net: add RAII socket and use it instead of bare SOCKET

Introduce a class to manage the lifetime of a socket - when the object
that contains the socket goes out of scope, the underlying socket will
be closed.

In addition, the new `Sock` class has a `Send()`, `Recv()` and `Wait()`
methods that can be overridden by unit tests to mock the socket
operations.

The `Wait()` method also hides the
`#ifdef USE_POLL poll() #else select() #endif` technique from higher
level code.

src/netbase.cpp

@@ -15,7 +15,9 @@
 
 #include <atomic>
 #include <cstdint>
+#include <functional>
 #include <limits>
+#include <memory>
 
 #ifndef WIN32
 #include <fcntl.h>
@@ -559,34 +561,28 @@ static bool Socks5(const std::string& strDest, int port, const ProxyCredentials
     return true;
 }
 
-/**
- * Try to create a socket file descriptor with specific properties in the
- * communications domain (address family) of the specified service.
- *
- * For details on the desired properties, see the inline comments in the source
- * code.
- */
-SOCKET CreateSocket(const CService &addrConnect)
+std::unique_ptr<Sock> CreateSockTCP(const CService& address_family)
 {
     // Create a sockaddr from the specified service.
     struct sockaddr_storage sockaddr;
     socklen_t len = sizeof(sockaddr);
-    if (!addrConnect.GetSockAddr((struct sockaddr*)&sockaddr, &len)) {
-        LogPrintf("Cannot create socket for %s: unsupported network\n", addrConnect.ToString());
-        return INVALID_SOCKET;
+    if (!address_family.GetSockAddr((struct sockaddr*)&sockaddr, &len)) {
+        LogPrintf("Cannot create socket for %s: unsupported network\n", address_family.ToString());
+        return nullptr;
     }
 
     // Create a TCP socket in the address family of the specified service.
     SOCKET hSocket = socket(((struct sockaddr*)&sockaddr)->sa_family, SOCK_STREAM, IPPROTO_TCP);
-    if (hSocket == INVALID_SOCKET)
-        return INVALID_SOCKET;
+    if (hSocket == INVALID_SOCKET) {
+        return nullptr;
+    }
 
     // Ensure that waiting for I/O on this socket won't result in undefined
     // behavior.
     if (!IsSelectableSocket(hSocket)) {
         CloseSocket(hSocket);
         LogPrintf("Cannot create connection: non-selectable socket created (fd >= FD_SETSIZE ?)\n");
-        return INVALID_SOCKET;
+        return nullptr;
     }
 
 #ifdef SO_NOSIGPIPE
@@ -602,11 +598,14 @@ SOCKET CreateSocket(const CService &addrConnect)
     // Set the non-blocking option on the socket.
     if (!SetSocketNonBlocking(hSocket, true)) {
         CloseSocket(hSocket);
-        LogPrintf("CreateSocket: Setting socket to non-blocking failed, error %s\n", NetworkErrorString(WSAGetLastError()));
+        LogPrintf("Error setting socket to non-blocking: %s\n", NetworkErrorString(WSAGetLastError()));
+        return nullptr;
     }
-    return hSocket;
+    return std::make_unique<Sock>(hSocket);
 }
 
+std::function<std::unique_ptr<Sock>(const CService&)> CreateSock = CreateSockTCP;
+
 template<typename... Args>
 static void LogConnectFailure(bool manual_connection, const char* fmt, const Args&... args) {
     std::string error_message = tfm::format(fmt, args...);