Static MATERIALIZE_PERMIT 

pub static MATERIALIZE_PERMIT: Semaphore

Serialises document materialisation across the whole process.

Materialising a document is memory-unbounded: export_all hands back the whole document set owned, before any of indexer::build_parts’ budgets apply. One at a time is affordable; several at once puts the node out of memory. The reindex sweep is already sequential — this bounds the scheduler-run IndexDocumentTasks, which run concurrently.

Held across the export → prune → build_parts span only, never across the meta-adapter write that follows.