From 975408227bc9e3d0f85cb11758b7049ec1eb46ed Mon Sep 17 00:00:00 2001
From: Ethan Dalool <git@voussoir.net>
Date: Fri, 4 Jun 2021 17:47:38 -0700
Subject: [PATCH] Improve cached_endpoint behavior with sentinel.

---
 .../etiquette_flask/backend/decorators.py     | 19 +++++++++++++++++--
 .../backend/endpoints/album_endpoints.py      |  2 +-
 .../backend/endpoints/tag_endpoints.py        |  2 +-
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/frontends/etiquette_flask/backend/decorators.py b/frontends/etiquette_flask/backend/decorators.py
index e25959a..a3963f6 100644
--- a/frontends/etiquette_flask/backend/decorators.py
+++ b/frontends/etiquette_flask/backend/decorators.py
@@ -4,11 +4,14 @@ import time
 
 from voussoirkit import dotdict
 from voussoirkit import passwordy
+from voussoirkit import sentinel
 
 import etiquette
 
 from . import jsonify
 
+NOT_CACHED = sentinel.Sentinel('not cached', truthyness=False)
+
 def cached_endpoint(max_age):
     '''
     The cached_endpoint decorator can be used on slow endpoints that don't need
@@ -29,12 +32,19 @@ def cached_endpoint(max_age):
     the previous return value (still using 200 or 304 as appropriate for the
     client's provided etag).
 
+    With max_age=0, the function will be run every time to check if the value
+    has changed, but if it hasn't changed then we can still send a 304 response,
+    saving bandwidth.
+
     An example use case would be large-sized data dumps that don't need to be
     precisely up to date every time.
     '''
+    if max_age < 0:
+        raise ValueError(f'max_age should be positive, not {max_age}.')
+
     state = dotdict.DotDict({
         'max_age': max_age,
-        'stored_value': None,
+        'stored_value': NOT_CACHED,
         'stored_etag': None,
         'headers': {'ETag': None, 'Cache-Control': f'max-age={max_age}'},
         'last_run': 0,
@@ -42,7 +52,12 @@ def cached_endpoint(max_age):
 
     def wrapper(function):
         def get_value(*args, **kwargs):
-            if state.max_age and (time.time() - state.last_run) > state.max_age:
+            can_bail = (
+                state.stored_value is not NOT_CACHED and
+                state.max_age != 0 and
+                (time.time() - state.last_run) < state.max_age
+            )
+            if can_bail:
                 return state.stored_value
 
             value = function(*args, **kwargs)
diff --git a/frontends/etiquette_flask/backend/endpoints/album_endpoints.py b/frontends/etiquette_flask/backend/endpoints/album_endpoints.py
index d8e54e4..f6d636b 100644
--- a/frontends/etiquette_flask/backend/endpoints/album_endpoints.py
+++ b/frontends/etiquette_flask/backend/endpoints/album_endpoints.py
@@ -195,7 +195,7 @@ def post_album_show_in_folder(album_id):
 # Album listings ###################################################################################
 
 @site.route('/all_albums.json')
-@decorators.cached_endpoint(max_age=0)
+@decorators.cached_endpoint(max_age=15)
 def get_all_album_names():
     all_albums = {album.display_name: album.id for album in common.P.get_albums()}
     response = {'albums': all_albums}
diff --git a/frontends/etiquette_flask/backend/endpoints/tag_endpoints.py b/frontends/etiquette_flask/backend/endpoints/tag_endpoints.py
index b2d5bfb..988a281 100644
--- a/frontends/etiquette_flask/backend/endpoints/tag_endpoints.py
+++ b/frontends/etiquette_flask/backend/endpoints/tag_endpoints.py
@@ -99,7 +99,7 @@ def post_tag_remove_synonym(tagname):
 # Tag listings #####################################################################################
 
 @site.route('/all_tags.json')
-@decorators.cached_endpoint(max_age=0)
+@decorators.cached_endpoint(max_age=15)
 def get_all_tag_names():
     all_tags = list(common.P.get_all_tag_names())
     all_synonyms = common.P.get_all_synonyms()