check cache prior to offloading iterator seek

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

src/database/map/rev_stream.rs

@@ -26,3 +26,20 @@ pub fn rev_raw_stream(&self) -> impl Stream<Item = Result<KeyVal<'_>>> + Send {
 	let opts = super::iter_options_default();
 	stream::ItemsRev::new(&self.db, &self.cf, opts).init(None)
 }
+
+#[tracing::instrument(
+    name = "cached",
+    level = "trace",
+    skip_all,
+    fields(%map),
+)]
+pub(super) fn _is_cached<P>(map: &super::Map) -> bool
+where
+	P: AsRef<[u8]> + ?Sized,
+{
+	let opts = super::cache_read_options_default();
+	let mut state = stream::State::new(&map.db, &map.cf, opts);
+
+	state.seek_rev();
+	!state.is_incomplete()
+}