feat: Rework to make robust to gaps (#1498)

letta/services/block_manager.py

@@ -252,17 +252,18 @@ class BlockManager:
     @enforce_types
     def undo_checkpoint_block(self, block_id: str, actor: PydanticUser, use_preloaded_block: Optional[BlockModel] = None) -> PydanticBlock:
         """
-        Move the block to the previous checkpoint (sequence_number - 1).
+        Move the block to the immediately previous checkpoint in BlockHistory.
+        If older sequences have been pruned, we jump to the largest sequence
+        number that is still < current_seq.
         """
         with self.session_maker() as session:
-            # 1) Load current block
+            # 1) Load the current block
             block = (
                 session.merge(use_preloaded_block)
                 if use_preloaded_block
                 else BlockModel.read(db_session=session, identifier=block_id, actor=actor)
             )
 
-            # 2) Ensure there's a current checkpoint to undo from
             if not block.current_history_entry_id:
                 raise ValueError(f"Block {block_id} has no history entry - cannot undo.")
 
@@ -271,37 +272,39 @@ class BlockManager:
                 raise NoResultFound(f"BlockHistory row not found for id={block.current_history_entry_id}")
 
             current_seq = current_entry.sequence_number
-            if current_seq <= 1:
-                raise ValueError(f"Block {block_id} is at the first checkpoint (seq=1). Cannot undo further.")
 
-            # 3) Move to the previous sequence
-            previous_seq = current_seq - 1
-            block = self._move_block_to_sequence(session, block, previous_seq, actor)
+            # 2) Find the largest sequence < current_seq
+            previous_entry = (
+                session.query(BlockHistory)
+                .filter(BlockHistory.block_id == block.id, BlockHistory.sequence_number < current_seq)
+                .order_by(BlockHistory.sequence_number.desc())
+                .first()
+            )
+            if not previous_entry:
+                # No earlier checkpoint available
+                raise ValueError(f"Block {block_id} is already at the earliest checkpoint (seq={current_seq}). Cannot undo further.")
 
-            # 4) Commit once at the end
+            # 3) Move to that sequence
+            block = self._move_block_to_sequence(session, block, previous_entry.sequence_number, actor)
+
+            # 4) Commit
             session.commit()
-            return block.to_pydantic()  # type: ignore
+            return block.to_pydantic()
 
     @enforce_types
     def redo_checkpoint_block(self, block_id: str, actor: PydanticUser, use_preloaded_block: Optional[BlockModel] = None) -> PydanticBlock:
         """
-        Move the block to the next checkpoint (sequence_number + 1).
-
-        Raises:
-            ValueError: if the block is not pointing to a known checkpoint,
-                        or if there's no higher sequence_number to move to.
-            NoResultFound: if the relevant BlockHistory row doesn't exist.
-            StaleDataError: on concurrency conflicts.
+        Move the block to the next checkpoint if it exists.
+        If some middle checkpoints have been pruned, we jump to the smallest
+        sequence > current_seq that remains.
         """
         with self.session_maker() as session:
-            # 1) Load current block
             block = (
                 session.merge(use_preloaded_block)
                 if use_preloaded_block
                 else BlockModel.read(db_session=session, identifier=block_id, actor=actor)
             )
 
-            # 2) If no current_history_entry_id, can't redo
             if not block.current_history_entry_id:
                 raise ValueError(f"Block {block_id} has no history entry - cannot redo.")
 
@@ -311,15 +314,17 @@ class BlockManager:
 
             current_seq = current_entry.sequence_number
 
-            # We'll move to next_seq = current_seq + 1
-            next_seq = current_seq + 1
-
-            # 3) Move to the next sequence using our helper
-            try:
-                block = self._move_block_to_sequence(session, block, next_seq, actor)
-            except NoResultFound:
+            # Find the smallest sequence that is > current_seq
+            next_entry = (
+                session.query(BlockHistory)
+                .filter(BlockHistory.block_id == block.id, BlockHistory.sequence_number > current_seq)
+                .order_by(BlockHistory.sequence_number.asc())
+                .first()
+            )
+            if not next_entry:
                 raise ValueError(f"Block {block_id} is at the highest checkpoint (seq={current_seq}). Cannot redo further.")
 
-            # 4) Commit once
+            block = self._move_block_to_sequence(session, block, next_entry.sequence_number, actor)
+
             session.commit()
-            return block.to_pydantic()  # type: ignore
+            return block.to_pydantic()