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Building bits of XenServer outside of Citrix has in the past been a bit of a challenging task, requiring careful construction of the build environment to replicate what 'XenBuilder', our internal build system, puts together. This has meant using custom DDK VMs or carefully installing by hand a set of packages taken from one of the XenServer ISOs. With XenServer Dundee, this will be a pain of the past, and making a build environment will be just a 'docker run' away.
Part of the work that's being done for XenServer Dundee has been moving things over to using standard build tools and packaging. In previous releases there have been a mix of RPMs, tarballs and patches for existing files, but for the Dundee project everything installed into dom0 is now packaged into an RPM. Taking inspiration and knowledge gained while working on xenserver/buildroot, we're building most of these dom0 packages now using mock. Mock is a standard tool for building RPM packages from source RPMs (SRPMS), and it works by constructing a completely clean chroot with only the dependencies defined by the SRPM. This means that everything needed to build a package must be in an RPM, and the dependencies defined by the SRPM must be correct too.
From the point of view of making reliably reproducible builds, using mock means there is very little possibility of having the build dependent upon the the environment. But there is also a side benefit of this work: If you actually want to rebuild a bit of XenServer you just need to have a yum repository with the XenServer RPMs in, and use 'yum-builddep' to put in place all of the build dependencies, and then building should be as simple as cloning the repository and typing 'make'.
The simplest place to do this would be in the dom0 environment itself, particularly now that the partition size has been bumped up to 20 gigs or so. However, that may well not be the most convenient. In fact, for a use case like this, the mighty Docker provides a perfect solution. Docker can quickly pull down a standard CentOS environment and then put in the reference to the XenServer yum repository, install gcc, OCaml, git, emacs and generally prepare the perfect build environment for development.
In fact, even better, Docker will actually do all of these bits for you! The docker hub has a facility for automatically building a Docker image provided everything required is in repository on Github. So we've prepared a repository containing a Dockerfile and associated gubbins that sets things up as above, and then the docker hub builds and hosts the resulting docker image.
Let's dive in with an example on how to use this. Say you have a desire to change some aspect of how networking works on XenServer, something that requires a change to the networking daemon itself, 'xcp-networkd'. We'll start by rebuilding that from the source RPM. Start the docker container and install the build dependencies:
$ docker run -i -t xenserver/xenserver-build-env [root@15729a23550b /]# yum-builddep -y xcp-networkd
this will now download and install everything required to be able to build the network daemon. Next, let's just download and build the SRPM:
[root@15729a23550b /]# yumdownloader --source xcp-networkd
At time of writing, this downloads the SRPM "xcp-networkd-0.9.6-1+s0+0.10.0+8+g96c3fcc.el7.centos.src.rpm". This will build correctly in our environment:
[root@15729a23550b /]# rpmbuild --rebuild xcp-networkd-* ... [root@15729a23550b /]# ls -l ~/rpmbuild/RPMS/x86_64/ total 2488 -rw-r--r-- 1 root root 1938536 Jan 7 11:15 xcp-networkd-0.9.6-1+s0+0.10.0+8+g96c3fcc.el7.centos.x86_64.rpm -rw-r--r-- 1 root root 604440 Jan 7 11:15 xcp-networkd-debuginfo-0.9.6-1+s0+0.10.0+8+g96c3fcc.el7.centos.x86_64.rpm
Alternatively, you can compile straight from the source. Most of our software is hosted on github, either under the xapi-project or xenserver organisations. xcp-networkd is a xapi-project repository, so we can clone it from there:
[root@15729a23550b /]# cd ~ [root@15729a23550b ~]# git clone git://github.com/xapi-project/xcp-networkd
Most of our RPMs have version numbers constructed automatically containing useful information about the source, and where the source is from git repositories the version information comes from 'git describe'.
[root@15729a23550b ~]# cd xcp-networkd [root@15729a23550b xcp-networkd]# git describe --tags v0.10.0-8-g96c3fcc
The important part here is the hash, in this case '96c3fcc'. Comparing with the SRPM version, we can see these are identical. We can now just type 'make' to build the binaries:
[root@15729a23550b xcp-networkd]# make
this networkd binary can then be put onto your XenServer and run.
The yum repository used by the container is being created directly from the snapshot ISOs uploaded to xenserver.org, using a simple bash script named update_xs_yum.sh available on github. The container default will be to use the most recently available release, but the script can be used by anyone to generate a repository from the daily snapshots too, if this is required. There's still a way to go before Dundee is released, and some aspect of this workflow are in flux – for example, the RPMs aren't currently signed. However, by the time Dundee is out the door we hope to make many improvements in this area. Certainly here in Citrix, many of us have switched to using this for our day-to-day build needs, because it's simply far more convenient than our old custom chroot generation mechanism.
There's still a way to go before Dundee is released, and some aspects of this still need some work before it's all totally seamless, for example the RPMs themselves are not currently signed. However, by the time Dundee is out the door we hope to make many improvements in this area. Certainly here in Citrix, many of us have switched to using this for our day-to-day build needs, because it's simply far more convenient than our old custom chroot generation mechanism.