tablets: Include pending replica in get_shard()

We need to move get_shard() from tablet_info to tablet_map in order to
have access to transition_info.

locator/tablets.cc

@@ -105,6 +105,23 @@ void tablet_map::set_tablet_transition_info(tablet_id id, tablet_transition_info
     _transitions.insert_or_assign(id, std::move(info));
 }
 
+std::optional<shard_id> tablet_map::get_shard(tablet_id tid, host_id host) const {
+    auto&& info = get_tablet_info(tid);
+
+    for (auto&& r : info.replicas) {
+        if (r.host == host) {
+            return r.shard;
+        }
+    }
+
+    auto tinfo = get_tablet_transition_info(tid);
+    if (tinfo && tinfo->pending_replica.host == host) {
+        return tinfo->pending_replica.shard;
+    }
+
+    return std::nullopt;
+}
+
 future<> tablet_map::clear_gently() {
     return utils::clear_gently(_tablets);
 }

locator/tablets.hh

@@ -52,15 +52,6 @@ using tablet_replica_set = utils::small_vector<tablet_replica, 3>;
 struct tablet_info {
     tablet_replica_set replicas;
 
-    std::optional<shard_id> get_shard(host_id host) const {
-        for (auto&& r : replicas) {
-            if (r.host == host) {
-                return r.shard;
-            }
-        }
-        return std::nullopt;
-    }
-
     bool operator==(const tablet_info&) const = default;
 };
 
@@ -148,6 +139,12 @@ public:
         return tablet_id(size_t(t) + 1);
     }
 
+    /// Returns shard id which is a replica for a given tablet on a given host.
+    /// If there is no replica on a given host, returns nullopt.
+    /// If the topology is transitional, also considers the new replica set.
+    /// The old replica set is preferred in case of ambiguity.
+    std::optional<shard_id> get_shard(tablet_id, host_id) const;
+
     const tablet_container& tablets() const {
         return _tablets;
     }