Struct HeaderSnapshot 

pub struct HeaderSnapshot {
    pub root_catalog: u64,
    pub freelist_head: u64,
    pub page_count: u64,
    pub view: HashMap<PageId, Arc<Page>>,
}

Snapshot of the page-0 header fields that the M5 catalog + freelist code mutates directly (not through the WAL).

Returned by Pager::header_snapshot at txn-begin; passed back to Pager::restore_header_snapshot on rollback so the on-disk header is rewound alongside the WAL pending buffer. M5 wrote these fields direct-to-disk for performance; the M6 transaction layer compensates by snapshotting them whenever a WriteTxn might roll back.

Fields

root_catalog: u64

Catalog B-tree root page id captured at snapshot time.

freelist_head: u64

Freelist head page id captured at snapshot time.

page_count: u64

File page count captured at snapshot time. Used so an alloc-then-rollback does not leak the appended page.

view: HashMap<PageId, Arc<Page>>

WAL “committed view” snapshot. See Pager::header_snapshot for the rationale — free_page removes per-page entries from the live view, and the rollback path needs to put them back.

#80: pages are held behind Arc so cloning this map (in the rollback snapshot path) is a set of refcount bumps rather than per-page 4 KiB memcpys, matching the live committed view.

Trait Implementations

impl Clone for HeaderSnapshot

fn clone(&self) -> HeaderSnapshot

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for HeaderSnapshot

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

Formats the value using the given formatter.