sockfilt.c: Fixed detection of client-side connection close

WINSOCK only:
Since FD_CLOSE is only signaled once, it may trigger at the same
time as FD_READ. Data actually being available makes it impossible
to detect that the connection was closed by checking that recv returns
zero. Another recv attempt could block the connection if it was
not closed. This workaround abuses exceptfds in conjunction with
readfds to signal that the connection has actually closed.

tests/server/sockfilt.c

@@ -580,7 +580,7 @@ static int select_ws(int nfds, fd_set *readfds, fd_set *writefds,
       networkevents |= FD_WRITE|FD_CONNECT;
 
     if(FD_ISSET(fds, exceptfds))
-      networkevents |= FD_OOB;
+      networkevents |= FD_OOB|FD_CLOSE;
 
     /* only wait for events for which we actually care */
     if(networkevents) {
@@ -695,8 +695,17 @@ static int select_ws(int nfds, fd_set *readfds, fd_set *writefds,
           if(!(wsanetevents.lNetworkEvents & (FD_WRITE|FD_CONNECT)))
             FD_CLR(sock, writefds);
 
+          /* HACK:
+           * use exceptfds together with readfds to signal
+           * that the connection was closed by the client.
+           *
+           * Reason: FD_CLOSE is only signaled once, sometimes
+           * at the same time as FD_READ with data being available.
+           * This means that recv/sread is not reliable to detect
+           * that the connection is closed.
+           */
           /* remove from descriptor set if not exceptional */
-          if(!(wsanetevents.lNetworkEvents & FD_OOB))
+          if(!(wsanetevents.lNetworkEvents & (FD_OOB|FD_CLOSE)))
             FD_CLR(sock, exceptfds);
         }
       }
@@ -802,6 +811,9 @@ static bool juggle(curl_socket_t *sockfdp,
     else {
       /* there's always a socket to wait for */
       FD_SET(sockfd, &fds_read);
+#ifdef USE_WINSOCK
+      FD_SET(sockfd, &fds_err);
+#endif
       maxfd = (int)sockfd;
     }
     break;
@@ -812,6 +824,9 @@ static bool juggle(curl_socket_t *sockfdp,
     /* sockfd turns CURL_SOCKET_BAD when our connection has been closed */
     if(CURL_SOCKET_BAD != sockfd) {
       FD_SET(sockfd, &fds_read);
+#ifdef USE_WINSOCK
+      FD_SET(sockfd, &fds_err);
+#endif
       maxfd = (int)sockfd;
     }
     else {
@@ -979,7 +994,22 @@ static bool juggle(curl_socket_t *sockfdp,
     /* read from socket, pass on data to stdout */
     nread_socket = sread(sockfd, buffer, sizeof(buffer));
 
-    if(nread_socket <= 0) {
+    if(nread_socket > 0) {
+      snprintf(data, sizeof(data), "DATA\n%04zx\n", nread_socket);
+      if(!write_stdout(data, 10))
+        return FALSE;
+      if(!write_stdout(buffer, nread_socket))
+        return FALSE;
+
+      logmsg("< %zd bytes data, client => server", nread_socket);
+      lograw(buffer, nread_socket);
+    }
+
+    if(nread_socket <= 0
+#ifdef USE_WINSOCK
+       || FD_ISSET(sockfd, &fds_err)
+#endif
+       ) {
       logmsg("====> Client disconnect");
       if(!write_stdout("DISC\n", 5))
         return FALSE;
@@ -991,15 +1021,6 @@ static bool juggle(curl_socket_t *sockfdp,
         *mode = ACTIVE_DISCONNECT;
       return TRUE;
     }
-
-    snprintf(data, sizeof(data), "DATA\n%04zx\n", nread_socket);
-    if(!write_stdout(data, 10))
-      return FALSE;
-    if(!write_stdout(buffer, nread_socket))
-      return FALSE;
-
-    logmsg("< %zd bytes data, client => server", nread_socket);
-    lograw(buffer, nread_socket);
   }
 
   return TRUE;