libssh port: immediate mode for ssh poll

This patch adds a switch to internal poll function in libssh that
allows to force this function to immediately return without actually
polling for data and in consequence processing this data. This switch
is used to avoid calling callback functions when flushing output
streams which caused locks due to recursive access to internal
ssh_terminal sessions registry.

Issue #4258

repos/libports/lib/symbols/libssh

@@ -228,6 +228,7 @@ ssh_event_add_connector T
 ssh_event_add_fd T
 ssh_event_add_poll T
 ssh_event_add_session T
+ssh_event_set_dopoll_immediate T
 ssh_event_dopoll T
 ssh_event_free T
 ssh_event_new T

repos/libports/ports/libssh.hash

@@ -1 +1 @@
-32ea34e5e531845ea1b155620b7685c668a9851d
+3abfbeacd8bfe69ba0e332a193fa62889a36ae0c

repos/libports/ports/libssh.port

@@ -11,5 +11,6 @@ DIR_CONTENT(include) := src/lib/libssh/include/libssh
 
 PATCHES   := src/lib/libssh/event_bind.patch \
              src/lib/libssh/sftp_support.patch \
-             src/lib/libssh/sftp_server_free.patch
+             src/lib/libssh/sftp_server_free.patch \
+             src/lib/libssh/event_immediate_poll.patch
 PATCH_OPT := -p1 -d src/lib/libssh

repos/libports/src/lib/libssh/event_immediate_poll.patch

@@ -0,0 +1,57 @@
+--- a/include/libssh/libssh.h
++++ b/include/libssh/libssh.h
+@@ -790,6 +790,7 @@
+                                     ssh_event_callback cb, void *userdata);
+ LIBSSH_API int ssh_event_add_session(ssh_event event, ssh_session session);
+ LIBSSH_API int ssh_event_add_connector(ssh_event event, ssh_connector connector);
++LIBSSH_API void ssh_event_set_dopoll_immediate(ssh_event event, int immediate);
+ LIBSSH_API int ssh_event_dopoll(ssh_event event, int timeout);
+ LIBSSH_API int ssh_event_remove_fd(ssh_event event, socket_t fd);
+ LIBSSH_API int ssh_event_remove_session(ssh_event event, ssh_session session);
+diff -r -u a/src/poll.c b/src/poll.c
+--- a/src/poll.c
++++ b/src/poll.c
+@@ -79,6 +79,9 @@
+   size_t polls_allocated;
+   size_t polls_used;
+   size_t chunk_size;
++  /* if set causes immediate exit of ssh_poll_ctx_dopoll with */
++  /* SSH_AGAIN to avoid any packet processing. */
++  int poll_immediate;
+ };
+ 
+ #ifdef HAVE_POLL
+@@ -604,6 +607,9 @@
+   if (!ctx->polls_used)
+     return SSH_ERROR;
+ 
++  if (ctx->poll_immediate)
++    return SSH_AGAIN;
++
+   ssh_timestamp_init(&ts);
+   do {
+     int tm = ssh_timeout_update(&ts, timeout);
+@@ -866,6 +872,23 @@
+ }
+ 
+ /**
++ * @brief Sets poll immediate mode.
++ *
++ * Effectively disables polling and causes immediate return with
++ * SSH_AGAIN.
++ *
++ * @param  event        The ssh_event object to set polling mode.
++ *
++ * @param  immediate    Immediate mode value.
++ */
++void ssh_event_set_dopoll_immediate(ssh_event event, int immediate) {
++    if(event == NULL || event->ctx == NULL) {
++        return;
++    }
++    event->ctx->poll_immediate = immediate;
++}
++
++/**
+  * @brief Poll all the sockets and sessions associated through an event object.i
+  *
+  * If any of the events are set after the poll, the call back functions of the