docs(prd): define authoritative failure boundaries

docs/prds/0082-authoritative-failure-boundaries.md

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+# PRD 0082: Authoritative failure boundaries
+
+- **Status:** Draft
+- **Author:** codex
+- **Created:** 2026-07-27
+- **Issue:** #444
+
+## Summary
+
+Make every security- or lifecycle-sensitive snapshot distinguish authoritative
+empty state from unavailable state, and make every destructive or
+resource-consuming boundary revalidate the assumptions it acts on. This
+finishes the focused quality work begun under #444 without broad rewrites:
+cleanup cannot act on stale identities, policy introspection cannot publish a
+fabricated empty policy, gateway servers bound untrusted work, and daemon
+shutdown does not emit uncaught background-thread failures.
+
+## Problem
+
+Several paths are individually fail-closed but compose into unsafe or
+misleading behavior:
+
+1. `cleanup` prepares a plan, waits indefinitely for operator confirmation,
+   then kills stored PIDs and removes stored paths without checking that those
+   identities still describe the same orphan. A PID may be reused or a run
+   directory may become active during the prompt.
+2. The supervisor reuses egress's deny-all fallback for
+   `list-egress-routes`. Deny-all is correct for enforcement, but presenting it
+   as a successful empty route table can cause a later replace-all proposal to
+   discard live routes.
+3. The supervisor and Git HTTP services accept bounded declared body sizes but
+   use blocking reads and unbounded request threads. An untrusted bottle can
+   exhaust the shared gateway with slow or parallel requests.
+4. macOS cleanup enumerates containers and networks independently and treats a
+   failed query as an empty class, so a partial snapshot can still become a
+   destructive plan.
+5. Gateway log-pump threads race stream closure during shutdown and emit
+   uncaught exceptions even when shutdown otherwise succeeds.
+
+These are one design problem: state used to authorize deletion, replacement,
+or resource allocation must be authoritative at the point of use.
+
+## Goals / Success Criteria
+
+- Cleanup never signals a PID or recursively deletes a path solely because it
+  appeared in a pre-confirmation snapshot.
+- Firecracker cleanup proves immediately before action that a PID is still the
+  same Firecracker process and that a run directory is still orphaned.
+- All backend cleanup discovery primitives raise a typed enumeration error on
+  operational failure. No backend may independently continue from a partial
+  snapshot.
+- Shared cleanup control flow lives in the backend layer; concrete backends
+  override resource-specific discovery and validation primitives rather than
+  each implementing a bespoke failure policy.
+- `list-egress-routes` returns an MCP error when attribution or policy
+  resolution is unavailable. A genuine, authoritatively resolved empty policy
+  remains a successful empty list.
+- Supervisor and Git HTTP request bodies have total read deadlines, and each
+  service bounds concurrent request work. Limits apply to authenticated
+  callers because bottles themselves are untrusted.
+- Gateway child-output pumping treats expected stream closure during shutdown
+  as completion while preserving diagnostics for unexpected failures.
+- Unit tests cover PID/path reuse, partial backend enumeration, transient
+  policy resolution failure, slow bodies, concurrency saturation, and stream
+  closure races.
+
+## Non-goals
+
+- Further decomposition solely to reduce module line counts.
+- Replacing gateway stdlib HTTP services with a web framework.
+- Changing egress matching, DLP decisions, proposal semantics, or backend
+  launch behavior beyond the synchronization required for safe cleanup.
+- Making cleanup silently skip uncertain resources. Uncertainty is an
+  operator-visible failure.
+
+## Design
+
+### Shared backend control flow
+
+Follow the backend architecture rule used by gateway attachment: shared
+behavior lives above concrete backends; subclasses provide primitives, not
+control flow.
+
+Cleanup remains previewable, but confirmation authorizes a *new authoritative
+evaluation*, not blind execution of the displayed object. The shared flow:
+
+1. asks each available backend for a preview;
+2. displays the union and asks for confirmation;
+3. refreshes each non-empty backend plan;
+4. validates destructive identities immediately before action;
+5. aborts loudly if the refreshed plan or any identity cannot be proven safe.
+
+Backend-specific primitives define how to identify a resource. Firecracker
+uses process start identity plus canonical config/run paths; container
+backends use authoritative CLI queries and stable resource names/labels.
+
+### Enforcement state versus introspection state
+
+Egress enforcement retains its deny-all fallback because uncertainty must not
+grant network access. Supervisor introspection uses a strict resolver path:
+unattributed callers and resolver failures become typed MCP errors, while a
+successfully resolved policy containing zero routes returns `routes: []`.
+
+### Gateway resource boundaries
+
+Both stdlib servers set a per-connection body deadline before reading and use a
+bounded request executor or semaphore. Saturated capacity fails quickly with a
+service-unavailable response. Existing size caps remain independent:
+supervisor proposals retain the 1 MiB cap and Git pack requests retain their
+larger protocol-appropriate cap.
+
+### Shutdown diagnostics
+
+The gateway output pump catches only stream-closure exceptions expected after
+the supervisor closes child pipes. Other I/O failures remain visible and are
+reported through the supervisor's normal diagnostic channel.
+
+## Implementation chunks
+
+1. Existing fail-closed security and backend enumeration fixes.
+2. FastAPI orchestrator transport and bounded control-plane bodies.
+3. Egress request-policy and outbound-DLP pipeline extraction.
+4. Supervisor MCP dispatch extraction.
+5. Shared cleanup refresh/revalidation plus authoritative macOS discovery.
+6. Strict supervisor introspection and bounded supervisor/Git HTTP work.
+7. Gateway shutdown log-pump closure handling.
+
+## Open questions
+
+None.