net: extend Sock with a method to check whether connected

This will be convenient in the I2P SAM implementation.

src/util/sock.cpp

@@ -250,6 +250,31 @@ std::string Sock::RecvUntilTerminator(uint8_t terminator,
     }
 }
 
+bool Sock::IsConnected(std::string& errmsg) const
+{
+    if (m_socket == INVALID_SOCKET) {
+        errmsg = "not connected";
+        return false;
+    }
+
+    char c;
+    switch (Recv(&c, sizeof(c), MSG_PEEK)) {
+    case -1: {
+        const int err = WSAGetLastError();
+        if (IOErrorIsPermanent(err)) {
+            errmsg = NetworkErrorString(err);
+            return false;
+        }
+        return true;
+    }
+    case 0:
+        errmsg = "closed";
+        return false;
+    default:
+        return true;
+    }
+}
+
 #ifdef WIN32
 std::string NetworkErrorString(int err)
 {

src/util/sock.h

@@ -143,6 +143,13 @@ public:
                                             std::chrono::milliseconds timeout,
                                             CThreadInterrupt& interrupt) const;
 
+    /**
+     * Check if still connected.
+     * @param[out] err The error string, if the socket has been disconnected.
+     * @return true if connected
+     */
+    virtual bool IsConnected(std::string& errmsg) const;
+
 private:
     /**
      * Contained socket. `INVALID_SOCKET` designates the object is empty.