  Create a new VM from each base-image, connect
+   to them with ``ssh``, and perform these steps as defined by the
+   ``libpod_images.json`` file.
+
+    1. Copy the current state of the repository into ``/tmp/libpod``.
+    2. Execute distribution-specific scripts to prepare the image for
+       use by the ``full_vm_testing`` task (above).
+    3. If successful, shut down each VM and create a new GCE Image
+       named after the base image and the commit sha of the merge.
+
+***Note:*** The ``.cirrus.yml`` file must be manually updated with the new
+images names, then the change sent in via a secondary pull-request.  This
+ensures that all the ``full_vm_testing`` tasks can pass with the new images,
+before subjecting all future PRs to them.  A workflow to automate this
+process is described in comments at the end of the ``.cirrus.yml`` file.