gloo/tcp/attr: hold on to bound socket to avoid port conflict issues (#429)

Summary:

This is a minor refactor to attr in the TCP backend for Gloo so that we don't close the bound fd between allocating it in `lookupAddrForHostname` and binding it in `Listener`. 

Listener sets `SO_REUSEADDR` so it's fine for us to bind twice to the same address but we will only listen on it in `Listener`.

Reviewed By: fduwjj

Differential Revision: D73148977

Author: d4l3k

gloo/transport/tcp/attr.h

@@ -11,11 +11,30 @@
 #include <string>
 
 #include <sys/socket.h>
+#include <unistd.h>
+#include <memory>
 
 namespace gloo {
 namespace transport {
 namespace tcp {
 
+// RAII for fds so they'll close when all references to them are freed.
+class FDHolder {
+ public:
+  explicit FDHolder(int fd) : fd_(fd) {}
+  ~FDHolder() {
+    close(fd_);
+  }
+
+  FDHolder(const FDHolder&) = delete;
+  FDHolder& operator=(const FDHolder&) = delete;
+  FDHolder(FDHolder&&) = delete;
+  FDHolder& operator=(FDHolder&&) = delete;
+
+ private:
+  const int fd_;
+};
+
 struct attr {
   attr() {}
   /* implicit */ attr(const char* ptr) : hostname(ptr) {}
@@ -31,6 +50,8 @@ struct attr {
   int ai_protocol;
   struct sockaddr_storage ai_addr;
   int ai_addrlen;
+
+  std::shared_ptr<FDHolder> fd{nullptr};
 };
 
 } // namespace tcp

gloo/transport/tcp/device.cc

@@ -117,7 +117,10 @@ static void lookupAddrForHostname(struct attr& attr) {
     attr.ai_protocol = rp->ai_protocol;
     memcpy(&attr.ai_addr, rp->ai_addr, rp->ai_addrlen);
     attr.ai_addrlen = rp->ai_addrlen;
-    close(fd);
+
+    // We explicitly don't close this FD here as we want to hold on to it until
+    // the listener binds the server socket to the port.
+    attr.fd = std::make_shared<FDHolder>(fd);
     break;
   }