added a lock on a critical path when regenerating pages. this should allow all images to load correctly

cbreader/settings/base.py

@@ -38,6 +38,7 @@ INSTALLED_APPS = (
     'bootstrap4',
     "comic",
     "comic_auth",
+    'db_mutex',
 )
 
 MIDDLEWARE = [

comic/models.py

@@ -13,7 +13,8 @@ from django.db.transaction import atomic
 from django.utils.http import urlsafe_base64_encode
 import PyPDF4
 import PyPDF4.utils
-
+from db_mutex import DBMutexError, DBMutexTimeoutError
+from db_mutex.db_mutex import db_mutex
 
 import rarfile
 
@@ -114,15 +115,17 @@ class ComicBook(models.Model):
         try:
             page_obj = ComicPage.objects.get(Comic=self, index=page)
         except ComicPage.MultipleObjectsReturned:
-            ComicPage.objects.filter(Comic=self).delete()
-            self.process_comic_pages(archive, self)
-            page_obj = ComicPage.objects.get(Comic=self, index=page)
+            with db_mutex('comicpage'):
+                ComicPage.objects.filter(Comic=self).delete()
+                self.process_comic_pages(archive, self)
+                page_obj = ComicPage.objects.get(Comic=self, index=page)
         try:
             out = (archive.open(page_obj.page_file_name), page_obj.content_type)
         except rarfile.NoRarEntry:
-            ComicPage.objects.filter(Comic=self).delete()
-            self.process_comic_pages(archive, self)
-            out = self.get_image(page)
+            with db_mutex('comicpage'):
+                ComicPage.objects.filter(Comic=self).delete()
+                self.process_comic_pages(archive, self)
+                out = self.get_image(page)
         return out
 
     def is_last_page(self, page):

poetry.lock

@@ -111,6 +111,17 @@ django = ">=2.2,<4.0"
 [package.extras]
 docs = ["sphinx (>=2.4,<3.0)", "sphinx_rtd_theme (>=0.4.3,<0.5.0)", "m2r2 (>=0.2.5,<0.3.0)"]
 
+[[package]]
+name = "django-db-mutex"
+version = "2.0.0"
+description = "Acquire a mutex via the DB in Django"
+category = "main"
+optional = false
+python-versions = "*"
+
+[package.dependencies]
+Django = ">=2.2"
+
 [[package]]
 name = "django-recaptcha2"
 version = "1.4.1"
@@ -363,7 +374,7 @@ dev = ["pytest (>=4.6.2)", "black (>=19.3b0)"]
 [metadata]
 lock-version = "1.1"
 python-versions = "^3.8"
-content-hash = "2cf2bf8371f6fa6dcc2a9d8d889387667eed3a994e46797c5113f8ea7e7ffd95"
+content-hash = "a4bc175a3b3f8918926aae3b74d4d5fee3111ddc61dcb3b37c353c0e99edc277"
 
 [metadata.files]
 asgiref = [
@@ -457,6 +468,10 @@ django-bootstrap4 = [
     {file = "django-bootstrap4-2.3.1.tar.gz", hash = "sha256:2c199020ac38866cdf8d1c5561ce7468116b9685b455a29843c0225ef8568879"},
     {file = "django_bootstrap4-2.3.1-py3-none-any.whl", hash = "sha256:b68f073b647b20ec7894a252a0ca4e06b7b8dafdbad995cb0cdc783d0bb4629d"},
 ]
+django-db-mutex = [
+    {file = "django-db-mutex-2.0.0.tar.gz", hash = "sha256:b8f3466611ac0045ec4d07f47ded159d08a3780ebc95c48c86af3c909fb2e3e6"},
+    {file = "django_db_mutex-2.0.0-py2.py3-none-any.whl", hash = "sha256:6d41176f4094c7a32c0ad6d157a0c268f23ca747ee5f32eadf7a18efe369715e"},
+]
 django-recaptcha2 = [
     {file = "django-recaptcha2-1.4.1.tar.gz", hash = "sha256:c0b43851b05c6bf6ebb5ecc890c13ccedacd9bb33d64b4291c74dd6fcbc89366"},
     {file = "django_recaptcha2-1.4.1-py3-none-any.whl", hash = "sha256:9ea90db0cec502741be1066c09ec1b8e02a73162a319a042e78e67c4605087af"},

pyproject.toml

@@ -23,6 +23,7 @@ mysqlclient = "^2.0.1"
 psycopg2 = "^2.8.6"
 rarfile = "^4.0"
 coverage = "^5.5"
+django-db-mutex = "^2.0.0"
 
 [tool.poetry.dev-dependencies]