SchOS/docs/plan/init.md

PID 1 becomes a role

What schinit is today is ../architecture/init.md. This is what it becomes, and it is the one item on this plan that deliberately breaks a Unix rule.

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The law being changed

Unix's rule is PID 1 is special and must never die. Every init since 1975 has honoured it by being careful.

SchOS's rule instead: PID 1 is a role. The kernel guarantees the role is filled. The role's state lives outside whichever process currently holds it.

That is a real change and it is worth making, because "must never die" is not a mechanism — it is a hope, and the thing it hopes about is the one program on the machine with no supervisor.

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The proposal this replaces

The original sketch was: init 1 starts init 2; if init 1 is killed, init 2 starts init 3 and revives init 1.

The instinct is right — init needs something watching it — and one half of the idea survives below. The specific arrangement has five problems, and four of them are structural rather than fixable. They are recorded because each one is a trap that any redundant-supervisor design falls into:

It defends against the attack that already fails. Userspace cannot kill PID 1 on this kernel. init_ignores() (kernel/proc/proc.c:620) drops SIGKILL to initproc. The threat is already handled.

Init 2 is the same binary as init 1. If init 1 died of its own bug — a null dereference parsing a manifest, an exec of a truncated binary — init 2 reaches the same line and dies the same way, and faster, because it just watched the input that caused it.

The chain has no bound. Init 2 makes init 3, which by symmetry watches init 2 and makes init 4. Without a cap it is a fork bomb whose author is the supervisor; with a cap, the cap is the real design and the chain is decoration.

Two supervisors is split brain. Both hold the job table. Both see a job's pid exit. Both restart it. Now the console shell exists twice, both reading /dev/console, and every keystroke goes to whichever woke first. This is the standard failure of redundant supervisors, and it is worse than no redundancy because it corrupts the state it was added to protect.

It only detects death. The common way an init breaks is not dying — it is hanging: deadlocked, spinning, or blocked forever on a disk that stopped answering. wait() never returns for a process that is merely stuck, so init 2 watches a wedged init 1 forever and reports everything normal. The scheme is armed against the rare failure and blind to the likely one.

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What gets built instead

Four parts. The first three are userspace; the fourth is the kernel change.

Part 1 — split by role, not by copy

schinit (PID 1) becomes small enough to be obviously correct and then stops changing: reap children, hold the job table in memory, supervise exactly one child. No manifest parsing. No filesystem walking after boot. No allocation. Its entire job is to be too boring to fail.

schinitd (PID 2) gets everything else — manifests, socket activation, respawn policy, backoff, and the eventual schinitctl. It is where the bugs will be, and it is restartable.

The detail that makes this work, and that the two-copy scheme cannot have:

Jobs are forked by PID 1, not by schinitd. schinitd decides and asks; PID 1 acts. So every job on the machine is a child of PID 1 at all times, and schinitd crashing orphans nothing, reparents nothing and restarts nothing. It comes back, reads the state, and finds the world exactly as it left it.

This keeps the good half of the original idea — two processes, one watching the other — with the roles made different, so the second is not a second copy of the first one's bug.

Part 2 — state outside the process

/proc/schinit: the serialised job table, written by PID 1 whenever it changes, readable by anyone. A resurrected supervisor reads it back and knows what is running without having started it.

procfs rather than a file on disk, for four reasons: it survives a read-only root, it survives a full disk, it costs no I/O, and it is already the pattern this kernel uses for exactly this kind of question.

The side benefit closes an item the architecture doc lists as missing — the job table is currently invisible. cat /proc/schinit is the observation.

Part 3 — a heartbeat, in both directions

The failure the original scheme cannot see.

PID 1 and schinitd hold a socketpair; each must be heard from within a deadline. Silence from schinitd means PID 1 kills and restarts it. Silence from PID 1 is the case part 4 exists for.

A hang is now as detectable as a death, which is the whole point.

Part 4 — proc_reincarnate()

The kernel change, and the actual law change.

Today exit() on initproc calls panic("init exiting") (kernel/proc/proc.c:277). Instead: tear down the address space, keep the struct proc, keep pid 1, and re-exec /sbin/schinit in place with a generation counter passed in.

No init 2, no init 3, no reparenting problem — because pid 1 never stops being pid 1. Every orphan the kernel already reparented to initproc is still correctly parented, initproc is still the pointer it always was, the children survive, and the state is in /proc/schinit.

This is strictly better than resurrection-by-sibling for every case, and it is the smaller change.

The escalation ladder

What makes it safe to have at all:

Generation	What happens
1	Normal boot.
2–3	Re-exec in place. Log it, keep the jobs, carry on.
4	Re-exec, but ignore /etc/schinit and start only a console shell. A manifest that kills init must not kill it forever.
5	Stop. panic() with stop code INIT_RESURRECTION_FAILED.

The last row is the honest one. A machine that cannot hold PID 1 up five times is not a machine that should keep trying, and PSOGM already renders a stop code, a call chain and a QR code of the whole report — which is a far better end than a silent loop. The crash screen is the correct place for this to arrive.

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Milestones

I0	Split the program. PID 1 supervises one child; schinitd does everything else. Jobs still forked by PID 1.
I1	/proc/schinit, written and read back. Kill schinitd by hand and watch it return without disturbing a single job.
I2	The heartbeat both ways, with a sysrq key that wedges schinitd on purpose so the timeout path is exercised rather than assumed.
I3	proc_reincarnate() and the ladder, with a sysrq key that kills PID 1 on purpose.

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What it costs

PID 1 and PID 2 must agree on a protocol, which is a compatibility surface where there was none — created by a plan whose own schapi.md §3 warns about exactly that. Keep it tiny, version it in its first message, and keep it entirely internal. It is not SchAPI and nothing else may speak it.

proc_reincarnate() is a genuinely dangerous function. It runs in the context of a process that is dying, and it must not take a lock that process may already hold. It belongs in proc.c next to exit(), written with the care of the panic path, and tested by a sysrq key rather than by reasoning.

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What it will still not do

• It does not make init correct, only survivable. A schinitd that starts the wrong jobs starts them again after every restart.
• It does not protect the jobs from each other. A job that fills the disk or exhausts the process table still takes the machine down, and the supervisor surviving does not help.
• A hang in PID 1 itself is only caught by schinitd noticing, and if both are wedged simultaneously nothing catches it. There is no hardware watchdog.
• Five generations is a guess, not a measured number.