Update notes

notes/buses.md

@@ -7,3 +7,6 @@ DBus - https://www.freedesktop.org/wiki/Software/dbus/
 ubus (openwrt) - https://oldwiki.archive.openwrt.org/doc/techref/ubus
  - ACLs
 
+plumber (plan9!) - http://man.cat-v.org/plan_9/4/plumber
+ - data format "plumb" http://man.cat-v.org/plan_9/6/plumb
+ - autostarts listeners on request

notes/control-commands.md

@@ -0,0 +1,93 @@
+---
+title: 'System control commands'
+---
+
+## systemctl
+
+Commands generally accept patterns to select subsets of the set of all
+units.
+
+ - list-units
+ - list-sockets
+ - list-timers
+ - is-active (predicate over units)
+ - is-failed
+ - status
+ - show
+ - cat (introspection on source files for units)
+ - help (this is interesting!)
+ - list-dependencies
+
+ - start
+ - stop
+ - reload ("please reload your configuration" - even though we are
+   mostly reactive, most services won't be, and sometimes config in
+   files exists etc)
+ - restart (stop (if running) and then start (unconditionally))
+ - try-restart (if running, stop and then start)
+ - reload-or-restart, try-reload-or-restart
+ - isolate (effectively set a new "supergoal", removing all others. nb `AllowIsolate` flag)
+ - kill
+ - clean (remove various cache/state/config/log data)
+ - freeze, thaw
+ - set-property (for us, change a config .pr file; no --runtime equivalent, though)
+ - reset-failed (resets the state variables of the supervisor)
+ - (several others)
+
+Enable vs start.
+
+Enable: `--system`, `--user`, `--runtime`, `--global`: various scopes for enabling a unit.
+
+Container management.
+
+Job management. (It has a job queue!)
+
+Environment management for spawned services.
+
+`daemon-reexec`, serializes state for an upgrade
+
+System lifecycle: startup -> running -> (shutdown | (maintenance -?-> running))
+
+At shutdown or sleep, "inhibitor locks" exist. The action can respect
+or ignore these. Interactive shutdown/sleep requests default to
+respecting; non-interactive, to ignoring.
+
+`isolate` targets:
+ - default
+ - rescue
+ - emergency
+ - halt
+ - poweroff
+ - reboot
+ - suspend
+ - hibernate
+ - hybrid-sleep
+ - suspend-then-hibernate
+
+`switch-root`
+
+## Service unit configuration
+
+https://www.freedesktop.org/software/systemd/man/systemd.service.html
+
+Service templates: can take a single argument to become instantiated.
+
+If a service unit config file doesn't exist, falls back to looking for
+an init script and synthesizes a unit on the fly.
+
+Default dependencies:
+ - requires sysinit.target
+ - starts after sysinit.target
+ - starts after basic.target
+ - conflicts with shutdown.target
+ - shuts down before shutdown.target (?)
+
+"slice units"? Oh ok, hierarchical supervision... and configuration of
+whole branches of the tree.
+
+Service lifecycle:
+ - states: active, activating, deactivating, dead, inactive, starting, ... more?
+
+Options:
+
+ - Type (it's complicated)

notes/init.md

@@ -77,3 +77,15 @@ and each service is aware of what it needs at the moment it starts.
 Services: are Syndicate programs that speak the protocol on stdio. To
 run other things, a supervisor program will listen to Syndicate and
 run a subprocess in an ad-hoc way.
+
+# pid-1 concerns
+
+PID-1 concerns:
+ - sessions https://man7.org/linux/man-pages/man2/setsid.2.html
+ - handling of SIGINT and SIGTERM is special
+    - (and how should SIGSTOP / SIGTSTP/SIGTTIN/SIGTTOU work?? there's
+      some detail here https://github.com/Yelp/dumb-init I don't
+      understand)
+    - the container case is different from the host case
+ - "pivoting" to shutdown mode
+ - reaping of orphaned processes

notes/logging.md

@@ -0,0 +1,18 @@
+---
+---
+
+from `ioctl_tty(2)`:
+
+       Redirecting console output
+           TIOCCONS
+                  Argument: void
+
+                  Redirect output that would have gone to /dev/console or
+                  /dev/tty0 to the given terminal.  If that was a
+                  pseudoterminal master, send it to the slave.  In Linux
+                  before version 2.6.10, anybody can do this as long as the
+                  output was not redirected yet; since version 2.6.10, only
+                  a process with the CAP_SYS_ADMIN capability may do this.
+                  If output was redirected already, then EBUSY is returned,
+                  but redirection can be stopped by using this ioctl with fd
+                  pointing at /dev/console or /dev/tty0.

notes/process-supervision.md

@@ -105,3 +105,12 @@ Logging in daemontools:
  - stdout is piped to stdin of the logger program
 
 runit - see https://docs.voidlinux.org/config/services/index.html
+
+2021-08-24 I asked on IRC: "I have a question about Erlang history, I
+wonder if any of the old timers are here. I want to know how and when
+supervisor.erl began its life. Joe's HOPL paper mentions "BOS" as a
+source of inspiration, but I want to know more..."
+
+> 15:03:25 < okeuday> tonyg: OTP behaviors were attributed to Lennart
+> Öhman (working at Sjöland & Thyselius Telecom AB) in the past, but
+> there are likely more details involved