Notes

notes/init.md

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+# init procedure
+
+    Summary of stages
+    -----------------
+
+    detect stage -> stage1 or stage2
+
+    stage1 (initramfs)
+     - mount various `tmpfs` and other kernel file systems
+     - get udev-like running? maybe not needed til stage2?
+     - mount rootfs
+     - move the `tmpfs`s etc over to the new mount
+     - pivot root
+
+    stage2 (real root)
+     - we don't need to mount the `tmpfs`s again
+     - the udev-like will be started by the supervision system
+     - so will all the other mounts
+     - we really just need to get to the point where the broker is
+       running and reacting to saved/configured system state
+     - we might be able to just `switch_root` straight into the broker??
+
+If the same `/init` script is to be used for initramfs and "stage
+two", then there needs to be a mechanism for distinguishing the two
+instances of `/init`. The experimental sketch I made uses presence of
+`/proc/mounts` as a signal we're in stage two.
+
+Filesystems on `/dev`; in Debian bookworm:
+
+    udev on /dev type devtmpfs (rw,nosuid,relatime,size=8088328k,nr_inodes=2022082,mode=755)
+    devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
+    tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
+    mqueue on /dev/mqueue type mqueue (rw,nosuid,nodev,noexec,relatime)
+    hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime,pagesize=2M)
+
+In my `/init` sketch:
+
+    mount -t devtmpfs none /dev
+    mkdir /dev/pts
+    mkdir /dev/shm
+    mount -t devpts none /dev/pts
+
+Note the differences in mount flags for the Debian case; e.g.
+`/dev/shm` has `nodev` set on it.
+
+In my sketch, I have a weak attempt at parallelism, `modprobe`ing
+`btrfs` at the same time as running `udhcpc`.
+
+To mount the root filesystem, we need all the necessary kernel
+modules. Maybe a udev-like "mount server" process could handle mount
+requests and modprobe as necessary?
+
+In my sketch, network interfaces have to be up before we try to mount
+the rootfs because it formats (!) the rootfs if it can't mount it (!!)
+and then installs Alpine on it.
+
+To swap over from initramfs to stage two: after the root is mounted,
+move the existing mounts (`/dev`, `/proc`, `/sys`, `/run`), kill all
+other processes, and `switch_root`.
+
+# broker and supervisor startup
+
+Now we're in "stage two", with no processes except the broker running.
+
+Let's use a rabbit-boot-like setup where we have a little
+logic-program-a-like determining process startup order. One difference
+will be the "supergoals" which will be configured in a couple of ways:
+a static "supergoal" of something which can get config from the file
+system and start other things, plus dynamic ones from user requests to
+start/stop stuff.
+
+Service classes & service instances: classes will be registered with
+the system and declare reverse dependencies. Forward dependencies are
+included with instances since they can be dynamic (? do we want this?)
+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.