M41: harden trust boundaries and delivery

backend/app/services/data_quality.py

@@ -5,7 +5,7 @@ from dataclasses import dataclass, field
 from datetime import UTC, datetime, timedelta
 from difflib import SequenceMatcher
 
-from sqlalchemy import select, update
+from sqlalchemy import func, select, update
 from sqlalchemy.orm import Session
 
 from app.core.errors import AppError
@@ -26,6 +26,7 @@ from app.services.vehicle_status import (
 REQUIRED_CUSTOMER_FIELDS = ("first_name", "last_name")
 REQUIRED_VEHICLE_FIELDS = ("registration_number", "make", "model", "location")
 DUPLICATE_THRESHOLD = 70
+DATA_QUALITY_SCAN_LOCK_ID = 6_138_493_717_091_029_491
 
 
 def issue_due_at(detected_at: datetime, severity: str) -> datetime:
@@ -356,6 +357,9 @@ def _scan_odometer_regressions(db: Session, scan: ScanResult) -> None:
 def run_scan(
     db: Session, *, actor_label: str | None = None, actor_type: str = "user"
 ) -> ScanResult:
+    # The check-then-insert work below spans several rules. Serialise whole scans at the
+    # database boundary so API and n8n triggers cannot both observe an empty condition.
+    db.scalar(select(func.pg_advisory_xact_lock(DATA_QUALITY_SCAN_LOCK_ID)))
     scan = ScanResult()
     _scan_duplicate_customers(db, scan)
     _scan_missing_required_fields(db, scan)
@@ -375,8 +379,13 @@ def run_scan(
     return scan
 
 
-def _load_open_issue(db: Session, public_ref: str) -> DataQualityIssue:
-    issue = db.scalar(select(DataQualityIssue).where(DataQualityIssue.public_ref == public_ref))
+def _load_open_issue(
+    db: Session, public_ref: str, *, lock: bool = True
+) -> DataQualityIssue:
+    statement = select(DataQualityIssue).where(DataQualityIssue.public_ref == public_ref)
+    if lock:
+        statement = statement.with_for_update()
+    issue = db.scalar(statement)
     if issue is None:
         raise AppError("ISSUE_NOT_FOUND", "Data quality issue not found.", status_code=404)
     if issue.status != "open":
@@ -707,8 +716,10 @@ def resolve_booking_overlap(
     return issue
 
 
-def _load_vehicle_status_conflict_issue(db: Session, public_ref: str) -> DataQualityIssue:
-    issue = _load_open_issue(db, public_ref)
+def _load_vehicle_status_conflict_issue(
+    db: Session, public_ref: str, *, lock: bool = True
+) -> DataQualityIssue:
+    issue = _load_open_issue(db, public_ref, lock=lock)
     if issue.rule_type != "vehicle_status_conflict":
         raise AppError(
             "NOT_A_STATUS_CONFLICT_ISSUE",
@@ -724,7 +735,7 @@ def preview_vehicle_status_recommendation(
     """Non-mutating: computes and returns the recommendation only. Never resolves the
     issue, never writes an audit event, never queues automation -- safe to call as often
     as the UI needs (e.g. every time the panel is opened) with zero side effects."""
-    issue = _load_vehicle_status_conflict_issue(db, public_ref)
+    issue = _load_vehicle_status_conflict_issue(db, public_ref, lock=False)
     vehicle = db.scalar(select(Vehicle).where(Vehicle.id == issue.entity_id))
     if vehicle is None:
         raise AppError(