Merge bitcoin/bitcoin#21879: refactor: wrap accept() and extend usage of Sock

6bf6e9fd9d net: change CreateNodeFromAcceptedSocket() to take Sock (Vasil Dimov)
9e3cbfca7c net: use Sock in CConnman::ListenSocket (Vasil Dimov)
f8bd13f85a net: add new method Sock::Accept() that wraps accept() (Vasil Dimov)

Pull request description:

  _This is a piece of https://github.com/bitcoin/bitcoin/pull/21878, chopped off to ease review._

  Introduce an `accept(2)` wrapper `Sock::Accept()` and extend the usage of `Sock` in `CConnman::ListenSocket` and `CreateNodeFromAcceptedSocket()`.

ACKs for top commit:
  laanwj:
    Code review ACK 6bf6e9fd9d
  jamesob:
    ACK 6bf6e9fd9d ([`jamesob/ackr/21879.2.vasild.wrap_accept_and_extend_u`](https://github.com/jamesob/bitcoin/tree/ackr/21879.2.vasild.wrap_accept_and_extend_u))
  jonatack:
    ACK 6bf6e9fd9d per `git range-diff ea989de 976f6e8 6bf6e9f` -- only change since my last review was `s/listen_socket.socket/listen_socket.sock->Get()/` in `src/net.cpp: CConnman::SocketHandlerListening()` -- re-read the code changes, rebase/debug build/ran units following my previous full review (https://github.com/bitcoin/bitcoin/pull/21879#pullrequestreview-761251278)
  w0xlt:
    tACK 6bf6e9f

Tree-SHA512: dc6d1acc4f255f1f7e8cf6dd74e97975cf3d5959e9fc2e689f74812ac3526d5ee8b6a32eca605925d10a4f7b6ff1ce5e900344311e587d19786b48c54d021b64

src/util/sock.cpp

@@ -10,6 +10,7 @@
 #include <util/system.h>
 #include <util/time.h>
 
+#include <memory>
 #include <stdexcept>
 #include <string>
 
@@ -73,6 +74,32 @@ int Sock::Connect(const sockaddr* addr, socklen_t addr_len) const
     return connect(m_socket, addr, addr_len);
 }
 
+std::unique_ptr<Sock> Sock::Accept(sockaddr* addr, socklen_t* addr_len) const
+{
+#ifdef WIN32
+    static constexpr auto ERR = INVALID_SOCKET;
+#else
+    static constexpr auto ERR = SOCKET_ERROR;
+#endif
+
+    std::unique_ptr<Sock> sock;
+
+    const auto socket = accept(m_socket, addr, addr_len);
+    if (socket != ERR) {
+        try {
+            sock = std::make_unique<Sock>(socket);
+        } catch (const std::exception&) {
+#ifdef WIN32
+            closesocket(socket);
+#else
+            close(socket);
+#endif
+        }
+    }
+
+    return sock;
+}
+
 int Sock::GetSockOpt(int level, int opt_name, void* opt_val, socklen_t* opt_len) const
 {
     return getsockopt(m_socket, level, opt_name, static_cast<char*>(opt_val), opt_len);

src/util/sock.h

@@ -10,6 +10,7 @@
 #include <util/time.h>
 
 #include <chrono>
+#include <memory>
 #include <string>
 
 /**
@@ -96,6 +97,14 @@ public:
      */
     [[nodiscard]] virtual int Connect(const sockaddr* addr, socklen_t addr_len) const;
 
+    /**
+     * accept(2) wrapper. Equivalent to `std::make_unique<Sock>(accept(this->Get(), addr, addr_len))`.
+     * Code that uses this wrapper can be unit tested if this method is overridden by a mock Sock
+     * implementation.
+     * The returned unique_ptr is empty if `accept()` failed in which case errno will be set.
+     */
+    [[nodiscard]] virtual std::unique_ptr<Sock> Accept(sockaddr* addr, socklen_t* addr_len) const;
+
     /**
      * getsockopt(2) wrapper. Equivalent to
      * `getsockopt(this->Get(), level, opt_name, opt_val, opt_len)`. Code that uses this