// file      : doc/manual.cli
// copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
// license   : MIT; see accompanying LICENSE file

"\name=build2-buildos-manual"
"\subject=buildos"
"\title=Operating System"

// NOTES
//
// - Maximum <pre> line is 70 characters.
//

"
\h0#preface|Preface|

This document describes \c{buildos}, the \c{build2} operating system.

\h1#intro|Introduction|

\c{buildos} is a Debian GNU/Linux-based in-memory network-booted operating
system specialized for autonomous building of software using the \c{build2}
toolchain. It's primary purpose is to run the \c{build2} build bot (\c{bbot}),
build slave (\c{bslave}), or both.

A machine that run an instance of \c{buildos} is called a \i{build host}. A
build host runs the \c{bbot} and/or \c{bslave} in the \i{agent mode}. The
actual building is performed in the virtual machines and/or containers. For
\c{bbot} it is normally one-shot virtual machines and for \c{bslave} it is
normally containers but can also be long-running virtual machines. Inside
virtual machines/containers, \c{bbot} and \c{bslave} run in the \i{worker
mode} and receive \i{build tasks} from their respective agents.

\h1#arch|Architecture|

Build OS root filesystem (\c{rootfs}) resides entirely in RAM with all changes
(such as installation of the \c{build2} toolchain} discarded on the next
reboot. A small amount of persistent (but not precious) state is stored in
\c{/state} (see \l{#config-storage-state State}).

After booting the kernel, the Build OS execution starts with a custom \c{init}
script which performs low-level configuration and setup and then hands off the
initialization to \c{systemd}. At the end of \c{systemd} startup the Build OS
monitor script (\c{buildos}) is started as a \c{systemd} service. On startup
this script bootstraps the \c{build2} toolchain, builds the \c{bbot} package,
and starts it (as another \c{systemd} service) in the agent mode. After that
the monitor starts monitoring for OS and toolchain changes. If either is
detected, the host is rebooted, which will trigger both booting the latest
OS and building of the latest \c{build2} toolchain and \c{bbot}.

@@ TODO: init steps.

The monitor service (and \c{bbot} that it starts) are executed as the
\c{build} user/group. The home directory of this user is \c{/build}. It has
the following subdirectories:

\ul|

\li|\n\c{/build/tftp}\n

  A size-limited \c{tmpfs} filesystem that is used to communicate with build
  machines as well as for build host log access.

  This directory is read-accessible via the TFTP server running on the default
  port. A \c{bbot} agent also makes sub-directories inside this directory
  temporarily write-accessible to build machines by running custom instances
  of the TFTP server on other ports.|

\li|\n\c{/build/machines}\n

  Contains virtual machines and containers. See \l{#config-storage-machines
  Machines} for details.||


\h1#boot|Booting|

\c{buildos} is normally booted from the network using PXE but can also be
booted locally from the kernel image and initrd directly.

\h#boot-reboot|Reboot|

Build OS can detect when the OS build has been updated and automatically
reboot the build host. This is achieved by polling the URL specified
with the \c{buildos.buildid_url} kernel command line parameter. It should
point to the \c{buildos-buildid} file that comes along the kernel image
and initrd. See \l{#boot-net Network Boot} for the usage example.

\h#boot-net|Network Boot|

Here we assume that you have already established your PXE setup using
PXELINUX. That is, you have configured a TFTP server that hosts the
\c{pxelinux} initial bootstrap program (NBP) and configured a DHCP
server to direct PXE client to this server/NBP.

To setup PXE boot of \c{buildos}, perform the following steps:

\ol|

\li|Copy the Build OS \c{-image}, \c{-initrd}, and \c{-buildid} files to the
TFTP server. For example:

\
# mkdir -p /var/lib/tftpboot/buildos
# cp buildos-image buildos-initrd buildos-buildid \
  /var/lib/tftpboot/buildos/
\

|

\li|Assuming the host MAC address is \c{de:ad:be:ef:b8:da}, create a
    host-specific configuration file (or use \c{default} as the last path
    component for a configuration that applies to all hosts):

\
# cat <<EOF >/var/lib/tftpboot/pxelinux.cfg/01-de-ad-be-ef-b8-da
default buildos
prompt 1
timeout 50

label buildos
  menu label buildos
  kernel /buildos/buildos-image
  initrd /buildos/buildos-initrd
  append buildos.smtp_relay=example.org buildos.admin_email=admin@example.org buildos.buildid_url=tftp://<tftp-host>/buildos/buildos-buildid
EOF
\

Where \c{<tftp-host>} in \c{buildid_url} is the address of the TFTP server
(the same address as returned by the DHCP server to PXE clients). Note that
all the parameters in \c{append} must be specified on a single line.|

\li|You can test the setup using QEMU/KVM, for example:

\
$ sudo kvm \
  -m 8G \
  -netdev tap,id=net0,script=./qemu-ifup \
  -device e1000,netdev=net0,mac=de:ad:be:ef:b8:da \
  -boot n
\

||

\h#boot-local|Local Boot|

During testing it is often useful to boot \c{buildos} directly from the
kernel image and initrd files. As an example, here is how this can be done
using QEMU/KVM:

\
sudo kvm \
  -m 8G \
  -netdev tap,id=net0,script=./qemu-ifup \
  -device e1000,netdev=net0,mac=de:ad:be:ef:b8:da \
  -kernel buildos-image -initrd buildos-initrd
\

\h1#config|Configuration|

\h#config-storage|Storage|

Build OS configures storage based on the labels assigned to disks and
partitions (collectively refered to as disks from now on). Build OS requires
storage for state as well as virtual machines and containers.

\h2#config-storage-state|State|

Build OS stores a small amount of state on a disk labeled \c{buildos.state}
(mounted as \c{/state}). This includes random number generator state, SSH
server host keys, and so on. While this state is persistent, it is not
precious.

The stored state is fairly small (hundreds of megabytes) and is not
performance-critical. While one can create a small state partition on the same
physical disk as used for machines (see below), having it on a separate disk
makes it easier to move machine disks around. Based on these requirements, a
small USB flash drive or flash card is a good option.

While any suitable filesystem can be used, \c{ext4} is a good choice, with
journaling disabled if used on a flash drive/card. For example:

\
mkfs.ext4 -L buildos.machines -O ^has_journal /dev/sdX
\

\h2#config-storage-machines|Machines|

For virtual machine and container storage we can use a single disk, in which
case it should be labeled \c{buildos.machines} or multiple disks, in which
case they should be labeled \c{buildos.machines.<volume>}. In both cases the
disks must be formatted as \c{btrfs}.

In a single disk configuration, the disk is mounted as \c{/build/machines}. In
a multi-disk configuration, each disk is mounted as
\c{/build/machines/<volume>}.

If no disks are found for required storage, then the boot process is
interrupted with a shell prompt where you can format and/or lable a suitable
disk. You can also view the storage configuration on a booted Build OS
instance by examining \c{/etc/fstab}.

As an example, let's consider the first boot of a clean machine that has an
SSD disk as \c{/dev/sda} and which we would like to use for virtual machine
storage. We would also like to over-provision this SSD by 10% to (potentially)
prolong its life and increase performance (you may want to skip this step if
you are using a datacenter-grade SSD that would normally already be generously
over-provisioned).

On the first boot we will be presented with a shell prompt which we use to
over-provision the disk:

\
# fdisk -l /dev/sda            # Query disk information.
# hdparm -N /dev/sda           # Query disk/host protection area sizes.
# hdparm -Np<COUNT> /dev/sda   # COUNT = sector count * 0.9
# hdparm -N /dev/sda           # Verify disk/host protection area sizes.
# ^D                           # Exit shell and reboot.
\

Note that this may not always work, depending on the disk controller used. An
alternative approach is to use the \c{mkfs.btrfs --byte-count} option when
formatting the disk to leave some disk space untouched and unused.

After the reboot we will be presented with a shell prompt again where we
confirm over-provisioning, format the disk as \c{btrfs}, and label it as
\c{buildos.machines}:

\
# fdisk -l /dev/sda            # Confirm disk size decreased by 10%.
# mkfs.btrfs -L buildos.machines -m single /dev/sda
# ^D                           # Exit shell and reboot.
\

\h#config-net|Network|

Network is configured via DHCP. Initially, all Ethernet interfaces that have
carrier are tried in (some) order and the first interface that is successfully
configured via DHCP is used.

Hostname is configured from the DHCP information. Failed that, a name is
generated based on the MAC address, in the form \c{build-xxxxxxxxxx}.
@@ Maybe also kernel cmdline?

Based on the discovery of the Ethernet interface, two bridge interfaces are
configured: \c{br0} is a public bridge that includes the Ethernet interface
and is configured via DHCP. \c{br1} is a private interface with NAT to \c{br0}
with \c{dnsmasq} configured as a DHCP on this interface.

Normally, \c{br0} is used for \c{bslave} virtual machines/container (since
they may need to be accessed directly) and \c{br1} \- for \c{bbot} virtual
machines. You can view the bridge configuration on a booted \c{buildos}
instance by examining \c{/etc/network/interfaces}.

@@ TODO: private network parameters.

\h#config-email|Email|

A \c{buildos} instance sends various notifications (including all messages to
\c{root}) to the admin email address. The admin email is specified with
the \c{buildos.admin_email} kernel command line parameter.

In order to deliver mail, the \c{postfix} MTA is configured to forward to a
relay. The relay host is specified with the \c{buildos.smtp_relay} kernel
command line parameter.

Note that no authentication of any kind is configured for relaying. This means
that the relay host should accept emails from build hosts either because of
their network location (for example, because they are on your organization's
local network and you are using your organization's relay) or because the
relay host accepts emails send to the admin address from anyone (which is
normally the case if the relay is the final destination for the admin
address, for example, \c{example.org} and \c{admin@example.org}).


\h#config-ssh|SSH|

Build OS runs an OpenSSH server with password authentication disabled. As a
result, the only way to login remotely is via a public key. To add a public
key into the \c{root} user's \c{authorized_keys} file we can use the
\c{buildos.ssh_key} kernel command line parameter. For example (note the
quotes):

\
buildos.ssh_key=\"ssh-rsa AAA...OA0DB user@host\"
\
"