Hypatia May
828c3f85c9
P1: both merge parents shipped SCHEMA_VERSION=3 for different layouts (surface columns vs seed_length), so an on-disk 3 was ambiguous and wrongly accepted. Bump to 4 (merged layout) so the version check rejects both sibling v3s. P2: a surface-eligible event with no surfaceOp lands in the log but vanishes from deriveMessages() (surface is the sole derivation path). The typed append overload enforces the marker only when the type arg is a literal; it collapses to optional when widened to the union (a caller iterating raw events). Guard at runtime in both append() and the seed constructor — no backward-compat for surface-less logs. Shared seed fixtures carry surfaceOp explicitly and the appendLog helper forwards it verbatim (no synthesized default). Exports isSurfaceEligibleType. Regression tests for all three, each verified to fail on the unfixed code. Gates: typecheck, test (1115), snapshot (14), doc-sync, lint, build, hygiene green.
76 lines
8.5 KiB
Markdown
76 lines
8.5 KiB
Markdown
# dsh-session
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Event-sourced session log and in-memory store. A `Session` is the append-only source of truth for an agent's whole interaction history — the LLM message history is *derived* from it. A **surface** layer (a linked list of message-producing events) is maintained on top of the raw log for efficient derivation and compaction.
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## Service: `SessionStore` (ctx key: `sessions`)
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Creates and holds event-sourced `Session` instances. Persistence is intentionally not implemented here — plugins subscribe to `session/event` and flush on `session/flush`.
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### Public API
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- `ctx.sessions.create(id?: SessionId, options?: { seed?: SessionEvent[]; meta?: { cwd?: string; parentSession?: SessionId; createdAt?: number } }): Session` — Create a session. `options.seed` replays/forks an existing event log; `options.meta` attaches creation metadata (validated absolute `cwd`, `parentSession` lineage) as the immutable `SessionHeader`. The store fills `version`/`id` and defaults `createdAt` to now; a caller reconstructing a persisted session passes the original `createdAt` to preserve it. Disposed with the calling fiber.
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- `ctx.sessions.get(id: SessionId): Session | undefined`
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- `ctx.sessions.list(): Session[]`
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#### Advanced: ordered-teardown lifecycle primitives
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`create()` covers the common case (the session is owned by the calling fiber). When a session must be torn down **in order with another resource** — so a final flush is captured before `onAppend` detaches — `create()`'s self-contained effect is wrong, because a fiber unload disposes sibling effects *concurrently*. For that, split the lifecycle and fold it into the owner's single effect:
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- `ctx.sessions.prepare(id?, options?): Session` — validate the id/cwd and construct the `Session`, WITHOUT entering it into the store. Same options as `create`.
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- `ctx.sessions.enter(session): () => void` — wire `onAppend` → `session/event` and add the session to the store; returns the DETACH disposer. Does NOT emit `session/created` (the caller yields the disposer first, then calls `announce`, so a throwing listener rolls the attach back). The id was already validated by `prepare`, which runs in the same synchronous sequence, so `enter` does not re-check.
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- `ctx.sessions.announce(session): void` — emit `session/created` for an entered session.
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`dsh-agent-loop`'s `AgentLoop.start` is the canonical consumer: it yields `enter`'s detach disposer, the registry unregister, and the loop-stop disposer into ONE composite effect, so teardown stops + awaits the loop (final flush captured) BEFORE detaching the session — whether the trigger is the `AgentHandle`'s `dispose()` or a fiber unload.
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### Events
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| Event | Mode | Purpose |
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|---|---|---|
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| `session/created` | emit | A session was created |
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| `session/event` | emit | An event was appended (sync, fire-and-forget) |
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| `session/flush` | parallel | Awaited durability checkpoint (persistence plugins drain buffers here) |
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### Class: `Session`
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Plain class (not a Cordis Service). Create via `ctx.sessions.create()`.
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- `session.append(type, data, opts?): SessionEvent` — synchronous, never blocks on I/O. **Throws** if `data` is not losslessly JSON-serializable (BigInt, function, symbol, undefined, non-finite number, circular ref, or an exotic object like Map/Set/Date) — the event log is the durable source of truth, so this invariant is enforced at the source (exported as `isJsonValue` for backends to reuse on their replay/fork entry points). A third parameter `opts: SurfaceIntent` carries surface metadata: `surfaceOp` controls how the event enters the surface linked list, and `sourceEventSeqs` records provenance (the seq numbers of events this one derives from). It is **required** for the five `SurfaceEventType` events (every message-producing event must declare how it joins the surface) and rejected by the compiler for non-surface types. The marker requirement is enforced two ways: the typed overload makes `opts` mandatory when `type` is a specific `SurfaceEventType` literal, AND `append` **throws** at runtime if a surface-eligible event arrives with no `surfaceOp` — covering the case where `type` widens to the `SessionEventType` union (a caller iterating raw events, where the conditional overload collapses to optional) so a marker-less message event can never silently land in the log and vanish from `deriveMessages()`.
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- `session.deriveMessages(): Message[]` — derive the LLM message history by walking the surface linked list (skipping non-surface events like chunks and boundaries; a `replace` shadows the nodes it covers). The surface is the single source of derived history — there is no raw-log fallback.
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- `session.surface: SurfaceManager` — the derived surface, lazily rebuilt from `surfaceOp` markers in the log. Processes only new events (delta) on each access — the log is append-only, so prior events never change.
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- `session.events`, `session.seq`, `session.id`
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- `session.header: SessionHeader` — immutable creation metadata (`version`, `id`, `createdAt`, optional `cwd`/`parentSession`). Kept out of the event log (a storage concern, not replayable state); a minimal header (stamped with the current `SESSION_FORMAT_VERSION`) is synthesized for bare `Session` construction.
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### Surface types
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- `SurfaceOp` — how a surface node entered the linked list: `'append'` (normal tail append) or `{ op: 'replace', start, end }` (replace nodes from `start` through `end` inclusive — both must be valid surface node seqs; `start === end` replaces a single node). Used by compaction to shadow old nodes without deleting them.
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- `SurfaceIntent` — `{ surfaceOp: SurfaceOp; sourceEventSeqs?: number[] }`, the required third parameter to `session.append()` for surface-eligible types.
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- `SurfaceNode` — `{ seq: number; prev: number | null; next: number | null }`, one node in the surface linked list.
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- `isSurfaceEvent(event)` / `isSurfaceEligibleType(type)` — the first narrows a `SessionEvent` to a fully-formed surface node (type is surface-eligible AND `surfaceOp` present); the second is the type-only check (is this one of the five `SurfaceEventType` values?), used to detect a surface-eligible event MISSING its marker — e.g. when validating a seed/load log.
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### Session event vocabulary (`types.ts`)
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The append-only log: `turn/start`, `turn/end`, `step/start`, `step/end`, `user/message`, `assistant/message`, `assistant/chunk`, `tool/call`, `tool/result`, `steering/message`, `context/message`. Token usage rides on `assistant/message.usage`; an operational error's step is on `turn/end.reason` for `kind: 'error'`.
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Merge-extensible via `SessionEventMap` — a compaction plugin adds `compaction/marker`, etc.
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Also defines `TurnTriggerMap` and `TurnEndReasonMap` (merge-extensible sum types for typed turn boundaries — `kind`-tagged instead of strings).
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Every `SessionEvent` carries two optional top-level fields (structural metadata):
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- `sourceEventSeqs?: number[]` — seq numbers of provenance sources (e.g., the `assistant/chunk` seqs behind an `assistant/message`, or the shadowed nodes behind a compaction marker).
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- `surfaceOp?: SurfaceOp` — how this event entered the surface. Absent for non-surface events (boundaries, chunks, usage, errors).
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### Metadata types (`types.ts`)
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- `SessionHeader` — immutable session metadata, written once: `{ version, id, createdAt, cwd?, parentSession? }`. Owned here (beside `SessionId`) because `Session.header` is typed by it; persistence backends re-export it rather than own it (which would force a package cycle).
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### Extension points
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- Persistence plugins: subscribe to `session/event` (write-behind) and drain on `session/flush` (awaited) and fiber dispose. A durable backend reads the log and reloads it into a live session; the metadata seam (`SessionHeader`, `session.header`) is what such a backend stores beside the log.
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- Replay/fork: `ctx.sessions.create(id, { seed })` seeds a new session with an existing event log. The surface rebuilds deterministically from `surfaceOp` markers in the seeded events. The seed is validated to the SAME invariants `append` enforces — including that every surface-eligible event (`SurfaceEventType`) carries a `surfaceOp` marker — so a marker-less message event is rejected at construction rather than silently vanishing from `deriveMessages()` (the surface is the sole derivation path) on resume.
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- Compaction: a future plugin appends a new event with `surfaceOp: { op: 'replace', start, end }` to shadow old surface nodes.
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### What is NOT here (TODO)
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- **Session branching/tree** (pi-style entry tree) — deferred unless needed beyond seed-based forking.
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