tuwunel_database/map/
open.rs

1use std::sync::Arc;
2
3use rocksdb::ColumnFamily;
4
5use crate::Engine;
6
7/// Acquires a stable column-family handle for a map.
8///
9/// The returned handle erases the engine-borrow lifetime carried by RocksDB.
10/// Its caller must retain the engine until after the handle is dropped, which
11/// `Map` guarantees by field ownership.
12///
13/// # Panics
14///
15/// Panics if the column family was not described before the engine opened.
16pub(super) fn open(engine: &Arc<Engine>, name: &str) -> Arc<ColumnFamily> {
17	let bounded_arc = engine.cf(name);
18	let bounded_ptr = Arc::into_raw(bounded_arc);
19	let cf_ptr = bounded_ptr.cast::<ColumnFamily>();
20
21	// SAFETY: Column family handles out of RocksDB are basic pointers and can
22	// be invalidated: 1. when the database closes. 2. when the column is dropped or
23	// closed. rust_rocksdb wraps this for us by storing handles in their own
24	// `RwLock<BTreeMap>` map and returning an Arc<BoundColumnFamily<'_>>` to
25	// provide expected safety. Similarly in "single-threaded mode" we would
26	// receive `&'_ ColumnFamily`.
27	//
28	// PROBLEM: We need to hold these handles in a field, otherwise we have to take
29	// a lock and get them by name from this map for every query, which is what
30	// conduit was doing, but we're not going to make a query for every query so we
31	// need to be holding it right. The lifetime parameter on these references makes
32	// that complicated. If this can be done without polluting the userspace
33	// with lifetimes on every instance of `Map` then this `unsafe` might not be
34	// necessary.
35	//
36	// SOLUTION: After investigating the underlying types it appears valid to
37	// Arc-swap `BoundColumnFamily<'_>` for `ColumnFamily`. They have the
38	// same inner data, the same Drop behavior, Deref, etc. We're just losing the
39	// lifetime parameter. We should not hold this handle, even in its Arc, after
40	// closing the database (dropping `Engine`). Since `Arc<Engine>` is a sibling
41	// member along with this handle in `Map`, that is prevented.
42	unsafe { Arc::from_raw(cf_ptr) }
43}
tuwunel_database/map/open.rs

tuwunel_database/map/
open.rs
