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Improve comments in BTreeCursor::compute_free_space()

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This commit is contained in:
jussisaurio
2024-12-24 10:30:27 +02:00
parent 91cca0d5b7
commit c6b7ddf77a
1 changed files with 20 additions and 11 deletions
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31
core/storage/btree.rs
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@@ -1441,10 +1441,14 @@ impl BTreeCursor {
let first_cell = (page.offset + 8 + child_pointer_size + (2 * ncell)) as u16;
// The amount of free space is the sum of:
// 1. 0..first_byte_in_cell_content (everything to the left of the cell content area pointer is unused free space)
// 2. fragmented_free_bytes.
// #1. space to the left of the cell content area pointer
// #2. fragmented_free_bytes (isolated 1-3 byte chunks of free space within the cell content area)
// #3. freeblocks (linked list of blocks of at least 4 bytes within the cell content area that are not in use due to e.g. deletions)
// #1 and #2 are known from the page header
let mut nfree = fragmented_free_bytes as usize + first_byte_in_cell_content as usize;
// #3 is computed by iterating over the freeblocks linked list
let mut pc = free_block_pointer as usize;
if pc > 0 {
if pc < first_byte_in_cell_content as usize {
@@ -1457,28 +1461,33 @@ impl BTreeCursor {
let mut size = 0;
loop {
// TODO: check corruption icellast
next = u16::from_be_bytes(buf[pc..pc + 2].try_into().unwrap()) as usize;
size = u16::from_be_bytes(buf[pc + 2..pc + 4].try_into().unwrap()) as usize;
next = u16::from_be_bytes(buf[pc..pc + 2].try_into().unwrap()) as usize; // first 2 bytes in freeblock = next freeblock pointer
size = u16::from_be_bytes(buf[pc + 2..pc + 4].try_into().unwrap()) as usize; // next 2 bytes in freeblock = size of current freeblock
nfree += size;
// Freeblocks are in order from left to right on the page,
// so next pointer should > current pointer + its size, or 0 if no next block exists.
if next <= pc + size + 3 {
break;
}
pc = next;
}
if next > 0 {
todo!("corrupted page ascending order");
}
// Next should always be 0 (NULL) at this point since we have reached the end of the freeblocks linked list
assert!(
next == 0,
"corrupted page: freeblocks list not in ascending order"
);
if pc + size > usable_space {
todo!("corrupted page last freeblock extends last page end");
}
assert!(
pc + size <= usable_space,
"corrupted page: last freeblock extends last page end"
);
}
// if( nFree>usableSize || nFree<iCellFirst ){
// return SQLITE_CORRUPT_PAGE(pPage);
// }
// don't count header and cell pointers?
// FIXME:don't count header and cell pointers?
nfree -= first_cell as usize;
nfree as u16
}