turso/core/mvcc/cursor.rs

use parking_lot::RwLock;

use crate::mvcc::clock::LogicalClock;
use crate::mvcc::database::{MVTableId, MvStore, Row, RowID, RowVersionState};
use crate::storage::btree::{BTreeCursor, BTreeKey, CursorTrait};
use crate::translate::plan::IterationDirection;
use crate::types::{IOResult, ImmutableRecord, RecordCursor, SeekKey, SeekOp, SeekResult};
use crate::{return_if_io, Result};
use crate::{Pager, Value};
use std::any::Any;
use std::cell::{Ref, RefCell};
use std::fmt::Debug;
use std::ops::Bound;
use std::sync::Arc;

#[derive(Debug, Copy, Clone)]
enum CursorPosition {
    /// We haven't loaded any row yet.
    BeforeFirst,
    /// We have loaded a row. This position points to a rowid in either MVCC index or in BTree.
    Loaded {
        row_id: RowID,
        /// Indicates whether the rowid is pointing BTreeCursor or MVCC index.
        in_btree: bool,
        /// Indicates whether the rowid poiting in BTreeCursor has been consumed.
        /// This is required to know whether `next` or `prev` has been called but not yet consumed.
        btree_consumed: bool,
    },
    /// We have reached the end of the table.
    End,
}

#[derive(Debug, Clone, Copy)]
enum NextState {
    NextBtree {
        new_position_in_mvcc: CursorPosition,
    },
}

#[derive(Debug, Clone, Copy)]
enum PrevState {
    PrevBtree {
        new_position_in_mvcc: CursorPosition,
    },
}
#[derive(Debug, Clone, Copy)]
enum MvccLazyCursorState {
    Next(NextState),
    Prev(PrevState),
}

pub struct MvccLazyCursor<Clock: LogicalClock> {
    pub db: Arc<MvStore<Clock>>,
    current_pos: RefCell<CursorPosition>,
    pub table_id: MVTableId,
    tx_id: u64,
    /// Reusable immutable record, used to allow better allocation strategy.
    reusable_immutable_record: RefCell<Option<ImmutableRecord>>,
    btree_cursor: Box<dyn CursorTrait>,
    null_flag: bool,
    record_cursor: RefCell<RecordCursor>,
    next_rowid_lock: Arc<RwLock<()>>,
    state: RefCell<Option<MvccLazyCursorState>>,
}

impl<Clock: LogicalClock + 'static> MvccLazyCursor<Clock> {
    pub fn new(
        db: Arc<MvStore<Clock>>,
        tx_id: u64,
        root_page_or_table_id: i64,
        btree_cursor: Box<dyn CursorTrait>,
    ) -> Result<MvccLazyCursor<Clock>> {
        assert!(
            (&*btree_cursor as &dyn Any).is::<BTreeCursor>(),
            "BTreeCursor expected for mvcc cursor"
        );
        let table_id = db.get_table_id_from_root_page(root_page_or_table_id);
        Ok(Self {
            db,
            tx_id,
            current_pos: RefCell::new(CursorPosition::BeforeFirst),
            table_id,
            reusable_immutable_record: RefCell::new(None),
            btree_cursor,
            null_flag: false,
            record_cursor: RefCell::new(RecordCursor::new()),
            next_rowid_lock: Arc::new(RwLock::new(())),
            state: RefCell::new(None),
        })
    }

    /// Returns the current row as an immutable record.
    pub fn current_row(
        &self,
    ) -> Result<IOResult<Option<std::cell::Ref<'_, crate::types::ImmutableRecord>>>> {
        match *self.current_pos.borrow() {
            CursorPosition::Loaded {
                row_id: _,
                in_btree,
                btree_consumed: _,
            } => {
                if in_btree {
                    let IOResult::Done(Some(record)) = self.btree_cursor.record()? else {
                        panic!("BTree should have returned record");
                    };
                    Ok(IOResult::Done(Some(record)))
                } else {
                    let Some(row) = self.read_mvcc_current_row()? else {
                        return Ok(IOResult::Done(None));
                    };
                    {
                        let mut record = self.get_immutable_record_or_create();
                        let record = record.as_mut().unwrap();
                        record.invalidate();
                        record.start_serialization(&row.data);
                    }

                    let record_ref =
                        Ref::filter_map(self.reusable_immutable_record.borrow(), |opt| {
                            opt.as_ref()
                        })
                        .unwrap();
                    Ok(IOResult::Done(Some(record_ref)))
                }
            }
            CursorPosition::BeforeFirst => {
                // Before first is not a valid position, so we return none.
                Ok(IOResult::Done(None))
            }
            CursorPosition::End => Ok(IOResult::Done(None)),
        }
    }

    pub fn read_mvcc_current_row(&self) -> Result<Option<Row>> {
        let row_id = match *self.current_pos.borrow() {
            CursorPosition::Loaded {
                row_id,
                in_btree,
                btree_consumed: _,
            } if !in_btree => row_id,
            _ => panic!("invalid position to read current mvcc row"),
        };
        self.db.read(self.tx_id, row_id)
    }

    pub fn close(self) -> Result<()> {
        Ok(())
    }

    pub fn get_next_rowid(&mut self) -> i64 {
        // lock so we don't get same two rowids
        let lock = self.next_rowid_lock.clone();
        let _lock = lock.write();
        let _ = self.last();
        match *self.current_pos.borrow() {
            CursorPosition::Loaded {
                row_id,
                in_btree: _,
                btree_consumed: _,
            } => row_id.row_id + 1,
            CursorPosition::BeforeFirst => 1,
            CursorPosition::End => 1,
        }
    }

    fn get_immutable_record_or_create(&self) -> std::cell::RefMut<'_, Option<ImmutableRecord>> {
        let mut reusable_immutable_record = self.reusable_immutable_record.borrow_mut();
        if reusable_immutable_record.is_none() {
            let record = ImmutableRecord::new(1024);
            reusable_immutable_record.replace(record);
        }
        reusable_immutable_record
    }

    fn get_current_pos(&self) -> CursorPosition {
        *self.current_pos.borrow()
    }

    /// Returns the new position of the cursor based on the new position in MVCC and the current rowid in BTree.
    /// If we are moving forwards -> choose smaller rowid (mvcc if mvcc < btree, else btree)
    /// If we are moving backwards -> choose larger rowid (mvcc if mvcc > btree, else btree)
    fn get_new_position_from_mvcc_and_btree(
        &mut self,
        new_position_in_mvcc: &Option<i64>,
        current_rowid_in_btree: &Option<i64>,
        direction: IterationDirection,
    ) -> CursorPosition {
        tracing::trace!("get_new_position_from_mvcc_and_btree(new_position_in_mvcc={:?}, current_rowid_in_btree={:?}, direction={:?})", new_position_in_mvcc, current_rowid_in_btree, direction);
        match (new_position_in_mvcc, current_rowid_in_btree) {
            (Some(mvcc_rowid), Some(btree_rowid)) => {
                // When forwards: choose smaller rowid (mvcc if mvcc < btree, else btree)
                // When backwards: choose larger rowid (mvcc if mvcc > btree, else btree)
                let use_mvcc = if direction == IterationDirection::Forwards {
                    mvcc_rowid < btree_rowid
                } else {
                    mvcc_rowid > btree_rowid
                };

                let (row_id, in_btree, btree_consumed) = if use_mvcc {
                    (*mvcc_rowid, false, false)
                } else {
                    (*btree_rowid, true, true)
                };

                CursorPosition::Loaded {
                    row_id: RowID {
                        table_id: self.table_id,
                        row_id,
                    },
                    in_btree,
                    btree_consumed,
                }
            }
            (None, Some(btree_rowid)) => CursorPosition::Loaded {
                row_id: RowID {
                    table_id: self.table_id,
                    row_id: *btree_rowid,
                },
                in_btree: true,
                btree_consumed: true,
            },
            (Some(mvcc_rowid), None) => CursorPosition::Loaded {
                row_id: RowID {
                    table_id: self.table_id,
                    row_id: *mvcc_rowid,
                },
                in_btree: false,
                btree_consumed: true,
            },
            (None, None) => CursorPosition::End,
        }
    }

    fn is_btree_allocated(&self) -> bool {
        let maybe_root_page = self.db.table_id_to_rootpage.get(&self.table_id);
        maybe_root_page.is_some_and(|entry| entry.value().is_some())
    }

    fn query_btree_version_is_valid(&self, row_id: i64) -> bool {
        // If the row is not found in MVCC index, this means row_id is valid in btree
        matches!(
            self.db
                .find_row_last_version_state(self.table_id, row_id, self.tx_id),
            RowVersionState::NotFound
        )
    }
}

impl<Clock: LogicalClock + 'static> CursorTrait for MvccLazyCursor<Clock> {
    fn last(&mut self) -> Result<IOResult<()>> {
        if self.is_btree_allocated() {
            return_if_io!(self.btree_cursor.last());
        }

        self.invalidate_record();
        self.current_pos.replace(CursorPosition::End);

        let position_in_mvcc = self.db.get_last_rowid(self.table_id);
        let position_in_btree = if self.is_btree_allocated() {
            let IOResult::Done(maybe_rowid) = self.btree_cursor.rowid()? else {
                panic!("BTree should have returned rowid after last");
            };
            maybe_rowid
        } else {
            None
        };
        let new_position = self.get_new_position_from_mvcc_and_btree(
            &position_in_mvcc,
            &position_in_btree,
            IterationDirection::Backwards,
        );
        self.current_pos.replace(new_position);
        self.invalidate_record();
        Ok(IOResult::Done(()))
    }

    /// Move the cursor to the next row. Returns true if the cursor moved to the next row, false if the cursor is at the end of the table.
    fn next(&mut self) -> Result<IOResult<bool>> {
        let current_state = *self.state.borrow();
        if current_state.is_none() {
            let before_first = matches!(self.get_current_pos(), CursorPosition::BeforeFirst);
            let min_id = match *self.current_pos.borrow() {
                CursorPosition::Loaded {
                    row_id,
                    in_btree: _,
                    btree_consumed: _,
                } => row_id.row_id + 1,
                // TODO: do we need to forward twice?
                CursorPosition::BeforeFirst => i64::MIN, // we need to find first row, so we look from the first id,
                CursorPosition::End => {
                    // let's keep same state, we reached the end so no point in moving forward.
                    return Ok(IOResult::Done(false));
                }
            };

            let new_position_in_mvcc =
                match self
                    .db
                    .get_next_row_id_for_table(self.table_id, min_id, self.tx_id)
                {
                    Some(id) => CursorPosition::Loaded {
                        row_id: id,
                        in_btree: false,
                        btree_consumed: false,
                    },
                    None => {
                        if before_first {
                            // if it wasn't loaded and we didn't find anything, it means the table is empty.
                            CursorPosition::BeforeFirst
                        } else {
                            // if we had something loaded, and we didn't find next key then it means we are at the end.
                            CursorPosition::End
                        }
                    }
                };
            self.state
                .replace(Some(MvccLazyCursorState::Next(NextState::NextBtree {
                    new_position_in_mvcc,
                })));
        }
        // Now we need to loop for next rowid in btree that is valid.
        // FIXME: this is quite unperformant, we should find a better way to do this.
        loop {
            let current_state = *self.state.borrow();
            let Some(MvccLazyCursorState::Next(NextState::NextBtree {
                new_position_in_mvcc,
            })) = current_state
            else {
                panic!("Invalid state {:?}", self.state.borrow());
            };

            // Check whether we have already consumed the rowid in btree. In BeforeFirst we can assume we haven't started calling next yet.
            // In End we can assume we have already called next and it returned false, so we can assume we have consumed the rowid.
            let btree_consumed = match self.get_current_pos() {
                CursorPosition::Loaded {
                    row_id: _,
                    in_btree: _,
                    btree_consumed,
                } => btree_consumed,
                CursorPosition::BeforeFirst => true,
                CursorPosition::End => true,
            };

            let found = if self.is_btree_allocated() {
                // If we have a functional btree, let's either find next value, or use the one pointed at by the cursor.
                if btree_consumed {
                    return_if_io!(self.btree_cursor.next())
                } else {
                    true
                }
            } else {
                // If we don't have a functional btree, we can't find next value, so we return false.
                false
            };
            // get current rowid in mvcc and in btree
            // compare both and set loaded to position of the one that is lesser
            let new_position_in_mvcc = match new_position_in_mvcc {
                CursorPosition::Loaded {
                    row_id,
                    in_btree: _,
                    btree_consumed: _,
                } => Some(row_id.row_id),
                CursorPosition::BeforeFirst => None,
                CursorPosition::End => None,
            };
            let current_rowid_in_btree = if found {
                let IOResult::Done(Some(rowid)) = self.btree_cursor.rowid()? else {
                    panic!("BTree should have returned rowid after next");
                };
                if self.query_btree_version_is_valid(rowid) {
                    Some(rowid)
                } else {
                    // if the row is not valid, we need to continue to the next rowid in btree.
                    //  We first set consumed to true so that next time we call next, we don't use the same rowid.
                    if let CursorPosition::Loaded { btree_consumed, .. } =
                        &mut *self.current_pos.borrow_mut()
                    {
                        *btree_consumed = true;
                    }
                    continue;
                }
            } else {
                None
            };
            let new_position = self.get_new_position_from_mvcc_and_btree(
                &new_position_in_mvcc,
                &current_rowid_in_btree,
                IterationDirection::Forwards,
            );
            self.current_pos.replace(new_position);
            self.invalidate_record();
            self.state.replace(None);

            return Ok(IOResult::Done(matches!(
                self.get_current_pos(),
                CursorPosition::Loaded { .. }
            )));
        }
    }

    fn prev(&mut self) -> Result<IOResult<bool>> {
        let current_state = *self.state.borrow();
        if current_state.is_none() {
            let max_id = match *self.current_pos.borrow() {
                CursorPosition::Loaded {
                    row_id,
                    in_btree: _,
                    btree_consumed: _,
                } => row_id.row_id,
                CursorPosition::BeforeFirst => {
                    return Ok(IOResult::Done(false));
                }
                CursorPosition::End => {
                    i64::MAX // we need to find last row, so we look from the last id,
                }
            };

            let new_position_in_mvcc =
                match self
                    .db
                    .get_prev_row_id_for_table(self.table_id, max_id, self.tx_id)
                {
                    Some(id) => CursorPosition::Loaded {
                        row_id: id,
                        in_btree: false,
                        btree_consumed: false,
                    },
                    None => CursorPosition::BeforeFirst,
                };
            self.state
                .replace(Some(MvccLazyCursorState::Prev(PrevState::PrevBtree {
                    new_position_in_mvcc,
                })));
        }
        // Now we need to loop for prev rowid in btree that is valid.
        // FIXME: this is quite unperformant, we should find a better way to do this.
        loop {
            let current_state = *self.state.borrow();
            let Some(MvccLazyCursorState::Prev(PrevState::PrevBtree {
                new_position_in_mvcc,
            })) = current_state
            else {
                panic!("Invalid state {:?}", self.state.borrow());
            };

            // Check whether we have already consumed the rowid in btree. In BeforeFirst we can assume we haven't started calling next yet.
            // In End we can assume we have already called next and it returned false, so we can assume we have consumed the rowid.
            let btree_consumed = match self.get_current_pos() {
                CursorPosition::Loaded {
                    row_id: _,
                    in_btree: _,
                    btree_consumed,
                } => btree_consumed,
                CursorPosition::BeforeFirst => true,
                CursorPosition::End => true,
            };

            let found = if self.is_btree_allocated() {
                // If we have a functional btree, let's either find next value, or use the one pointed at by the cursor.
                if btree_consumed {
                    return_if_io!(self.btree_cursor.prev())
                } else {
                    true
                }
            } else {
                // If we don't have a functional btree, we can't find next value, so we return false.
                false
            };
            // get current rowid in mvcc and in btree
            let maybe_rowid_mvcc = match new_position_in_mvcc {
                CursorPosition::Loaded {
                    row_id,
                    in_btree: _,
                    btree_consumed: _,
                } => Some(row_id.row_id),
                CursorPosition::BeforeFirst => None,
                CursorPosition::End => None,
            };
            let maybe_rowid_in_btree = if found {
                let IOResult::Done(Some(rowid)) = self.btree_cursor.rowid()? else {
                    panic!("BTree should have returned rowid after prev because we found a valid rowid after calling btree_cursor.prev()");
                };
                if self.query_btree_version_is_valid(rowid) {
                    Some(rowid)
                } else {
                    // if the row is not valid, we need to continue to the next rowid in btree.
                    //  We first set consumed to true so that next time we call next, we don't use the same rowid.
                    if let CursorPosition::Loaded { btree_consumed, .. } =
                        &mut *self.current_pos.borrow_mut()
                    {
                        *btree_consumed = true;
                    }
                    continue;
                }
            } else {
                None
            };
            // Update based on direction.
            let new_position = self.get_new_position_from_mvcc_and_btree(
                &maybe_rowid_mvcc,
                &maybe_rowid_in_btree,
                IterationDirection::Backwards,
            );
            self.current_pos.replace(new_position);
            self.invalidate_record();
            self.state.replace(None);

            return Ok(IOResult::Done(matches!(
                self.get_current_pos(),
                CursorPosition::Loaded { .. }
            )));
        }
    }

    fn rowid(&self) -> Result<IOResult<Option<i64>>> {
        let rowid = match self.get_current_pos() {
            CursorPosition::Loaded {
                row_id,
                in_btree: _,
                btree_consumed: _,
            } => Some(row_id.row_id),
            CursorPosition::BeforeFirst => None,
            CursorPosition::End => None,
        };
        Ok(IOResult::Done(rowid))
    }

    fn record(
        &self,
    ) -> Result<IOResult<Option<std::cell::Ref<'_, crate::types::ImmutableRecord>>>> {
        self.current_row()
    }

    fn seek(&mut self, seek_key: SeekKey<'_>, op: SeekOp) -> Result<IOResult<SeekResult>> {
        let row_id = match seek_key {
            SeekKey::TableRowId(row_id) => row_id,
            SeekKey::IndexKey(_) => {
                todo!();
            }
        };
        // gt -> lower_bound bound excluded, we want first row after row_id
        // ge -> lower_bound bound included, we want first row equal to row_id or first row after row_id
        // lt -> upper_bound bound excluded, we want last row before row_id
        // le -> upper_bound bound included, we want last row equal to row_id or first row before row_id
        let rowid = RowID {
            table_id: self.table_id,
            row_id,
        };
        let (bound, lower_bound) = match op {
            SeekOp::GT => (Bound::Excluded(&rowid), true),
            SeekOp::GE { eq_only: _ } => (Bound::Included(&rowid), true),
            SeekOp::LT => (Bound::Excluded(&rowid), false),
            SeekOp::LE { eq_only: _ } => (Bound::Included(&rowid), false),
        };
        self.invalidate_record();
        let rowid = self.db.seek_rowid(bound, lower_bound, self.tx_id);
        if let Some(rowid) = rowid {
            self.current_pos.replace(CursorPosition::Loaded {
                row_id: rowid,
                in_btree: false,
                btree_consumed: false,
            });
            if op.eq_only() {
                if rowid.row_id == row_id {
                    Ok(IOResult::Done(SeekResult::Found))
                } else {
                    Ok(IOResult::Done(SeekResult::NotFound))
                }
            } else {
                Ok(IOResult::Done(SeekResult::Found))
            }
        } else {
            let forwards = matches!(op, SeekOp::GE { eq_only: _ } | SeekOp::GT);
            if forwards {
                let _ = self.last()?;
            } else {
                let _ = self.rewind()?;
            }
            Ok(IOResult::Done(SeekResult::NotFound))
        }
    }

    /// Insert a row into the table.
    /// Sets the cursor to the inserted row.
    fn insert(&mut self, key: &BTreeKey) -> Result<IOResult<()>> {
        let Some(rowid) = key.maybe_rowid() else {
            todo!()
        };
        let row_id = RowID::new(self.table_id, rowid);
        let record_buf = key.get_record().unwrap().get_payload().to_vec();
        let num_columns = match key {
            BTreeKey::IndexKey(record) => record.column_count(),
            BTreeKey::TableRowId((_, record)) => record.as_ref().unwrap().column_count(),
        };
        let row = crate::mvcc::database::Row::new(row_id, record_buf, num_columns);

        self.current_pos.replace(CursorPosition::Loaded {
            row_id: row.id,
            in_btree: false,
            btree_consumed: true,
        });
        // FIXME: set btree to somewhere close to this rowid?
        if self.db.read(self.tx_id, row.id)?.is_some() {
            self.db.update(self.tx_id, row).inspect_err(|_| {
                self.current_pos.replace(CursorPosition::BeforeFirst);
            })?;
        } else {
            self.db.insert(self.tx_id, row).inspect_err(|_| {
                self.current_pos.replace(CursorPosition::BeforeFirst);
            })?;
        }
        self.invalidate_record();
        Ok(IOResult::Done(()))
    }

    fn delete(&mut self) -> Result<IOResult<()>> {
        let IOResult::Done(Some(rowid)) = self.rowid()? else {
            todo!();
        };
        let rowid = RowID::new(self.table_id, rowid);
        self.db.delete(self.tx_id, rowid)?;
        self.invalidate_record();
        Ok(IOResult::Done(()))
    }

    fn set_null_flag(&mut self, flag: bool) {
        self.null_flag = flag;
    }

    fn get_null_flag(&self) -> bool {
        self.null_flag
    }

    fn exists(&mut self, key: &Value) -> Result<IOResult<bool>> {
        self.invalidate_record();
        let int_key = match key {
            Value::Integer(i) => i,
            _ => unreachable!("btree tables are indexed by integers!"),
        };
        let rowid = self.db.seek_rowid(
            Bound::Included(&RowID {
                table_id: self.table_id,
                row_id: *int_key,
            }),
            true,
            self.tx_id,
        );
        tracing::trace!("found {rowid:?}");
        let exists = if let Some(rowid) = rowid {
            rowid.row_id == *int_key
        } else {
            false
        };
        if exists {
            self.current_pos.replace(CursorPosition::Loaded {
                row_id: RowID {
                    table_id: self.table_id,
                    row_id: *int_key,
                },
                in_btree: false,
                btree_consumed: false,
            });
        }
        Ok(IOResult::Done(exists))
    }

    fn clear_btree(&mut self) -> Result<IOResult<Option<usize>>> {
        todo!()
    }

    fn btree_destroy(&mut self) -> Result<IOResult<Option<usize>>> {
        todo!()
    }

    fn count(&mut self) -> Result<IOResult<usize>> {
        todo!()
    }

    /// Returns true if the is not pointing to any row.
    fn is_empty(&self) -> bool {
        // If we reached the end of the table, it means we traversed the whole table therefore there must be something in the table.
        // If we have loaded a row, it means there is something in the table.
        match self.get_current_pos() {
            CursorPosition::Loaded { .. } => false,
            CursorPosition::BeforeFirst => true,
            CursorPosition::End => true,
        }
    }

    fn root_page(&self) -> i64 {
        self.table_id.into()
    }

    fn rewind(&mut self) -> Result<IOResult<()>> {
        // First run btree_cursor rewind so that we don't need a explicit state machine.
        if self.is_btree_allocated() {
            return_if_io!(self.btree_cursor.rewind());
        }

        self.invalidate_record();
        if !matches!(self.get_current_pos(), CursorPosition::BeforeFirst) {
            self.current_pos.replace(CursorPosition::BeforeFirst);
        }

        let new_position_in_mvcc = self
            .db
            .get_next_row_id_for_table(self.table_id, i64::MIN, self.tx_id)
            .map(|id| id.row_id);

        let IOResult::Done(maybe_rowid_in_btree) = self.btree_cursor.rowid()? else {
            panic!("BTree should have returned rowid after rewind because we called btree_cursor.rewind()");
        };
        let maybe_rowid_in_btree =
            maybe_rowid_in_btree.filter(|rowid| self.query_btree_version_is_valid(*rowid));
        let new_position = self.get_new_position_from_mvcc_and_btree(
            &new_position_in_mvcc,
            &maybe_rowid_in_btree,
            IterationDirection::Forwards,
        );
        self.current_pos.replace(new_position);
        Ok(IOResult::Done(()))
    }

    fn has_record(&self) -> bool {
        todo!()
    }

    fn set_has_record(&self, _has_record: bool) {
        todo!()
    }

    fn get_index_info(&self) -> &crate::types::IndexInfo {
        todo!()
    }

    fn seek_end(&mut self) -> Result<IOResult<()>> {
        todo!()
    }

    fn seek_to_last(&mut self) -> Result<IOResult<()>> {
        self.invalidate_record();
        let max_rowid = RowID {
            table_id: self.table_id,
            row_id: i64::MAX,
        };
        let bound = Bound::Included(&max_rowid);
        let lower_bound = false;

        let rowid = self.db.seek_rowid(bound, lower_bound, self.tx_id);
        if let Some(rowid) = rowid {
            self.current_pos.replace(CursorPosition::Loaded {
                row_id: rowid,
                in_btree: false,
                btree_consumed: false,
            });
        } else {
            self.current_pos.replace(CursorPosition::End);
        }
        Ok(IOResult::Done(()))
    }

    fn invalidate_record(&mut self) {
        self.get_immutable_record_or_create()
            .as_mut()
            .unwrap()
            .invalidate();
        self.record_cursor.borrow_mut().invalidate();
    }

    fn has_rowid(&self) -> bool {
        todo!()
    }

    fn record_cursor_mut(&self) -> std::cell::RefMut<'_, crate::types::RecordCursor> {
        self.record_cursor.borrow_mut()
    }

    fn get_pager(&self) -> Arc<Pager> {
        todo!()
    }

    fn get_skip_advance(&self) -> bool {
        todo!()
    }
}

impl<Clock: LogicalClock> Debug for MvccLazyCursor<Clock> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("MvccLazyCursor")
            .field("current_pos", &self.current_pos)
            .field("table_id", &self.table_id)
            .field("tx_id", &self.tx_id)
            .field("reusable_immutable_record", &self.reusable_immutable_record)
            .field("btree_cursor", &())
            .finish()
    }
}