Backward scan of a table wasn't implemented yet in MVCC so this achieves
that. I added simple test for mixed btree and mvcc backward scan but I
should add more intense testing for this.
<!-- CURSOR_SUMMARY -->
---
> [!NOTE]
> Implements backward scanning and last() in MVCC lazy cursor and adds
directional rowid iteration in the MVCC store, with new tests for mixed
MVCC+B-Tree backward scans.
>
> - **MVCC Cursor (`core/mvcc/cursor.rs`)**:
> - Implement `prev()` and `last()` with mixed MVCC/B-Tree
coordination using `IterationDirection`.
> - Add `PrevState` and extend state machine to handle backward
iteration.
> - Update `get_new_position_from_mvcc_and_btree(...)` to choose
rowids based on direction.
> - Integrate B-Tree cursor calls (`last`, `prev`) and adjust
`rewind`/rowid selection; tweak next-rowid when at `End`.
> - **MVCC Store (`core/mvcc/database/mod.rs`)**:
> - Add `get_prev_row_id_for_table(...)` and generalized
`get_row_id_for_table_in_direction(...)` supporting forward/backward
scans.
> - Add tracing and minor refactors around next/prev rowid retrieval.
> - **Tests (`core/mvcc/database/tests.rs`)**:
> - Add test for backward scan combining B-Tree and MVCC and an
ignored test covering delete during backward scan.
>
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<!-- /CURSOR_SUMMARY -->
Closes#3980
This PR add proper program abort in case of unfinished statement reset
and interruption.
Also, this PR makes rollback methods non-failing because otherwise of
their callers usually unclear (if rollback failed - what is the state of
statement/connection/transaction?)
Reviewed-by: Preston Thorpe <preston@turso.tech>
Closes#3591
MVCC is like the annoying younger cousin (I know because I was him) that
needs to be treated differently. MVCC requires us to use root_pages that
might not be allocated yet, and the plan is to use negative root_pages
for that case. Therefore, we need i64 in order to fit this change.
TransitionResult is an internal implementation detail that tells
an invocation of StateMachine::step() to continue looping, but it
is of no use to other callers.
For this reason, just return an IOResult from StateMachine::step()
which simplifies the result handling.
Currently header changes are tracked through pager by reading page 1.
MVCC has it's own layer to track changes during txn so this commit makes
it so that headers are tracked by each txn separately.
On commit we update the _global_ header which is used to update
`database_size` because pager commits require it to be up to date. This
also makes it _simpler_ to keep track of header updates and update
pager's header accordingly.
The `run_once()` name is just a historical accident. Furthermore, it now
started to appear elsewhere as well, so let's just call it IO::step() as we
should have from the beginning.
