portfwd: stop double-closing the forwarded socket

The LOCAL_CLEANUP handler closes the target socket, and portfwd_worker()
closed its own copy of the same descriptor again at teardown. That was
unreachable while the library never emitted the action, which is why the
report against it was set aside; it is reachable now.

- record the cleanup in the forwarding state and skip the second close
- reset the record when a fresh forward's socket is adopted, or one left
  by an earlier refused open skips closing a live socket at exit
- drop the handler comment saying the action is never emitted

Author: ejohnstown

examples/portfwd/portfwd.c

@@ -249,6 +249,7 @@ typedef struct PortfwdState {
     SOCKET_T appFd;         /* socket to the local target, -1 when idle */
     word32 channelId;       /* id of the inbound forwarded-tcpip channel */
     int pending;            /* a new channel is waiting to be wired up */
+    int cleanupRxd;         /* LOCAL_CLEANUP closed appFd for us */
     int replied;            /* peer answered the tcpip-forward request */
     int refused;            /* ...and the answer was a refusal */
     int badPort;            /* ...or named a port outside 1..65535 */
@@ -319,15 +320,17 @@ static int portfwdRemoteFwdCb(WS_FwdCbAction action, void* ctx,
             st->pending = 1;
             break;
         case WOLFSSH_FWD_LOCAL_CLEANUP:
-            /* The library does not currently emit this action, so this branch
-             * never runs. The target socket is closed when portfwd_worker()
-             * leaves its loop. Kept so the handler is right if that changes. */
+            /* Paired with the LOCAL_SETUP that opened the target socket.
+             * portfwd_worker() keeps its own copy of the descriptor, so tell
+             * it not to close what has already been closed. The closing
+             * channel's id arrives in the port argument. */
             (void)address;
             (void)port;
             if (st->appFd != (SOCKET_T)-1) {
                 WCLOSESOCKET(st->appFd);
                 st->appFd = (SOCKET_T)-1;
             }
+            st->cleanupRxd = 1;
             break;
         case WOLFSSH_FWD_REMOTE_SETUP:
         case WOLFSSH_FWD_REMOTE_CLEANUP:
@@ -746,6 +749,9 @@ THREAD_RETURN WOLFSSH_THREAD portfwd_worker(void* args)
                         WS_CHANNEL_ID_SELF);
                 if (fwdState.appFd != (SOCKET_T)-1 && newChannel != NULL) {
                     appFd = fwdState.appFd;
+                    /* The latch describes this descriptor now, not one an
+                     * earlier failed open already cleaned up. */
+                    fwdState.cleanupRxd = 0;
                     fwdChannel = newChannel;
                     fwdChannelId = fwdState.channelId;
                     FD_SET(appFd, &templateFds);
@@ -885,7 +891,12 @@ THREAD_RETURN WOLFSSH_THREAD portfwd_worker(void* args)
     WCLOSESOCKET(sshFd);
     if (listenFd != (SOCKET_T)-1)
         WCLOSESOCKET(listenFd);
-    WCLOSESOCKET(appFd);
+    /* Skip a descriptor the cleanup callback already closed; closing it
+     * twice can take down whatever has been handed the number since. */
+    if (fwdState.cleanupRxd)
+        appFd = (SOCKET_T)-1;
+    if (appFd != (SOCKET_T)-1)
+        WCLOSESOCKET(appFd);
     wolfSSH_free(ssh);
     wolfSSH_CTX_free(ctx);
 #ifdef WOLFSSH_SMALL_STACK