Struct Meta 

pub struct Meta {
    pub format_version: u16,
    pub roots: [u32; 8],
    pub next_lsn: u64,
    pub generation: u64,
}

Fields

format_version: u16

roots: [u32; 8]

next_lsn: u64

LSN high-water mark, persisted so a rotation followed by a restart does not renumber log records from 1. Nothing today compares a page’s lsn against a record’s, but a later idempotence check would be silently wrong if LSNs repeated, and that is not a defect worth discovering from a corrupted store.

generation: u64

Monotone publication counter (2f). Chooses the live slot on open and the victim slot on write. 0 = the create-time publication.

Implementations

impl Meta

pub fn write(&self, pool: &BufferPool) -> Result<()>

pub fn write_slot(&self, pool: &BufferPool) -> Result<()>

Write this Meta into the slot its generation selects (2f). The caller flushes data pages and BARRIERS before calling, and flushes again after: the flip must reach the medium only once everything it names is already there.

pub fn from_page(p: &PageRef<'_>) -> Result<Meta>

Decode a Meta from an already-verified page. Pure – no BufferPool involved – so a caller holding a raw, already-CRC- checked PageRef over a single buffer can read it without needing a whole pool wrapped around one page. read below is a thin wrapper over this.

Fallible (Task 17 re-review, R2): a page can CRC-verify – its own bytes are exactly what was last written – while still holding zero slots, which Meta::write’s own error path can produce (see its doc comment) and nothing stops a future writer from producing another way. PageRef::slot(0) on such a page returns an EMPTY slice, not an error – verification and “has a slot 0 at all” are different questions – so the previous, infallible version of this function indexed straight into it and panicked. A malformed superblock is exactly the kind of damage this crate’s own recover() exists to repair; a decoder that panics on it instead of returning Err takes that repair path down with it, which is precisely what R2 measured.

pub fn read(pool: &BufferPool) -> Result<Meta>

pub fn read_latest(pool: &BufferPool) -> Result<Meta>

2f: read BOTH slots, adopt the newest valid one. A slot that fails its page checksum or does not parse is normally the loser – a torn flip leaves the previous publication standing. An intact unsupported logical version must refuse instead of falling back to stale metadata. A page-1 slot that is a valid page of any OTHER kind is a pre-2f file and is refused outright: silently adopting slot 0 would let the next checkpoint overwrite a live tree page.

pub fn init_slot_b(pool: &BufferPool) -> Result<()>

Initialise slot B as an EMPTY Meta page (valid page, no record): recognisably a slot – so read_latest never mistakes this for a pre-2f file – but never adoptable until a real flip writes it.

Trait Implementations

impl Clone for Meta

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for Meta

impl Debug for Meta

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.