Allow ports to be reused in gloo (#97677)

Summary:
X-link: pytorch/pytorch#97677

Pull Request resolved: #353

ProcessGroupGloo and gloo seem to be opening and closing sockets without allowing the port to be reused. We see this issue pop up in larger training jobs "Address already in use" and we assume it to be because all the ephemeral ports are exhausted.

This diff allows ports to be reused, we see a reduced number of ports being in `TIME_WAIT` state.

context: https://fb.workplace.com/groups/319878845696681/permalink/5988899781205532/

another issue: https://fb.workplace.com/groups/319878845696681/permalink/958768178474408/

Differential Revision: D44029927

fbshipit-source-id: 1f83e9288776a6ec6e2f2b1ea356739ae057d4a6

gloo/transport/tcp/device.cc

@@ -101,6 +101,15 @@ static void lookupAddrForHostname(struct attr& attr) {
   struct addrinfo* rp;
   for (rp = result; rp != nullptr; rp = rp->ai_next) {
     auto fd = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol);
+
+    // Set SO_REUSEADDR to signal that reuse of the listening port is OK.
+    int on = 1;
+    rv = setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, reinterpret_cast<const char*>(&on), sizeof(on));
+    if (rv == -1) {
+      close(fd);
+      GLOO_ENFORCE_NE(rv, -1);
+    }
+
     if (fd == -1) {
       continue;
     }