299 lines
12 KiB
Plaintext
299 lines
12 KiB
Plaintext
A distro which already uses runit
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I installed Void Linux, in order to see what do they have.
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Xfce desktop looks fairly okay, network is up.
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ps tells me they did put X, dbus, NM and udev into runsvdir-supervised tree:
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1 ? 00:00:01 runit
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623 ? 00:00:00 runsvdir
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629 ? 00:00:00 runsv
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650 tty1 00:00:00 agetty
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630 ? 00:00:00 runsv
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644 ? 00:00:09 NetworkManager
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1737 ? 00:00:00 dhclient
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631 ? 00:00:00 runsv
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639 tty4 00:00:00 agetty
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632 ? 00:00:00 runsv
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640 ? 00:00:00 sshd
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1804 ? 00:00:00 sshd
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1809 pts/3 00:00:00 sh
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1818 pts/3 00:00:00 ps
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633 ? 00:00:00 runsv
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637 tty5 00:00:00 agetty
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634 ? 00:00:00 runsv
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796 ? 00:00:00 dhclient
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635 ? 00:00:00 runsv
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649 ? 00:00:00 uuidd
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636 ? 00:00:00 runsv
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647 ? 00:00:00 acpid
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638 ? 00:00:00 runsv
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652 ? 00:00:00 console-kit-dae
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641 ? 00:00:00 runsv
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651 tty6 00:00:00 agetty
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642 ? 00:00:00 runsv
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660 tty2 00:00:00 agetty
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643 ? 00:00:00 runsv
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657 ? 00:00:02 dbus-daemon
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645 ? 00:00:00 runsv
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658 ? 00:00:00 cgmanager
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648 ? 00:00:00 runsv
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656 tty3 00:00:00 agetty
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653 ? 00:00:00 runsv
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655 ? 00:00:00 lxdm-binary
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698 tty7 00:00:14 Xorg
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729 ? 00:00:00 lxdm-session
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956 ? 00:00:00 sh
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982 ? 00:00:00 xfce4-session
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1006 ? 00:00:04 nm-applet
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654 ? 00:00:00 runsv
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659 ? 00:00:00 udevd
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Here is a link to Void Linux's wiki:
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https://wiki.voidlinux.eu/Runit
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Void Linux packages install their services as subdirectories of /etc/rc,
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such as /etc/sv/sshd, with a script file, "run", and a link
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"supervise" -> /run/runit/supervise.sshd
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For sshd, "run" contains:
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#!/bin/sh
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ssh-keygen -A >/dev/null 2>&1 # generate host keys if they don't exist
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[ -r conf ] && . ./conf
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exec /usr/bin/sshd -D $OPTS
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That's it from the POV of the packager.
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This is pretty minimalistic, and yet, it is already distro-specific:
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the link to /run/runit/* is conceptually wrong, it requires packagers
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to know that /etc/rc should not be mutable and thus they need to use
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a different location in filesystem for supervise/ directory.
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I think a good thing would be to require just one file: the "run" script.
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The rest should be handled by distro tooling, not by packager.
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A similar issue is arising with logging. It would be ideal if packagers
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would not need to know how a particular distro manages logs.
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Whatever their daemons print to stdout/stderr, should be automagically logged
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in a way distro prefers.
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* * * * * * * *
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Proposed "standard" on how distros should use runit
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The original idea of services-as-directories belongs to D.J.Bernstein (djb),
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and his project to implement it is daemontools: https://cr.yp.to/daemontools.html
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There are several reimplementations of daemontools:
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- runit: by Gerrit Pape, http://smarden.org/runit/
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(busybox has it included)
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- s6: by Laurent Bercot, http://skarnet.org/software/s6/
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It is not required that a specific clone should be used. Let evolution work.
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Terminology
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daemon: any long running background program. Common examples are sshd, getty,
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ntpd, dhcp client...
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service: daemon controlled by a service monitor.
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service directory: a directory with an executable file (script) named "run"
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which (usually) execs some daemon, possibly after some preparatory steps.
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It should start it not as a child or daemonized process, but by exec'ing it
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(inheriting the same PID and the place in the process tree).
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service monitor: a tool which watches a set of service directories.
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In daemontools package, it is called "svscan". In runit, it is called
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"runsvdir". In s6, it is called "s6-svscan".
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Service monitor starts a supervisor for each service directory.
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If it dies, it restarts it. If service directory disappears,
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service monitor will not be restarted if it dies.
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runit's service monitor (runsvdir) sends SIGTERM to supervisors
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whose directories disappeared.
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supervisor: a tool which monitors one service directory.
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It runs "run" script as its child. It restarts it if it dies.
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It can be instructed to start/stop/signal its child.
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In daemontools package, it is called "supervise". In runit, it is called
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"runsv". In s6, it is called "s6-supervise".
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Conceptually, a daemontools clone can be designed such that it does not *have*
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the supervisor component: service monitor can directly monitor all its daemons
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(for example, this may be a good idea for memory-constrained systems).
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However all three existing projects (daemontools/runit/s6) do have a per-service
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supervisor process.
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log service: a service which is exclusively tasked with logging
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the output of another service. It is implemented as log/ subdirectory
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in a service directory. It has the same structure as "normal"
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service dirs: it has a "run" script which starts a logging tool.
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If log service exists, stdout of its "main" service is piped
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to log service. Stops/restarts of either of them do not sever the pipe
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between them.
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If log service exists, daemontools and s6 run a pair of supervisors
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(one for the daemon, one for the logger); runit runs only one supervisor
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per service, which is handling both of them (presumably this is done
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to use fewer processes and thus, fewer resources).
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User API
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"Users" of service monitoring are authors of software which has daemons.
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They need to package their daemons to be installed as services at package
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install time. And they need to do this for many distros.
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The less distros diverge, the easier users' lives are.
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System-wide service dirs reside in a distro-specific location.
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The recommended location is /var/service. (However, since it is not
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a mandatory location, avoid depending on it in your run scripts.
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Void Linux wanted to have it somewhere in /run/*, and they solved this
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by making /var/service a symlink).
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The install location for service dirs is /etc/rc:
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when e.g. ntpd daemon is installed, it creates the /etc/rc/ntpd
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directory with (minimally) one executable file (script) named "run"
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which starts ntpd daemon. It can have other files there.
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At boot, distro should copy /etc/rc/* to a suitable writable
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directory (common choice are /var/service, /run/service etc).
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It should create log/ directories in each subdirectory
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and create "run" files in them with suitable (for this particular distro)
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logging tool invocation, unless this directory chose to channel
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all logging from all daemons through service monitor process
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and log all of them into one file/database/whatever,
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in which case log/ directories should not be created.
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It is allowable for a distro to directly use /etc/rc/ as the only
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location of its service directories. (For example,
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/var/service may be a symlink to /etc/rc).
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However, it poses some problems:
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(1) Supervision tools will need to write to subdirectories:
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the control of running daemons is implemented via some files and fifos
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in automatically created supervise/ subdirectory in each /etc/rc/DIR.
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(2) Creation of a new service can race with the rescanning of /etc/rc/
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by service monitor: service monitor may see a directory with only some files
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present. If it attempts to start the service in this state, all sorts
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of bad things may happen. This may be worked around by various
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heuristics in service monitor which give new service a few seconds
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of "grace time" to be fully populated; but this is not yet
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implemented in any of three packages.
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This also may be worked around by creating a .dotdir (a directory
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whose name starts with a dot), populating it, and then renaming;
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but packaging tools usually do not have an option to do this
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automatically - additional install scripting in packages will be needed.
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Daemons' output file descriptors are handled somewhat awkwardly
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by various daemontools implementations. For example, for runit tools,
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daemons' stdout goes to wherever runsvdir's stdout was directed;
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stderr goes to runsvdir, which in turn "rotates" it on its command line
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(which is visible in ps output).
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Hopefully this get changed/standardized; while it is not, the "run" file
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should start with a
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exec 2>&1
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command, making stderr equivalent to stdout.
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An especially primitive service which does not want its output to be logged
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with standard tools can do
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exec >LOGFILE 2>&1
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or even
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exec >/dev/null 2>&1
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To prevent creation of distro-specific log/ directory, a service directory
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in /etc/rc can contain an empty "log" file.
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Controlling daemons
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The "svc" tool is available for admins and scripts to control services.
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In particular, often one service needs to control another:
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e.g. ifplugd can detect that the network cable was just plugged in,
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and it needs to (re)start DHCP service for this network device.
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The name of this tool is not standard either, which is an obvious problem.
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I propose to fix this by implementing a tool with fixed name and API by all
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daemontools clones. Lets use original daemontools name and API. Thus:
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The following form must work:
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svc -udopchaitkx DIR
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Options map to up/down/once/STOP/CONT/HUP/ALRM/INT/TERM/KILL/exit
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commands to the daemon being controlled.
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The form with one option letter must work. If multiple-option form
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is supported, there is no guarantee in which order they take effect:
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svc -it DIR can deliver TERM and INT in any order.
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If more than one DIR can be specified (which is not a requirement),
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there is no guarantee in which order commands are sent to them.
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If DIR has no slash and is not "." or "..", it is assumed to be
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relative to the system-wide service directory.
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[Currently, "svc" exists only in daemontools and in busybox.
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This proposal asks developers of other daemontools implementations
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to add "svc" command to their projects]
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The "svok DIR" tool exits 0 if service is running, and nonzero if not.
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Other tools with different names and APIs may exist; however
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for portability scripts should use the above tools.
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Creation of a new service on a running system should be done atomically.
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To this end, first create and populate a new /etc/rc/DIR.
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Then "activate" it by running ??????? - this copies (or symlinks,
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depending on the distro) its files to the "live" service directory,
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wherever it is located on this distro.
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Removal of the service should be done as follows:
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svc -d DIR [DIR/log], then remove the service directory:
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this makes service monitor SIGTERM per-directory supervisors
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(if they exist in the implementation).
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Implementation details
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Top-level service monitor program name is not standardized
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[svscan, runsvdir, s6-svscan ...] - it does not need to be,
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as far as daemon packagers are concerned.
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It may run one per-directory supervisor, or two supervisors
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(one for DIR/ and one for DIR/log/); for memory-constrained systems
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an implementation is possible which itself controls all services, without
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intermediate supervisors.
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[runsvdir runs one "runsv DIR" per DIR, runsv handles DIR/log/ if that exists]
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[svscan runs a pair of "supervise DIR" and "supervise DIR/log"]
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Directories are remembered by device+inode numbers, not names. Renaming a directory
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does not affect the running service (unless it is renamed to a .dotdir).
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Removal (or .dotdiring) of a directory sends SIGTERM to any running services.
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Standard output of non-logged services goes to standard output of service monitor.
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Standard output of logger services goes to standard output of service monitor.
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Standard error of them always goes to standard error of service monitor.
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If you want to log standard error of your logged service along with its stdout, use
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"exec 2>&1" in the beginning of your "run" script.
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Whether stdout/stderr of service monitor is discarded (>/dev/null)
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or logged in some way is system-dependent.
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Containers
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[What do containers need?]
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