This format is based on previous discussions:
1. Log header
```rust
/// Log's Header, this will be the 64 bytes in any logical log file.
/// Log header is 64 bytes at maximum, fields added must not exceed that size. If it doesn't exceed
/// it, any bytes missing will be padded with zeroes.
struct LogHeader {
version: u8,
salt: u64,
encrypted: u8, // 0 is no
}
```
2. Transaction format:
* Transaction id
* Checksum u64
* Byte size of all rows combined
* Rows
* End marker (offset position after appending buffer)
3. Row format:
```rust
/// Serialize a row_version into on disk format.
/// Format of a "row" (maybe we could change the name because row is not general enough for
/// future type of values):
///
/// * table_id (root page) -> u64
/// * row type -> u8
///
/// (by row type)
/// Delete:
/// * Payload length -> u64
/// * Rowid -> varint
///
/// Insert:
/// * Payload length -> u64
/// * Data size -> varint
/// * Rowid -> varint
/// * Data -> [u8] (data size length)
fn serialize(&self, buffer: &mut Vec<u8>, row_version: &RowVersion) {
```
Closes#3245
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.
MVCC checkpoints are always TRUNCATE, plus they block all other transactions.
This guarantees that never need to let transactions read from the SQLite WAL.
In MVCC, the checkpoint procedure is roughly as follows:
- Take the blocking_checkpoint_lock
- Write everything in the logical log to the pager, and from there commit to the SQLite WAL.
- Immediately TRUNCATE checkpoint the WAL into the database file.
- Release the blocking_checkpoint_lock.
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.
this is only used for returning LimboResult::Busy, and we already
have LimboError::Busy, so it only adds confusion.
Moreover, the current busy handler was not handling LimboError::Busy,
because it's returned as an error, not as Ok. So this may fix the
"busy handler not working" issue in the perf thrpt benchmark.
