Prevent duplicative merge attempts.

web/main/models.py

@@ -34,7 +34,7 @@
 from django.core.exceptions import ValidationError
 from django.core.paginator import Paginator, Page
 from django.core.validators import MaxLengthValidator, validate_unicode_slug
-from django.db import ProgrammingError, connection, models, transaction
+from django.db import DatabaseError, ProgrammingError, connection, models, transaction
 from django.db.models import Count, F, JSONField, Q, QuerySet
 
 from django.template.defaultfilters import truncatechars
@@ -2964,10 +2964,17 @@ def merge_draft(self):
         See main/test/test_drafts.py
         """
         # set up variables
-        draft = self
-        if not self.is_draft:
-            raise ValueError("Only draft casebooks may be merged")
-        parent = self.draft_of
+        try:
+            # Technique from https://github.com/harvard-lil/capstone/blob/0f7fb80f26e753e36e0c7a6a199b8fdccdd318be/capstone/capapi/serializers.py#L121
+            #
+            # Fetch casebooks here inside a transaction, using select_for_update
+            # to lock the rows so we don't collide with any simultaneous requests
+            draft = Casebook.objects.select_for_update(nowait=True).get(pk=self.pk)
+            if not draft.is_draft:
+                raise ValueError("Only draft casebooks may be merged")
+            parent = Casebook.objects.select_for_update(nowait=True).get(draft=draft.id)
+        except DatabaseError:
+            raise ValueError("This casebook's draft is already being merged.")
 
         # swap all attributes
 

web/main/test/test_drafts.py

@@ -1,7 +1,11 @@
+from concurrent.futures import ThreadPoolExecutor
 from main.models import User, Section, Resource
+from django.db import connections
 
 from test.test_helpers import dump_casebook_outline
 
+import pytest
+
 
 def test_merge_drafts(reset_sequences, full_casebook, assert_num_queries, legal_document_factory):
 
@@ -59,3 +63,26 @@ def test_merge_drafts(reset_sequences, full_casebook, assert_num_queries, legal_
 
     # Clones of the original casebook have proper attribution
     assert elena in second_casebook.attributed_authors
+
+
+@pytest.mark.django_db(transaction=True)
+def test_duplicative_merge_prevented(full_casebook_with_draft):
+    """ Fetch two jobs at the same time in threads and make sure same job isn't returned to both. """
+    draft = full_casebook_with_draft.draft
+
+    def attempt_merge(i):
+        try:
+            draft.merge_draft()
+            return True
+        except Exception as e:
+            return e
+        finally:
+            for connection in connections.all():
+                connection.close()
+
+    with ThreadPoolExecutor(max_workers=2) as e:
+        results = e.map(attempt_merge, range(2))
+
+    first, second = list(results)
+    assert (first is True and "already being merged" in str(second)) or \
+           (second is True and "already being merged" in str(first))