Allow to set fallback secret keys (#4482)

doc/admin/config.rst

@@ -294,6 +294,10 @@ Example::
     setting is not provided, pretix will generate a random secret on the first start
     and will store it in the filesystem for later usage.
 
+``secret_fallback0`` ... ``secret_fallback9``
+    Prior versions of the secret to be used by Django for signing and verification purposes that will still
+    be accepted but no longer be used for new signing.
+
 ``debug``
     Whether or not to run in debug mode. Default is ``False``.
 

src/pretix/settings.py

@@ -94,6 +94,13 @@ else:
                 pass  # os.chown is not available on Windows
             f.write(SECRET_KEY)
 
+
+SECRET_KEY_FALLBACKS = []
+for i in range(10):
+    if config.has_option('django', f'secret_fallback{i}'):
+        SECRET_KEY_FALLBACKS.append(config.get('django', f'secret_fallback{i}'))
+
+
 # Adjustable settings
 
 debug_fallback = "runserver" in sys.argv or "runserver_plus" in sys.argv