[rabit] Small cleanup to tracker initialization. (#9524)

- Remove recover related code.
- Clean startup, no need to consider previously connected nodes.

python-package/xgboost/tracker.py

@@ -137,15 +137,9 @@ class WorkerEntry:
         return self._get_remote(wait_conn, nnset)
 
     def _get_remote(
-        self, wait_conn: Dict[int, "WorkerEntry"], nnset: Set[int]
+        self, wait_conn: Dict[int, "WorkerEntry"], badset: Set[int]
     ) -> List[int]:
         while True:
-            ngood = self.sock.recvint()
-            goodset = set()
-            for _ in range(ngood):
-                goodset.add(self.sock.recvint())
-            assert goodset.issubset(nnset)
-            badset = nnset - goodset
             conset = []
             for r in badset:
                 if r in wait_conn:
@@ -343,7 +337,7 @@ class RabitTracker:
                 shutdown[s.rank] = s
                 logging.debug("Received %s signal from %d", s.cmd, s.rank)
                 continue
-            assert s.cmd in ("start", "recover")
+            assert s.cmd == "start"
             # lazily initialize the workers
             if tree_map is None:
                 assert s.cmd == "start"

rabit/src/allreduce_base.cc

@@ -318,21 +318,10 @@ bool AllreduceBase::ReConnectLinks(const char *cmd) {
     // get number of to connect and number of to accept nodes from tracker
     int num_conn, num_accept, num_error = 1;
     do {
-      // send over good links
-      std::vector<int> good_link;
       for (auto & all_link : all_links) {
-        if (!all_link.sock.BadSocket()) {
+        all_link.sock.Close();
-          good_link.push_back(static_cast<int>(all_link.rank));
-        } else {
-          if (!all_link.sock.IsClosed()) all_link.sock.Close();
-        }
       }
-      int ngood = static_cast<int>(good_link.size());
       // tracker construct goodset
-      Assert(tracker.SendAll(&ngood, sizeof(ngood)) == sizeof(ngood), "ReConnectLink failure 5");
-      for (int &i : good_link) {
-        Assert(tracker.SendAll(&i, sizeof(i)) == sizeof(i), "ReConnectLink failure 6");
-      }
       Assert(tracker.RecvAll(&num_conn, sizeof(num_conn)) == sizeof(num_conn),
              "ReConnectLink failure 7");
       Assert(tracker.RecvAll(&num_accept, sizeof(num_accept)) == sizeof(num_accept),