Module readers 

2n: the reader table – how the writer learns the oldest generation any live snapshot reader still needs, so page recycling never pulls a page out from under one.

LMDB’s shape, flock-based so it cannot go stale: each snapshot reader creates readers/r-<pid>-<n> holding its pinned generation and KEEPS an exclusive flock on it for the snapshot’s lifetime. Close or crash, the kernel drops the lock with the fd. The writer, when it wants the oldest, walks the directory: a file whose lock it can grab is a dead reader’s leavings (unlinked on the spot); a file whose lock is busy is a live reader. Its generation is stored in two identical framed copies, each with magic, version and CRC. Any malformed copy or disagreement means generation zero: ambiguity leaks space but cannot recycle a live snapshot’s pages.

Failure posture (the 2n risk ladder): an unreadable directory or file means “assume a reader at generation 0” – recycling stops, the file grows like it always did, nothing can be wrongly reused.

Structs

ReaderSlot

Held by a snapshot reader for its lifetime; the registration disappears (unlink + lock release) on drop, and the LOCK disappears even on crash.

Functions

oldest_live_reader

The oldest generation any live reader pins, or u64::MAX when none. Dead readers’ files are swept as they are met. Any ambiguity – a file that cannot be opened or parsed while its lock is busy – reports generation 0: recycling halts rather than guesses.