Watchbound

Watchbound

Build filesystem consumers around conservative invalidations and explicit coverage. Preserve the distinction between what Watchbound observed and what it can safely claim.

Establish the applicable contract

Before changing an integration:

1. Inspect the installed watchbound version, its declarations, and its exported capabilities.
2. Check capabilities.support.currentRuntime.targetCompatible, the matching entry in capabilities.support.targets, and qualifyRoot(root).state. Matching diagnostic runtime facts alone do not widen the declared support target. In the current API, the older capabilities.support.currentRuntime.supported field is removed: targetCompatible covers only the selected packaged target, while qualifyRoot(root) supplies the required host and root decision. Older packages may still expose the removed field or only the legacy single-target fields.
3. Confirm that the consumer can rescan after a conservative root invalidation and can operate honestly with partial or uncertain coverage.
4. Prefer another watcher when the consumer needs a cross-platform package, exact event history, typed create/update/delete events, or cannot own joined native disposal.

When a task starts from @parcel/watcher, keep current upstream public API facts separate from Watchbound's retained 2.5.6 tagged-source and reproduced conformance evidence. Do not project those observations onto another Parcel version without new evidence. In a Watchbound source checkout, read docs/parcel-watcher-limitations-linux.md for the decision guide and evidence boundary.

The source repository can be ahead of immutable registry releases. Do not use a source-candidate API merely because this skill describes it; verify that the selected package version exports the capability.

Treat support as deliberately evidence-bound. Use the installed package's capabilities and immutable registry, tag, and release records instead of a hard-coded release number. The checked-in source matrix declares supported x64 and ARM64 GNU/Linux targets with a kernel 5.15/glibc 2.35 baseline, Node >=18.15.0, and Node-API 6 or newer; exact native, distro, Electron, Nix, reproducibility, kernel-floor, separately supervised overflow, and registry smoke evidence backs those declarations. Immutable release records identify whether the selected package also includes the exact ARMv7 hard-float target and its deterministic cross-build/package, QEMU-user Electron, system-QEMU kernel-floor, and registry evidence. ARMv7 has no native-hardware or performance evidence.

An environment with recognized container evidence cannot qualify. Treat container-unknown as unknown evidence, never as a negative container result; not-detected covers only the designated probes and is not exhaustive. Detected WSL and network, FUSE, or overlay roots cannot qualify. Musl, soft-float/unknown/non-v7 ARM, big-endian ARM, and non-Linux hosts are unsupported. Exact GNU/Linux ARMv7 hard-float retains both declared emulated execution lanes as continuing support gates. Distro recognition, successful loading, and runtime facts never widen the selected package's exact target status.

Preserve the semantic model

Interpret invalidated paths as places whose derived state may need recomputing, not as an exact filesystem journal. Never infer create, update, delete, or rename history from a batch.

Handle every coverage variant:

• complete: Watchbound claims coverage of the included tree. Paths remain conservative invalidations.
• partial: Some logical directories are not watched. Surface degraded coverage or provide an ongoing fallback; a one-time rescan does not remove the future blind spot.
• uncertain: Delivered detail may be incomplete. Rebuild from a safe boundary, normally the root, and use reconciliation or explicit root recovery where applicable.

Use invalidatedPathBytes when exact Linux path bytes matter. If pathEncoding is root-collapsed, treat the physical string root invalidation as the safe boundary rather than reconstructing an unrepresentable child. If it is bytes-only, invalidatedPaths is intentionally empty and the exact byte paths are the only authoritative invalidations; never substitute a lexical alias.

Keep cardinality bounded throughout the consumer. Bound pending work, coalesce duplicate invalidations, and prefer one root rebuild over an unbounded queue of possibly incomplete detail.

Integrate the JavaScript API

Use the lazy top-level subscribe() for a single default configuration. Use createEngine({ nativeWatchBudget }) when the process must explicitly bound unique native watches or inspect process-global runtime statistics. watchLimit counts subscription-local logical directories; the native watch budget counts process-wide unique native watches. A null limit means no Watchbound-imposed limit, not unlimited kernel resources.

Start from this lifecycle shape, adapting the recovery policy to the consumer:

import { subscribe } from "watchbound";

const subscription = await subscribe(
  workspaceRoot,
  async (batch, { signal }) => {
    if (batch.coverage.state === "uncertain") {
      await rebuildWorkspace(workspaceRoot, { signal });
      return;
    }

    if (batch.coverage.state === "partial") {
      reportDegradedCoverage(batch.coverage);
    }

    await invalidateDerivedState(batch.invalidatedPaths, { signal });
  },
  {
    signal: establishmentAbortController.signal,
    initialExclusions: ["generated"],
    excludedDirectoryNames: [".git", "node_modules"],
    observedExcludedPaths: [".git"],
    watchLimit: 50_000,
    maxBatchPaths: 2_048,
    outputQueueCapacity: 8,
  },
);

try {
  await runConsumer(subscription);
} finally {
  await subscription.dispose();
}

Treat those option values as examples. Read accepted bounds and defaults from capabilities.options for the selected build.

Use initialExclusions when excluded directories are already known before subscription. Prefixes are exact normalized root-relative directory paths, not globs or basenames, and are applied before the generation-zero topology scan. An empty prefix excludes the root. Watchbound does not discover Git ignores or application policy; consumers must compute and update that complete prefix set.

When capabilities.features.directoryNameExclusions and capabilities.features.observedExcludedPaths are true, use excludedDirectoryNames for exact directory components that must be pruned at every depth, including future and renamed-in directories. Use observedExcludedPaths for nonempty exact root-relative excluded boundaries whose creation, deletion, rename, replacement, or identity change should trigger policy refresh. Observation overrides only boundary delivery: descendants remain unwatched and symlinks are not followed. Both options join initialExclusions at generation zero.

subscription.replaceExclusions(generation, policy) atomically replaces all three sets. Omitted object fields are empty. The legacy array form remains a prefix-only replacement and clears the newer sets, so an integration using the new behavior should always send the complete policy object.

Check initialCoverage immediately after establishment. Use ordered callback batches as the authoritative JavaScript observation boundary. observedState, operation results, and native-backed getters intentionally can describe different points in time; do not optimistically advance observed state after an operation acknowledgement.

Callbacks are promise-aware and serialized per subscription. Keep them cooperative with the callback context's signal, and make their workload bounded. Synchronous throws and rejections are contained and counted, but still represent consumer failures.

The SubscriptionOptions.signal cancels establishment only. After subscribe() resolves, stop with subscription.dispose(). Inside a callback, call context.stop() to request disposal and join it later from outside the current callback. Never await subscription.dispose() from that callback, await a later callback from the same subscription, or wait for observed state that requires the current callback credit.

Retain the subscription strongly and dispose it explicitly. Joined disposal is idempotent, waits for an already admitted Promise-like callback, and guarantees that no callback begins after the promise resolves. A callback that never settles can therefore keep disposal pending unless it observes its signal.

Recover coverage deliberately

Call reconcile() only for recoverable uncertainty: event-overflow, topology-race, or consumer-backpressure. It rebuilds topology and emits a conservative root boundary; it never reconstructs lost events. Automatic reconciliation is opt-in, bounded, and does not make a replacement-root decision.

Treat root-replaced separately. Call recoverRoot({ identityPolicy: "original-only" }) when the original root identity must return, or explicitly choose "accept-replacement" when the application authorizes adopting the candidate at the same lexical path. Do not silently choose identity policy on the user's behalf.

Expect reconciliation, exclusion replacement, and root recovery to share a topology transaction gate. Handle structured WatchboundError fields and retryAfter; never parse diagnostic message text. Keep retries bounded and wait for the named condition to change.

Translate exclusions at the consumer boundary

Supply the complete replacement exclusion policy with a strictly increasing bigint generation. Prefixes and observed boundaries are normalized root-relative paths; directory names are exact single components at every depth. Every value is compared with exact Linux bytes. They are not globs, Git-ignore rules, workspace mappings, or application defaults.

Keep ignore-policy translation in the consumer. Do not move it into the Rust engine or Node binding. Preserve caller-supplied root components until native symlink-ancestry validation; do not normalize away symlink/.. before calling Watchbound. Never follow directory symlinks.

Contribute to the repository

Read AGENTS.md and CONTRIBUTING.md before editing. Use docs/README.md to find the relevant contract, evidence, or decision records. Maintain these ownership boundaries:

• Put filesystem semantics and the Linux inotify state machine in engine/.
• Keep node/ to representation translation and lifecycle bridging.
• Put ergonomic API policy, callback handling, and automatic reconciliation in js/.
• Keep consumer ignore rules, UI policy, and logical workspace mapping outside the engine.

Use test-driven changes for watcher semantics and lifecycle. Run focused tests while iterating, then run:

pnpm test
pnpm check

After public declaration or API-documentation changes, also run:

deno doc --lint js/index.d.ts

Before handing off declaration, entrypoint, package-surface, or package documentation changes, also run:

pnpm test:packages

Preserve bounded queues, exact Linux path bytes, explicit watch limits, no directory-symlink following, and the no-later-callback disposal guarantee. On loss or pressure, prefer a root invalidation and explicit uncertainty over over-detailed claims.

Do not record final benchmark readings until the user confirms the host is quiet and prepared. Do not run forced-overflow conformance casually. Do not publish packages or integrate Watchbound into Codex Desktop unless the user explicitly expands the scope.