From ee8742f6f6c59e9fcc92df2d98d4d51335952d67 Mon Sep 17 00:00:00 2001
From: Benjamin Dauvergne <bdauvergne@entrouvert.com>
Date: Thu, 22 Apr 2021 10:12:36 +0200
Subject: [PATCH 2/2] api: restart fillslot on IntegrityError (#53367)

---
 chrono/api/views.py | 45 ++++++++++++++++++++++++++++++---------------
 1 file changed, 30 insertions(+), 15 deletions(-)

diff --git a/chrono/api/views.py b/chrono/api/views.py
index 6622d8c..7b1c660 100644
--- a/chrono/api/views.py
+++ b/chrono/api/views.py
@@ -19,7 +19,7 @@ import datetime
 import itertools
 import uuid
 
-from django.db import transaction
+from django.db import IntegrityError, transaction
 from django.db.models import Prefetch, Q
 from django.http import Http404, HttpResponse
 from django.shortcuts import get_object_or_404
@@ -918,7 +918,7 @@ class Fillslots(APIView):
     def post(self, request, agenda_identifier=None, event_identifier=None, format=None):
         return self.fillslot(request=request, agenda_identifier=agenda_identifier, format=format)
 
-    def fillslot(self, request, agenda_identifier=None, slots=[], format=None):
+    def fillslot(self, request, agenda_identifier=None, slots=[], format=None, retry=False):
         multiple_booking = bool(not slots)
         try:
             agenda = Agenda.objects.get(slug=agenda_identifier)
@@ -1138,19 +1138,34 @@ class Fillslots(APIView):
             # booking requires real Event objects (not lazy Timeslots);
             # create them now, with data from the slots and the desk we found.
             events = []
-            for start_datetime in datetimes:
-                event = Event.objects.create(
-                    agenda=available_desk.agenda,
-                    slug=str(uuid.uuid4()),  # set slug to avoid queries during slug generation
-                    meeting_type=meeting_type,
-                    start_datetime=start_datetime,
-                    full=False,
-                    places=1,
-                    desk=available_desk,
-                )
-                if resources:
-                    event.resources.add(*resources)
-                events.append(event)
+            try:
+                with transaction.atomic():
+                    for start_datetime in datetimes:
+                        event = Event.objects.create(
+                            agenda=available_desk.agenda,
+                            slug=str(uuid.uuid4()),  # set slug to avoid queries during slug generation
+                            meeting_type=meeting_type,
+                            start_datetime=start_datetime,
+                            full=False,
+                            places=1,
+                            desk=available_desk,
+                        )
+                        if resources:
+                            event.resources.add(*resources)
+                        events.append(event)
+            except IntegrityError:
+                # "optimistic concurrency control", between our availability
+                # check with get_all_slots() and now, new event can have been
+                # created and conflict with the events we want to create, and
+                # so we get an IntegrityError exception. In this case we
+                # restart the fillslot() from the begginning to redo the
+                # availability check and return a proper error to the client.
+                #
+                # To prevent looping, we still raise the IntegrityError during
+                # the second run of fillslot().
+                if retry:
+                    raise
+                return self.fillslot(request, agenda_identifier=agenda_identifier, slots=slots, retry=True)
         else:
             # convert event recurrence identifiers to real event slugs
             for i, slot in enumerate(slots.copy()):
-- 
2.31.0