2022-02-11 09:48:49 +00:00
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# System overview
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Synit uses the Linux kernel as a hardware abstraction and virtualisation layer.
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All processes in the system are arranged into a [supervision
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tree](../glossary.md#supervision-tree), conceptually rooted at the [system
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bus](./system-bus.md) (NB. not at PID 1).
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2022-02-11 12:43:39 +00:00
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```ditaa system-supervision-tree
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(Example)
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+----------------------------------+
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|Root System Bus (syndicate–server)|
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+----------------+-----------------+
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+--------+--------+---------+----------+---------------+
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| | | | | |
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+--+--+ +---+---+ +--+--+ +----+----+ +---+---+ +-----+-----+
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|init | |console| |udevd| |Network | |Wifi | |Session bus|
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+-----+ |getty | +-----+ |Interface| |Daemon | ... +-----+-----+
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+-------+ |Monitor | |(wlan0)| |
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|Daemon | +-------+ |
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+---------+ |
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+----------+----------------------------+
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+---+---+ +--+--+ +----+---+
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|Browser| |Email| . . . |X Server|
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+-------+ +-----+ +--------+
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```
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Here's an example of `ps` output from a Synit prototype running on a mobile phone:
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2022-02-11 09:48:49 +00:00
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PID TTY STAT TIME COMMAND
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1 ? Ssl 0:00 /sbin/synit-pid1
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1034 ? Sl 0:00 /usr/bin/syndicate-server --inferior --config /etc/syndicate/boot
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1045 ttyS0 Ss+ 0:00 \_ getty 0 /dev/console
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1052 ? S 0:01 \_ /sbin/udevd --children-max=5
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1113 ? Sl 0:11 \_ python3 /usr/lib/synit/interface-monitor
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1129 ? S 0:00 \_ sshd: /usr/sbin/sshd -D [listener] 0 of 10-100 startups
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1286 ? S 0:00 \_ wpa_supplicant -Dnl80211,wext -C/run/wpa_supplicant -i wlan0
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1287 ? S 0:00 \_ udhcpc -i usb0 -fR -s /usr/lib/synit/udhcpc.script
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1390 ? S 0:00 \_ ntpd -d -n -p pool.ntp.org
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1431 ? S 0:01 \_ python3 /usr/lib/synit/user-settings-daemon
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1497 ? Sl 0:01 \_ python3 /usr/lib/synit/wifi-daemon
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1516 ? S 0:00 \_ udhcpc -i wlan0 -fR -s /usr/lib/synit/udhcpc.script
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1035 ? S 0:00 s6-log t s999999 n500 /var/log/synit
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## Boot process
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The kernel first loads the stock PostmarketOS `initrd`, which performs a number of important
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tasks and then delegates to `/sbin/init`.
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### /sbin/init = synit-init.sh
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- Source code: [`[synit]/packaging/packages/synit-config/files/sbin/synit-init.sh`](https://git.syndicate-lang.org/synit/synit/src/branch/main/packaging/packages/synit-config/files/sbin/synit-init.sh)
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The [`synit-config` package](../install.md#build-the-packages) overrides the usual contents of
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`/sbin/init`, replacing it with a short shell script, `synit-init.sh`. This script, in turn,
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takes care of a few boring tasks such as mounting `/dev`, `/proc`, `/run`, etc., ensuring that
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a few important directories exist, and remounting `/` as read-write before `exec`ing
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`/sbin/synit-pid1`.
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For the remainder of the lifetime of the system, `/sbin/synit-pid1` is the PID 1 `init`
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process.
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### /sbin/synit-pid1
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- Source code: [`[synit]/synit-pid1/`](https://git.syndicate-lang.org/synit/synit/src/branch/main/synit-pid1)
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- Packaging: [`[synit]/packaging/packages/synit-pid1/`](https://git.syndicate-lang.org/synit/synit/src/branch/main/packaging/packages/synit-pid1)
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The `synit-pid1` program starts by spawning the [system bus](./system-bus.md)
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(`syndicate-server` in the process tree above) and the program `/sbin/synit-log`, connecting
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`stderr` of the former to `stdin` of the latter.
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It then goes on to perform two tasks concurrently: the first is the Unix
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[`init`](https://en.wikipedia.org/wiki/Init) role, reaping zombie processes, and the second is
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to interact with the system bus as an ordinary system service.
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The latter allows the system to treat `init` just like any other part of the system, accessing
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its abilities to reboot or power off the system using messages and assertions in the system
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dataspace as usual.
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Even though `synit-pid1` is, to the kernel, a *parent* process of `syndicate-server`, it is
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logically a *child* process.
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### /sbin/synit-log
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- Source code: [`[synit]/packaging/packages/synit-pid1/synit-log`](https://git.syndicate-lang.org/synit/synit/src/branch/main/packaging/packages/synit-pid1/synit-log)
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This short shell script invokes the [S6](../glossary.md#s6) program `s6-log` to capture log
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output from the system bus, directing it to files in `/var/log/synit/`.
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