session UPDATE wait for ssh channel close

Wait until we receiver SSH EOF from the peer,
which avoids needlessly printing socket exception callbacks from libssh.
100ms should be more than enough to receive this message, if not
just free as before. Also possibly fix some race conditions
by locking io_lock before sending the EOF.

Fixes #589

Author: Roytak

src/session.c

@@ -796,18 +796,37 @@ nc_session_free_transport(struct nc_session *session, int *multisession)
 #ifdef NC_ENABLED_SSH_TLS
     case NC_TI_SSH: {
         int r;
+        struct timespec ts;
 
-        if (connected) {
-            ssh_channel_send_eof(session->ti.libssh.channel);
-            ssh_channel_free(session->ti.libssh.channel);
-        }
         /* There can be multiple NETCONF sessions on the same SSH session (NETCONF session maps to
          * SSH channel). So destroy the SSH session only if there is no other NETCONF session using
          * it. Also, avoid concurrent free by multiple threads of sessions that share the SSH session.
          */
         /* SESSION IO LOCK */
         r = nc_mutex_lock(session->io_lock, NC_SESSION_FREE_LOCK_TIMEOUT, __func__);
 
+        if (connected) {
+            /* send EOF to the peer, but do not close the channel yet, wait for the peer to send EOF too.
+             * 100ms timeout should be enough for the peer to react and send EOF,
+             * if not, just continue with freeing the session and closing the channel.
+             * This is done to avoid libssh WRN log about reading from a closed channel */
+            ssh_channel_send_eof(session->ti.libssh.channel);
+            nc_timeouttime_get(&ts, 100);
+            while (!ssh_channel_is_eof(session->ti.libssh.channel)) {
+                /* poll for the EOF, non-blocking */
+                if (ssh_channel_poll(session->ti.libssh.channel, 0) == SSH_ERROR) {
+                    /* if poll fails, just break and continue with freeing the session, it will be closed anyway */
+                    break;
+                }
+                if (nc_timeouttime_cur_diff(&ts) < 1) {
+                    /* waited long enough, continue with freeing */
+                    break;
+                }
+                usleep(NC_TIMEOUT_STEP);
+            }
+            ssh_channel_free(session->ti.libssh.channel);
+        }
+
         if (session->ti.libssh.next) {
             for (siter = session->ti.libssh.next; siter != session; siter = siter->ti.libssh.next) {
                 if (siter->status != NC_STATUS_STARTING) {
@@ -869,7 +888,7 @@ nc_session_free_transport(struct nc_session *session, int *multisession)
         sock = nc_tls_get_fd_wrap(session);
 
         if (connected) {
-            /* notify the peer that we're shutting down */
+            /* notify the peer that we're shutting down, we don't need to wait for the peer's response */
             nc_tls_close_notify_wrap(session->ti.tls.session);
         }