Limit number of stored failures

Failures are messages shown on the access captcha screen when the
captcha answer was not accepted for whatever reason.

anonstream/__init__.py

@@ -38,7 +38,7 @@ def create_app(toml_config):
     app.captcha_factory = create_captcha_factory(app.config['CAPTCHA_FONTS'])
     app.captcha_signer = create_captcha_signer(app.config['SECRET_KEY'])
 
-    app.failures = {}
+    app.failures = OrderedDict()
 
     # State for tasks
     app.users_update_buffer = set()

anonstream/access.py

@@ -2,6 +2,7 @@ import time
 
 from quart import current_app
 
+CONFIG = current_app.config
 FAILURES = current_app.failures
 
 def add_failure(message):
@@ -9,6 +10,10 @@ def add_failure(message):
     while timestamp in FAILURES:
         timestamp += 1
     FAILURES[timestamp] = message
+
+    while len(FAILURES) > CONFIG['MAX_FAILURES']:
+        FAILURES.popitem(last=False)
+
     return timestamp
 
 def pop_failure(failure_id):

anonstream/config.py

@@ -84,11 +84,13 @@ def toml_to_flask_section_names(config):
 def toml_to_flask_section_memory(config):
     cfg = config['memory']
     assert cfg['states'] >= 0
+    assert cfg['failures'] >= 0
     assert cfg['chat_scrollback'] >= 0
     assert cfg['chat_messages'] >= cfg['chat_scrollback']
     return {
         'MAX_STATES': cfg['states'],
         'MAX_CAPTCHAS': cfg['captchas'],
+        'MAX_FAILURES': cfg['failures'],
         'MAX_CHAT_MESSAGES': cfg['chat_messages'],
         'MAX_CHAT_SCROLLBACK': cfg['chat_scrollback'],
     }

config.toml

@@ -38,6 +38,7 @@ foreground_color = "#dddddd"
 [memory]
 states = 32
 captchas = 256
+failures = 256
 chat_messages = 8192
 chat_scrollback = 256