Report the disconnect with no channel to drop

wolfSSH_shutdown() set ssh->error to WS_DISCONNECT only inside the
channel branch, so a flush that emptied the buffer with the channel
already retired left behind the WS_WANT_WRITE that queued it. echoserver
and sftpclient read that error and burn ten wolfSSH_worker() calls on a
write that is already done. TestShutdownFlushesWithNoChannel asserts it.

Issue: F-8837

Author: ejohnstown

src/ssh.c

@@ -1161,13 +1161,16 @@ int wolfSSH_shutdown(WOLFSSH* ssh)
      * and the wait for a close that will not come. RFC 4253 section 11.1. */
     if (channel != NULL && ssh->disconnected) {
         WLOG(WS_LOG_DEBUG, "Session already disconnected, nothing to send");
-        /* An unfinished flush owns ssh->error. Callers gate their retry on
-         * WS_WANT_WRITE, so overwriting it strands the queued disconnect. */
-        if (flushRet == WS_SUCCESS)
-            ssh->error = WS_DISCONNECT;
         channel = NULL;
     }
 
+    /* Report the dead session with or without a channel to drop: callers
+     * gate their retry on ssh->error, and the flush above may have just
+     * emptied the output buffer they would be retrying for. An unfinished
+     * flush owns the error instead, since that retry is still owed. */
+    if (ssh != NULL && ssh->disconnected && flushRet == WS_SUCCESS)
+        ssh->error = WS_DISCONNECT;
+
     /* if channel close was not already sent then send it */
     if (channel != NULL && !channel->closeTxd) {
        if (ret == WS_SUCCESS) {

tests/regress.c

@@ -3583,6 +3583,9 @@ static void TestShutdownFlushesWithNoChannel(void)
     AssertIntEQ(ret, WS_CHANNEL_CLOSED);
     AssertFalse(wolfSSH_OutputPending(ssh));
     AssertIntEQ(out[LENGTH_SZ + 1], MSGID_DISCONNECT);
+    /* The flush finished, so the WS_WANT_WRITE that queued it is stale.
+     * Leaving it sends the caller back for a write that is already done. */
+    AssertIntEQ(wolfSSH_get_error(ssh), WS_DISCONNECT);
 
     wolfSSH_free(ssh);
     wolfSSH_CTX_free(ctx);