chore: better epoch checking

Author: jcnelson

stackslib/src/chainstate/burn/db/sortdb.rs

@@ -2859,6 +2859,39 @@ impl SortitionDB {
         if existing_epochs == epochs {
             return Ok(());
         }
+
+        let tip = SortitionDB::get_canonical_burn_chain_tip(db_tx)?;
+        let existing_epoch_idx = StacksEpoch::find_epoch(&existing_epochs, tip.block_height)
+            .unwrap_or_else(|| {
+                panic!("FATAL: Sortition tip {} has no epoch in its existing epochs table", tip.block_height);
+            });
+
+        let new_epoch_idx = StacksEpoch::find_epoch(&epochs, tip.block_height)
+            .unwrap_or_else(|| {
+                panic!("FATAL: Sortition tip {} has no epoch in the configured epochs list", tip.block_height);
+            });
+
+        // can't retcon epochs -- all epochs up to (but excluding) the tip's epoch in both epoch
+        // lists must be the same.
+        for i in 0..existing_epoch_idx.min(new_epoch_idx) {
+            if existing_epochs[i] != epochs[i] {
+                panic!("FATAL: tried to retcon epoch {:?} into epoch {:?}", &existing_epochs[i], &epochs[i]);
+            }
+        }
+
+        // can't change parameters of the current epoch in either epoch list,
+        // except for the end height (and only if it hasn't been reached yet)
+        let mut diff_epoch = existing_epochs[existing_epoch_idx].clone();
+        diff_epoch.end_height = epochs[new_epoch_idx].end_height;
+
+        if diff_epoch != epochs[new_epoch_idx] {
+            panic!("FATAL: tried to change current epoch {:?} into {:?}", &existing_epochs[existing_epoch_idx], &epochs[new_epoch_idx]);
+        }
+
+        if tip.block_height >= epochs[new_epoch_idx].end_height {
+            panic!("FATAL: tip has reached or passed the end of the configured epoch");
+        }
+
         info!("Replace existing epochs with new epochs");
         db_tx.execute("DELETE FROM epochs;", NO_PARAMS)?;
         for epoch in epochs.into_iter() {