REST API: Add support for idempotency keys

src/pretix/api/middleware.py

@@ -0,0 +1,77 @@
+import json
+from hashlib import sha1
+
+from django.conf import settings
+from django.db import transaction
+from django.http import HttpRequest, HttpResponse, JsonResponse
+from django.utils.timezone import now
+from rest_framework import status
+
+from pretix.api.models import ApiCall
+
+
+class IdempotencyMiddleware:
+    def __init__(self, get_response):
+        self.get_response = get_response
+
+    def __call__(self, request: HttpRequest):
+        if request.method in ('GET', 'HEAD', 'OPTIONS'):
+            return self.get_response(request)
+
+        if not request.path.startswith('/api/'):
+            return self.get_response(request)
+
+        if not request.META.get('HTTP_X_IDEMPOTENCY_KEY'):
+            return self.get_response(request)
+
+        auth_hash_parts = '{}:{}'.format(
+            request.META.get('HTTP_AUTHORIZATION', ''),
+            request.COOKIES.get(settings.SESSION_COOKIE_NAME, '')
+        )
+        auth_hash = sha1(auth_hash_parts.encode()).hexdigest()
+        idempotency_key = request.META.get('HTTP_X_IDEMPOTENCY_KEY', '')
+
+        with transaction.atomic():
+            call, created = ApiCall.objects.select_for_update().get_or_create(
+                auth_hash=auth_hash,
+                idempotency_key=idempotency_key,
+                defaults={
+                    'locked': now(),
+                    'request_method': request.method,
+                    'request_path': request.path,
+                    'response_code': 0,
+                    'response_headers': '{}',
+                    'response_body': b''
+                }
+            )
+
+        if created:
+            resp = self.get_response(request)
+            with transaction.atomic():
+                if resp.status_code in (409, 429, 503):
+                    # This is the exception: These calls are *meant* to be retried!
+                    call.delete()
+                else:
+                    call.response_code = resp.status_code
+                    call.response_body = resp.content.encode() if isinstance(resp.content, str) else resp.content
+                    call.response_headers = json.dumps(resp._headers)
+                    call.locked = None
+                    call.save(update_fields=['locked', 'response_code', 'response_headers',
+                                             'response_body'])
+            return resp
+        else:
+            if call.locked:
+                r = JsonResponse(
+                    {'detail': 'Concurrent request with idempotency key.'},
+                    status=status.HTTP_409_CONFLICT,
+                )
+                r['Retry-After'] = 5
+                return r
+
+            r = HttpResponse(
+                content=call.response_body,
+                status=call.response_code,
+            )
+            for k, v in json.loads(call.response_headers).values():
+                r[k] = v
+            return r