echoserver: close the agent socket on reset and handle a rekey

The worker drops back to APP_STATE_LISTEN when an agent connection ends,
but never closes the socket. The next accept() overwrites agentFd, so
every agent connection after the first leaks the previous descriptor. The
forward path has the same gap on its connection-reset arm, where the
socket is closed but fwdFd keeps the closed number.

A rekey was treated as a read failure and ended the session. It cannot
just be skipped either: wolfSSH_worker() reports WS_REKEYING in place of
WS_CHAN_RXD while keying, and nothing raises the data report again, so
ignoring it strands whatever arrived in that call and the peer waits on
an answer that never comes. This is the hazard the library already calls
out for WS_EXTDATA, which is exempted from the same override.

- close agentFd and clear it on both the read-zero and the
  ECONNRESET/ECONNABORTED arms
- clear fwdFd on the forward reset arm, matching the read-zero arm
- clear agentCtx.appFd and fwdCtx.appFd wherever the worker closes the
  socket, so the stored copy cannot outlive the descriptor
- drain the channel on WS_REKEYING as well as WS_CHAN_RXD, and take an
  empty read as "nothing buffered" rather than a failure on that path.
  wolfSSH_ChannelIdRead() has no isKeying gate and the window credit it
  owes is parked until the rekey completes

Author: ejohnstown

examples/echoserver/echoserver.c

@@ -999,14 +999,25 @@ static int ssh_worker(thread_ctx_t* threadCtx)
                 cnt_r = wolfSSH_worker(ssh, &lastChannel);
                 if (cnt_r < 0) {
                     rc = wolfSSH_get_error(ssh);
-                    if (rc == WS_CHAN_RXD) {
+                    /* wolfSSH_worker() reports WS_REKEYING in place of
+                     * WS_CHAN_RXD while a rekey is in flight, and the data
+                     * report is never raised again, so drain on both or the
+                     * buffered bytes sit there and the peer waits forever.
+                     * wolfSSH_ChannelIdRead() has no isKeying gate; the window
+                     * credit it owes is parked until the rekey finishes. */
+                    if (rc == WS_CHAN_RXD || rc == WS_REKEYING) {
                         if (lastChannel == threadCtx->shellCtx.channelId) {
                             cnt_r = wolfSSH_ChannelIdRead(ssh,
                                     threadCtx->shellCtx.channelId,
                                     threadCtx->channelBuffer,
                                     sizeof threadCtx->channelBuffer);
-                            if (cnt_r <= 0)
+                            if (cnt_r <= 0) {
+                                /* Nothing was buffered. Only an actual data
+                                 * report makes that a failure. */
+                                if (rc == WS_REKEYING)
+                                    continue;
                                 break;
+                            }
                             #ifdef SHELL_DEBUG
                                 buf_dump(threadCtx->channelBuffer, cnt_r);
                             #endif
@@ -1044,8 +1055,13 @@ static int ssh_worker(thread_ctx_t* threadCtx)
                             cnt_r = wolfSSH_ChannelIdRead(ssh, agentChannelId,
                                     threadCtx->channelBuffer,
                                     sizeof threadCtx->channelBuffer);
-                            if (cnt_r <= 0)
+                            if (cnt_r <= 0) {
+                                /* Nothing was buffered. Only an actual data
+                                 * report makes that a failure. */
+                                if (rc == WS_REKEYING)
+                                    continue;
                                 break;
+                            }
                             #ifdef SHELL_DEBUG
                                 buf_dump(threadCtx->channelBuffer, cnt_r);
                             #endif
@@ -1063,8 +1079,13 @@ static int ssh_worker(thread_ctx_t* threadCtx)
                                     threadCtx->fwdCtx.channelId,
                                     threadCtx->channelBuffer,
                                     sizeof threadCtx->channelBuffer);
-                            if (cnt_r <= 0)
+                            if (cnt_r <= 0) {
+                                /* Nothing was buffered. Only an actual data
+                                 * report makes that a failure. */
+                                if (rc == WS_REKEYING)
+                                    continue;
                                 break;
+                            }
                             #ifdef SHELL_DEBUG
                                 buf_dump(threadCtx->channelBuffer, cnt_r);
                             #endif
@@ -1084,6 +1105,7 @@ static int ssh_worker(thread_ctx_t* threadCtx)
                             if (fwdFd != -1) {
                                 WCLOSESOCKET(fwdFd);
                                 fwdFd = -1;
+                                threadCtx->fwdCtx.appFd = -1;
                             }
                             if (threadCtx->fwdCbCtx.originName != NULL) {
                                 WFREE(threadCtx->fwdCbCtx.originName,
@@ -1151,6 +1173,9 @@ static int ssh_worker(thread_ctx_t* threadCtx)
                     if (cnt_r == 0) {
                         /* Read zero-returned. Socket is closed. Go back
                            to listening. */
+                        WCLOSESOCKET(agentFd);
+                        agentFd = -1;
+                        threadCtx->agentCtx.appFd = -1;
                         threadCtx->agentCtx.state = APP_STATE_LISTEN;
                         continue;
                     }
@@ -1164,6 +1189,9 @@ static int ssh_worker(thread_ctx_t* threadCtx)
                                 err == SOCKET_ECONNABORTED) {
                             /* Connection reset. Socket is closed.
                              * Go back to listening. */
+                            WCLOSESOCKET(agentFd);
+                            agentFd = -1;
+                            threadCtx->agentCtx.appFd = -1;
                             threadCtx->agentCtx.state = APP_STATE_LISTEN;
                             continue;
                         }
@@ -1215,6 +1243,7 @@ static int ssh_worker(thread_ctx_t* threadCtx)
                            to listening. */
                         WCLOSESOCKET(fwdFd);
                         fwdFd = -1;
+                        threadCtx->fwdCtx.appFd = -1;
                         if (threadCtx->fwdCbCtx.hostName != NULL) {
                             WFREE(threadCtx->fwdCbCtx.hostName,
                                     NULL, 0);
@@ -1235,6 +1264,8 @@ static int ssh_worker(thread_ctx_t* threadCtx)
                             /* Connection reset. Socket is closed.
                              * Go back to listening. */
                             WCLOSESOCKET(fwdFd);
+                            fwdFd = -1;
+                            threadCtx->fwdCtx.appFd = -1;
                             threadCtx->fwdCtx.state = APP_STATE_LISTEN;
                             continue;
                         }