tor-netdir: Provide relay_by_rs_idx

crates/tor-netdir/src/lib.rs

@@ -843,6 +843,22 @@ impl NetDir {
         assert!(answer.has_identity(id));
         Some(answer)
     }
+    /// Obtain a `Relay` given a `RouterStatusIdx`
+    ///
+    /// Differs from `relay_from_rs_and_rsi` as follow:
+    ///  * That function expects the caller to already have an `MdConsensusRouterStatus`;
+    ///    it checks with `debug_assert` that the relay in the netdir matches.
+    ///  * That function panics if the `RouterStatusIdx` is invalid; this one returns `None`.
+    ///  * That function returns an `UncheckedRelay`; this one a `Relay`.
+    ///
+    /// `None` could be returned here, even with a valid `rsi`,
+    /// if `rsi` refers to an unuseable relay.
+    #[cfg_attr(not(feature = "hs-common"), allow(dead_code))]
+    pub(crate) fn relay_by_rs_idx(&self, rs_idx: RouterStatusIdx) -> Option<Relay<'_>> {
+        let rs = self.c_relays().get(rs_idx)?;
+        let md = self.mds.get(rs_idx)?.as_deref();
+        UncheckedRelay { rs, md }.into_relay()
+    }
 
     /// Return a relay with the same identities as those in `target`, if one
     /// exists.