issue.stored_event_count: consequences for 'irrelevance'

document & assert

events/retention.py

@@ -83,6 +83,9 @@ def get_random_irrelevance(stored_event_count):
     if `cnt` "hovers" around a certain value (which is likely to happen when there's repeated eviction/fill-up). ×2 is
     simply to correct for random() (which returns .5 on average).
     """
+    # assert as a tripwire to check our assumptions; note that the actual calculation actually "succeeds" for < 1, but
+    # it becomes non-sensical, so I'd rather have it fail. The present event is part of the count, i.e. always >= 1
+    assert stored_event_count >= 1
     return nonzero_leading_bits(round(random() * stored_event_count * 2))
 
 

issues/migrations/0013_fix_issue_stored_event_counts.py

@@ -18,6 +18,10 @@ def fill_stored_event_count(apps, schema_editor):
             issue.stored_event_count = correct_value
             issue.save()
 
+    # Note: because "irrelevances" have been calculated based on stored_event_count, those will be wrong too... but
+    # there doesn't seem to be an easy way to recover from that, so we'll just let the problem fix itself over time.
+    # easy meaning: both easy-enough to write a datamigration for, and not super-expensive to run.
+
 
 class Migration(migrations.Migration):