From 19d3cd2b1def4654bd5803b141a6c3bd6689e267 Mon Sep 17 00:00:00 2001 From: sami Date: Thu, 10 Sep 2026 00:00:08 +0400 Subject: [PATCH] =?UTF-8?q?daemon:=20draft=20ADR=200013=20=E2=80=94=20task?= =?UTF-8?q?=20state-machine=20ownership=20(D1)?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Drafts the CORE/DAEMON split PROTO raised as D1 and CORE already agreed to in contracts/proto-answers-m1.md, since neither lane would write it down alone. DAEMON owns new/queued and pause-for-schedule; CORE owns probing through complete|failed and cancelled-from-anywhere is DAEMON-driven; paused is shared. Ties into ADR 0011 in two places: - retry_wait looks identical to segment starvation from the budget accessor's point of view (zero segments, deliberately) and must be excluded from tasks_starved by construction, not by DAEMON guessing from timing. - restart handling: CORE holds no persistent state, so any CORE-owned TaskState reloads as queued and re-admits through the scheduler; paused tasks reload with their pauseReason intact. Surfaces one real contract gap while drafting, not just an open question: event.task.state's error field is schema-scoped to failed/retry_wait only, so CORE auto-pausing for auth_required or server_file_changed currently has no wire signal telling DAEMON why — needed before the resume-must-not-cross-reasons rule (§3) can be implemented at all. Filed as a PROTO follow-up in the ADR itself. Status: proposed, needs CORE + PROTO sign-off (four open items at the end) before daemon/src/sched/'s pause/resume logic is written against it. Co-Authored-By: Claude Sonnet 5 Claude-Session: https://claude.ai/code/session_01Upd9WhG9oppieig5nRDLig --- docs/adr/0013-task-state-machine-ownership.md | 204 ++++++++++++++++++ 1 file changed, 204 insertions(+) create mode 100644 docs/adr/0013-task-state-machine-ownership.md diff --git a/docs/adr/0013-task-state-machine-ownership.md b/docs/adr/0013-task-state-machine-ownership.md new file mode 100644 index 0000000..09b1736 --- /dev/null +++ b/docs/adr/0013-task-state-machine-ownership.md @@ -0,0 +1,204 @@ +# ADR 0013 — Task state-machine ownership: CORE, DAEMON, and the shared `paused` + +**Status:** proposed · **Date:** 2026-09-09 · **Lane:** DAEMON, drafted at PROTO's request +(D1 in `core/docs/proto-requests-m1.md`), for CORE and PROTO to review and sign off. +**Needs:** explicit accept from CORE and PROTO before `daemon/src/sched/` calls into CORE's +task-start API, since the calling convention below is derived from this split. + +## Context + +`docs/04` §1 hands CORE the download lifecycle (`probing → connecting → downloading ⇄ +paused → retry_wait → assembling → verifying → complete | failed`), but `queued` and every +scheduler transition are DAEMON's, and neither brief says where the seam is. PROTO raised +this as D1 and proposed a split; CORE agreed (`contracts/proto-answers-m1.md` D1); both +declined to write it down alone — a two-lane decision recorded by the lane that owns +neither half is exactly the failure mode `contracts/README.md` rule 4 exists to prevent. +DAEMON drafts it because DAEMON is the one lane touching both halves (it owns the SQL row +and the RPC surface; it calls into CORE). + +The wire side is already frozen and does not need to change: `TaskState` +(`contracts/schema/types/TaskState.schema.json`) has the twelve values with no field +naming who drove a transition, and `event.task.state` carries `previousState` — a client +renders what it's told and keeps no state machine of its own. This ADR is about who +*decides* a transition, not the wire shape of one. + +This is also where ADR 0011 (admission control) and the state machine meet: `queued → +connecting` is the admission event, and `retry_wait` looks identical to starvation from +inside the segment-budget accounting. Both are addressed below because getting them wrong +independently produces the same false alarm from two different directions. + +## Decision + +### 1. Ownership, by state + +| State | Driven by | Entered from | Notes | +|---|---|---|---| +| `new` | DAEMON | (creation) | `download.add` persists the row before CORE knows the task exists. | +| `queued` | DAEMON | `new`, `paused` (resume, subject to §3) | Scheduler admission pool. CORE has no concept of "queued" — `retry_wait` re-enters `connecting` directly inside CORE and never passes back through `queued`. | +| `probing` | CORE | `queued` | DAEMON calls CORE's `start()` once admitted (ADR 0011 §1); CORE owns everything from here until it hands back a terminal or `paused` state. | +| `connecting` | CORE | `probing`, `retry_wait` | | +| `downloading` | CORE | `connecting` | | +| `paused` | **shared** | any CORE state, `queued` | See §2 — this is the one state either side may enter unilaterally. | +| `retry_wait` | CORE | `connecting`, `downloading` | CORE's own backoff timer (`docs/04` §7); DAEMON does not schedule retries. | +| `assembling` | CORE | `downloading` | Normally a no-op rename; a real state for HLS/DASH muxing. | +| `verifying` | CORE | `assembling` | Checksum verification when requested. | +| `complete` | CORE | `verifying` | Terminal. | +| `failed` | CORE | any CORE state | Terminal. `max_retries_exhausted` after `retry_wait`, or any non-retryable error. | +| `cancelled` | DAEMON | any state | Terminal. Always user- or policy-initiated (`download.cancel`, category/queue removal) — CORE never cancels on its own; it only executes the teardown DAEMON asked for and confirms. | + +Reading the table as a picture: + +``` +DAEMON: new ──▶ queued ──▶[admit]──▶ (hand to CORE) + ▲ │ + │ resume CORE: probing ─▶ connecting ─▶ downloading + pause────┤ │ │ ▲ │ │ + (either side)│ retry_wait◀────────┘ │ │ │ + ▼ │ (backoff done)│ ▼ ▼ + paused ◀──────────(auto-pause)─────┘ assembling paused + │ (auto) + verifying + │ + complete + (from anywhere, DAEMON-driven) ──────────────────────▶ cancelled + (from any CORE state, CORE-driven, non-retryable) ───▶ failed +``` + +### 2. `paused` is shared, and idempotency is the whole contract + +Both sides can put a task in `paused`, for disjoint reasons: + +- **DAEMON-initiated**: user clicks pause, a schedule window closes, a queue is stopped, + `Queue.onComplete`, or the admission governor reconciling a lowered `maxActiveSegments` + (ADR 0011 §2 — pausing the lowest-priority excess). DAEMON calls CORE's `pause(TaskId)`. +- **CORE-initiated ("auto-pause")**: `auth_required` (401/407, `docs/04` §7), + `server_file_changed` (F1's carrier, `contracts/proto-answers-m1.md`), disk full. CORE + transitions to `paused` on its own and reports it up through the existing + state-change callback — the same path every other CORE-driven transition uses. This is + not a new mechanism. + +The contract that makes this safe: + +- **`pause()` is idempotent.** DAEMON calling `pause()` on a task CORE already + auto-paused is a no-op, not an error — DAEMON does not need to know CORE got there + first. Symmetrically, if CORE were ever mid-transition to `paused` when DAEMON's pause + request arrives, the result is still `paused`, once. +- **DAEMON persists *why* a task is paused**, in the `tasks` table, not in `TaskState` + itself (the wire type stays a flat enum — this is DAEMON-local bookkeeping, not a + contract change). A `pauseReason` distinguishing at least `user`, `schedule`, + `queue_stopped`, `admission_reconcile`, and CORE's `error.code` when auto-paused. This + is what makes §3's resume rule possible — but it needs a contract fix first: see the + gap in open item 3 below. `error.code` is not currently carried on a transition into + `paused` at all. +- **CORE does not need to track why it's paused past the current occurrence.** Once + paused, CORE's job is done; DAEMON is the only side that later decides whether to + resume, and DAEMON is also the only side with persistent storage to remember the + reason across a restart. + +### 3. Resume must not cross reasons + +The bug this section exists to prevent: a schedule window opens, DAEMON blindly resumes +every `paused` task in that queue, and it resumes a task CORE paused because it's waiting +on credentials that were never provided. The task immediately re-fails or re-pauses, looks +like a flapping bug, and burns a retry. + +Rule: **DAEMON resumes a task only when the reason it recorded matches the event that +justifies resuming.** A schedule window opening resumes tasks paused for `schedule`. A +user clicking "resume" resumes anything (explicit user intent overrides any reason). +`auth_required` and `server_file_changed` are resumed only by the paths that actually +address them — `download.provideAuth` (F2) and the user's restart-decision response (F1) — +never by the scheduler. This means `queued` is not the only state a schedule can put a +task back into a run cycle from; the scheduler's "should this task be running right now" +check must skip tasks paused for a reason it doesn't own. + +### 4. `retry_wait` is not starvation + +ADR 0011 §3.6 defines a starved task as a running task with `segments_active(id) == 0`, +and asserts `tasks_starved == 0` in steady state past a bounded delay. A task in +`retry_wait` also holds zero segments, deliberately, for up to 60 s (`docs/04` §7's backoff +cap) — and it is **not** admission-starved, it is CORE's own policy holding it idle. + +Resolution: `retry_wait` must be distinguishable from true starvation without DAEMON +guessing from timing. CORE's `starved_tasks()` (ADR 0011) excludes any task CORE itself +holds at zero by policy — `retry_wait` and CORE-initiated `paused` both fall outside +`tasks_starved` by construction, because that accessor is about the segment allocator +failing to grant a slot to a task that wants one, not about a task that isn't asking. A +task in `retry_wait` still counts against `connection.maxConcurrentDownloads` from +DAEMON's side (it is running, not requeued) but contributes nothing to the segment +allocator's guarantee pass until it re-enters `connecting`. + +### 5. Restart — CORE holds no persistent state, DAEMON reloads to `queued` + +Per the M1 DoD ("kill and restart the daemon mid-download: all tasks reload with correct +state and resume") and the layering rule (CORE has no SQL, no state survives a CORE +restart except what's on disk in `.veloxpart.meta`, which CORE reads back itself): on +daemon startup, DAEMON loads every non-terminal task from SQLite. Any task whose persisted +`TaskState` was a CORE-owned state (`probing` through `verifying`) is **rewritten to +`queued`** in memory before the scheduler sees it — the on-disk `TaskState` is a +last-known-value, not a resumable position, because CORE's in-process state died with the +process. The scheduler re-admits it exactly like any other queued task; CORE re-derives +where to actually resume from `.veloxpart.meta` and re-validates with `If-Range` +(`docs/04` §5), independent of what SQL said the state was. + +`paused` tasks reload as `paused`, with their `pauseReason` intact, and are not +auto-admitted — §3 applies identically after a restart as it does live. + +## Consequences + +- `daemon/src/sched/` calls `start(TaskId)` exactly once per admission (`queued → + probing`), never re-enters a CORE-owned state directly, and treats every CORE-owned + state as opaque past that call except for reading it back for projection. +- The `tasks` table needs a `pauseReason` column (DAEMON-local; not a wire type) before + `sched/`'s pause/resume logic can be written correctly — flagging as a concrete + follow-up, not blocking this ADR's acceptance. +- CORE's `starved_tasks()` (ADR 0011) must exclude `retry_wait` and CORE-auto-paused tasks + by construction; if that isn't already true in CORE's accessor, it needs to be before + DAEMON relies on the starvation invariant, since otherwise every backoff cycle would + trip DAEMON's governor-invariant warning as a false positive. +- No `contracts/` change. `TaskState`, `event.task.state`, and `previousState` are already + sufficient; this ADR is entirely about which process calls which function when. + +## Alternatives considered + +**A single owner drives every transition (CORE, told about queues).** Rejected — this is +the D1 problem restated with CORE holding the SQL-shaped concepts (`queued`, schedules, +priority) that the layering rule (`CLAUDE.md` §3) forbids it from touching. + +**DAEMON drives every transition, treating CORE as a dumb byte-mover.** Rejected — CORE's +internal states (`retry_wait`, `assembling`, `verifying`) depend on engine internals +(backoff timers, mux completion, streaming hash state) DAEMON has no visibility into +without CORE reporting them; forcing DAEMON to poll or reimplement that timing duplicates +`docs/04` §7 in two places and they will drift. + +**A `pauseReason` on the wire (`TaskState` split into `paused_user` / `paused_auto` / +etc.).** Rejected — it roughly doubles the enum for a fact only DAEMON's resume logic +needs, and adding wire cardinality for internal bookkeeping is the kind of thing that +becomes a compatibility problem the moment a client starts branching on it. The value +DAEMON needs (CORE's reason) can travel on the existing `error` field instead — see the +contract gap in open item 3 — without touching `TaskState` itself. + +## A contract gap this ADR surfaces, not just an open question + +`event.task.state.schema.json`'s own description scopes `error` to "whenever the new +state is failed or retry_wait" — **not** `paused`. The one fixture +(`event.task.state.json`) only exercises the `failed` case. So today, when CORE +auto-pauses for `auth_required` or `server_file_changed`, DAEMON has no signal on the wire +telling it *why* — §2/§3 of this ADR are unbuildable without one. This needs a PROTO +follow-up (minor: widening an existing field's presence condition, per +`contracts/README.md` rule 4 — no new field, no retype) to also populate `error` when +`state == "paused"` and the pause was CORE-initiated. DAEMON is not asking for a way to +tell CORE-paused from user-paused on the wire in general — `error: null` on a +DAEMON-initiated pause is sufficient, since DAEMON already knows it just did that. + +## Open, for CORE and PROTO to confirm or amend + +1. Does CORE's `starved_tasks()` already exclude `retry_wait` and auto-paused tasks, or + does this ADR ask for a behavior change there (§4)? +2. Is `pause()` idempotent today, or does calling it on an already-paused task currently + return an error CORE needs to relax? +3. PROTO: land the `error`-on-`paused` widening above before DAEMON writes the resume + logic in §3 — otherwise DAEMON has no correctness-preserving way to implement it and + would have to guess from timing, which is the exact failure mode ADR 0011 was written + to rule out for a different pair of governors. +4. Naming: is "auto-pause" the term CORE already uses internally, or does CORE have an + existing name for this that this ADR should adopt instead of introducing a new one?