[Coll] Implement get host address in libxgboost. (#9644)

- Port `xgboost.tracker.get_host_ip` in C++.

src/collective/tracker.cc

@@ -0,0 +1,76 @@
+/**
+ * Copyright 2023, XGBoost Contributors
+ */
+#if defined(__unix__) || defined(__APPLE__)
+#include <netdb.h>       // gethostbyname
+#include <sys/socket.h>  // socket, AF_INET6, AF_INET, connect, getsockname
+#endif                   // defined(__unix__) || defined(__APPLE__)
+
+#if !defined(NOMINMAX) && defined(_WIN32)
+#define NOMINMAX
+#endif  // !defined(NOMINMAX)
+
+#if defined(_WIN32)
+#include <winsock2.h>
+#include <ws2tcpip.h>
+#endif  // defined(_WIN32)
+
+#include <string>  // for string
+
+#include "xgboost/collective/result.h"  // for Result, Fail, Success
+#include "xgboost/collective/socket.h"  // for GetHostName, FailWithCode, MakeSockAddress, ...
+
+namespace xgboost::collective {
+[[nodiscard]] Result GetHostAddress(std::string* out) {
+  auto rc = GetHostName(out);
+  if (!rc.OK()) {
+    return rc;
+  }
+  auto host = gethostbyname(out->c_str());
+
+  // get ip address from host
+  std::string ip;
+  rc = INetNToP(host, &ip);
+  if (!rc.OK()) {
+    return rc;
+  }
+
+  if (!(ip.size() >= 4 && ip.substr(0, 4) == "127.")) {
+    // return if this is a public IP address.
+    // not entirely accurate, we have other reserved IPs
+    *out = ip;
+    return Success();
+  }
+
+  // Create an UDP socket to prob the public IP address, it's fine even if it's
+  // unreachable.
+  auto sock = socket(AF_INET, SOCK_DGRAM, 0);
+  if (sock == -1) {
+    return Fail("Failed to create socket.");
+  }
+
+  auto paddr = MakeSockAddress(StringView{"10.255.255.255"}, 1);
+  sockaddr const* addr_handle = reinterpret_cast<const sockaddr*>(&paddr.V4().Handle());
+  socklen_t addr_len{sizeof(paddr.V4().Handle())};
+  auto err = connect(sock, addr_handle, addr_len);
+  if (err != 0) {
+    return system::FailWithCode("Failed to find IP address.");
+  }
+
+  // get the IP address from socket desrciptor
+  struct sockaddr_in addr;
+  socklen_t len = sizeof(addr);
+  if (getsockname(sock, reinterpret_cast<struct sockaddr*>(&addr), &len) == -1) {
+    return Fail("Failed to get sock name.");
+  }
+  ip = inet_ntoa(addr.sin_addr);
+
+  err = system::CloseSocket(sock);
+  if (err != 0) {
+    return system::FailWithCode("Failed to close socket.");
+  }
+
+  *out = ip;
+  return Success();
+}
+}  // namespace xgboost::collective

src/collective/tracker.h

@@ -0,0 +1,15 @@
+/**
+ * Copyright 2023, XGBoost Contributors
+ */
+#pragma once
+#include <string>  // for string
+
+#include "xgboost/collective/result.h"  // for Result
+
+namespace xgboost::collective {
+// Prob the public IP address of the host, need a better method.
+//
+// This is directly translated from the previous Python implementation, we should find a
+// more riguous approach, can use some expertise in network programming.
+[[nodiscard]] Result GetHostAddress(std::string* out);
+}  // namespace xgboost::collective