This PR introduce simple fuzz test for BTree insertion algorithm and
fixes few bugs found by fuzzer
- BTree algorithm returned early although there were overflow pages on
stack and more rebalances were needed
- BTree balancing algorithm worked under assumption that single page
will be enough for rebalance - although this is not always true (if page
were tightly packed with relatively big cells, insertion of new very big
cell can require 3 split pages to distribute the content between them)
- `overflow_cells` wasn't cleared properly during rebalancing
- insertions of dividers to the parent node were implemented incorrectly
- `defragment_page` didn't reset
`PAGE_HEADER_OFFSET_FRAGMENTED_BYTES_COUNT` field which can lead to
suboptimal usage of pages
Closes#951
- if we have page which is tightly packed with relatively big cells, we
will be unable to balance its content if we will insert very big
(~page size) cell in the middle (because nothing can't be merged with
new cell - so we will need to split 1 page into 3)
We really need to make the WAL lock less expensive, but switching to
`parking_lot` is anyway something we should do.
Before:
```
Execute `SELECT 1`/Limbo
time: [56.230 ns 56.463 ns 56.688 ns]
```
After:
```
Execute `SELECT 1`/Limbo
time: [52.003 ns 52.132 ns 52.287 ns]
```
### What?
adding checkpoint result returning number of pages in wal and num pages
checkpointed.
Part of #696
### Context
SQLite returns in checkpoint result of calling `pragma wal_checkpoint;`
`0|3|3` while limbo returns `0|0|0`.
https://sqlite.org/pragma.html#pragma_wal_checkpoint
- 1st col: 1 (checkpoint SQLITE_BUSY) or 0 (not busy).
- 2nd col: # modified pages written to wal file
- 3rd col: # pages moved to db after checkpoint
This PR aims to add 2nd and 3rd column to the checkpoint result.
SQLite
```
sqlite3 test.db
sqlite> pragma journal_mode=wal;
wal
sqlite> pragma journal_mode;
wal
sqlite> create table t1 (id text);
sqlite> insert into t1(id) values (1),(2);
sqlite> select * from t1;
1
2
sqlite> pragma wal_checkpoint;
0|3|3
```
Limbo
```
./target/debug/limbo test.db
Limbo v0.0.13
Enter ".help" for usage hints.
limbo> pragma journal_mode;
wal
limbo> create table t1(id text);
limbo> insert into t1(id) values (1),(2);
limbo> select * from t1;
1
2
# current the 2nd and 3rd columns are hard coded in limbo to 0
limbo> pragma wal_checkpoint;
0|0|0
```
Closes#827
