refactor database engine/options; add column descriptors

Signed-off-by: Jason Volk <jason@zemos.net>

src/database/map/open.rs

@@ -0,0 +1,37 @@
+use std::sync::Arc;
+
+use rocksdb::ColumnFamily;
+
+use crate::Engine;
+
+pub(super) fn open(db: &Arc<Engine>, name: &str) -> Arc<ColumnFamily> {
+	let bounded_arc = db.cf(name);
+	let bounded_ptr = Arc::into_raw(bounded_arc);
+	let cf_ptr = bounded_ptr.cast::<ColumnFamily>();
+
+	// SAFETY: Column family handles out of RocksDB are basic pointers and can
+	// be invalidated: 1. when the database closes. 2. when the column is dropped or
+	// closed. rust_rocksdb wraps this for us by storing handles in their own
+	// `RwLock<BTreeMap>` map and returning an Arc<BoundColumnFamily<'_>>` to
+	// provide expected safety. Similarly in "single-threaded mode" we would
+	// receive `&'_ ColumnFamily`.
+	//
+	// PROBLEM: We need to hold these handles in a field, otherwise we have to take
+	// a lock and get them by name from this map for every query, which is what
+	// conduit was doing, but we're not going to make a query for every query so we
+	// need to be holding it right. The lifetime parameter on these references makes
+	// that complicated. If this can be done without polluting the userspace
+	// with lifetimes on every instance of `Map` then this `unsafe` might not be
+	// necessary.
+	//
+	// SOLUTION: After investigating the underlying types it appears valid to
+	// Arc-swap `BoundColumnFamily<'_>` for `ColumnFamily`. They have the
+	// same inner data, the same Drop behavior, Deref, etc. We're just losing the
+	// lifetime parameter. We should not hold this handle, even in its Arc, after
+	// closing the database (dropping `Engine`). Since `Arc<Engine>` is a sibling
+	// member along with this handle in `Map`, that is prevented.
+	unsafe {
+		Arc::increment_strong_count(cf_ptr);
+		Arc::from_raw(cf_ptr)
+	}
+}

src/database/map/options.rs

@@ -0,0 +1,26 @@
+use rocksdb::{ReadOptions, ReadTier, WriteOptions};
+
+#[inline]
+pub(crate) fn iter_options_default() -> ReadOptions {
+	let mut read_options = read_options_default();
+	read_options.set_background_purge_on_iterator_cleanup(true);
+	//read_options.set_pin_data(true);
+	read_options
+}
+
+#[inline]
+pub(crate) fn cache_read_options_default() -> ReadOptions {
+	let mut read_options = read_options_default();
+	read_options.set_read_tier(ReadTier::BlockCache);
+	read_options
+}
+
+#[inline]
+pub(crate) fn read_options_default() -> ReadOptions {
+	let mut read_options = ReadOptions::default();
+	read_options.set_total_order_seek(true);
+	read_options
+}
+
+#[inline]
+pub(crate) fn write_options_default() -> WriteOptions { WriteOptions::default() }