storage_service, tablets: Move get_leaving_replica() to tablets.cc

For better encapsulation of tablet-specific code.

locator/tablets.cc

@@ -75,6 +75,20 @@ tablet_transition_info::tablet_transition_info(tablet_transition_stage stage, ta
     , reads(get_selector_for_reads(stage))
 { }
 
+tablet_replica get_leaving_replica(const tablet_info& tinfo, const tablet_transition_info& trinfo) {
+    std::unordered_set<tablet_replica> leaving(tinfo.replicas.begin(), tinfo.replicas.end());
+    for (auto&& r : trinfo.next) {
+        leaving.erase(r);
+    }
+    if (leaving.empty()) {
+        throw std::runtime_error(format("No leaving replicas"));
+    }
+    if (leaving.size() > 1) {
+        throw std::runtime_error(format("More than one leaving replica"));
+    }
+    return *leaving.begin();
+}
+
 const tablet_map& tablet_metadata::get_tablet_map(table_id id) const {
     try {
         return _tablets.at(id);

locator/tablets.hh

@@ -168,6 +168,9 @@ struct tablet_transition_info {
     bool operator==(const tablet_transition_info&) const = default;
 };
 
+// Returns the leaving replica for a given transition.
+tablet_replica get_leaving_replica(const tablet_info&, const tablet_transition_info&);
+
 /// Stores information about tablets of a single table.
 ///
 /// The map contains a constant number of tablets, tablet_count().