---
phase: 04-folders-sharing-quotas-document-ux
plan: "04"
subsystem: sharing-api
tags: [sharing, idor, audit, fastapi, sqlalchemy]
dependency_graph:
  requires:
    - "04-01"  # Folder API (models present)
    - "04-02"  # Document-move API
    - "04-03"  # Audit service (write_audit_log)
  provides:
    - sharing-api-endpoints
  affects:
    - backend/main.py
tech_stack:
  added: []
  patterns:
    - FastAPI router with prefix /api/shares
    - SQLAlchemy async join (Share + User for handle lookup)
    - IntegrityError → HTTP 409 (duplicate share)
    - IDOR protection via owner assertion → 404 (not 403)
key_files:
  created:
    - backend/api/shares.py
  modified:
    - backend/main.py
decisions:
  - "GET /received defined before DELETE /{share_id} to prevent FastAPI path parameter conflict"
  - "DELETE wrong-owner returns 404 not 403 (T-04-04-02 — prevents share ID enumeration)"
  - "write_audit_log uses session.flush within caller transaction (D-14 pattern)"
  - "No quota table touched in shares.py — recipient quota isolation (T-04-04-04)"
metrics:
  duration_seconds: 145
  completed_date: "2026-05-25"
  tasks_total: 1
  tasks_completed: 1
  files_created: 1
  files_modified: 1
---

# Phase 04 Plan 04: Sharing API Summary

## One-liner

JWT-authenticated document sharing API with IDOR-safe revoke (404 on wrong-owner), handle-based recipient lookup, and metadata-only "received" virtual folder.

## What was built

Created `backend/api/shares.py` implementing four endpoints:

1. **POST /api/shares** — grant share by recipient handle; 400 self-share, 404 bad UUID/unknown doc/unknown user, 409 duplicate (UniqueConstraint → IntegrityError)
2. **GET /api/shares?document_id=** — list shares owned by current user for a document, with recipient handle via JOIN
3. **GET /api/shares/received** — virtual "Shared with me" folder; returns metadata only (id, filename, content_type, size_bytes, created_at, owner_handle) — never extracted_text
4. **DELETE /api/shares/{share_id}** — revoke with IDOR protection: `share.owner_id != current_user.id → 404 "Share not found"`

Registered in `backend/main.py` as `shares_router` under the Phase 4 routers section.

## Commits

| Hash | Description |
|------|-------------|
| 964128e | feat(phase-4): Sharing API (SHARE-01..05) — grant by handle, received folder, IDOR-safe revoke |

## Task Results

| Task | Name | Commit | Files |
|------|------|--------|-------|
| 1 | Create backend/api/shares.py — full sharing API | 964128e | backend/api/shares.py (created), backend/main.py (modified) |

## Verification Results

```
tests/test_shares.py — 7 xfailed (all stubs — implementations in 04-05)
Full suite: 1 failed (pre-existing test_extract_docx ModuleNotFoundError), 122 passed, 7 skipped, 39 xfailed
```

The pre-existing failure (`test_extract_docx — ModuleNotFoundError: No module named 'docx'`) is completely unrelated to this plan (missing python-docx package in the local environment). It was already failing before this plan.

## Security Invariants Verified

| Threat | Mitigation | Verified |
|--------|-----------|---------|
| T-04-04-02: Share IDOR on DELETE | `share.owner_id != current_user.id → 404` | grep line 246 |
| T-04-04-03: extracted_text leak | Not included in received response | grep: absent from return dict |
| T-04-04-04: Quota modification | No quotas table touched in shares.py | grep: no `quotas` reference |
| T-04-04-05: Duplicate share DoS | IntegrityError → 409 on UniqueConstraint | line 103-106 |
| T-04-04-06: Doc existence leak | Ownership assertion → 404 (not 403) | line 99-100 |

## Acceptance Criteria

- [x] backend/api/shares.py exists with all four endpoint functions
- [x] GET /api/shares/received defined before DELETE /{share_id} (line 187 vs 228)
- [x] DELETE checks `share.owner_id != current_user.id` → 404
- [x] IntegrityError → 409 for duplicate share
- [x] write_audit_log called for share.granted and share.revoked (lines 110, 255)
- [x] GET /received does NOT include extracted_text
- [x] `python3 -c "from api.shares import router"` exits 0
- [x] test_share_revoke_wrong_owner_404 is xfail (not FAILED)
- [x] Full suite has zero FAILED from this plan's changes

## Deviations from Plan

None — plan executed exactly as written. Route ordering, IDOR pattern, audit log calls, and quota isolation all implemented per specification.

## Known Stubs

None — all four endpoints are fully implemented. Test stubs in test_shares.py are intentional xfail stubs that will be implemented in plan 04-05 (per the test file docstring).

## Threat Flags

None — all trust boundaries and STRIDE threats from the plan's threat model are mitigated in the implementation.

## Self-Check: PASSED

- [x] backend/api/shares.py exists
- [x] backend/main.py includes shares_router
- [x] Commit 964128e exists in git log
- [x] All 7 share tests are xfail (zero FAILED)