net_ssh: Handle SSH session termination more cleanly and sanely.

Previously, ssh_channel_close would sometimes hang when
called at session termination. To work around this, preliminary
changes were made to set the channel to nonblocking mode and
try to cleanly end the session using SSH EOF. This can lead
to busy loops if done simplistically, since the client may
not end the session cleanly. As a compromise, try to do so
if we can, but use a loose loop and give up after a minute or so.

LBBS-57 #close

nets/net_ssh.c

@@ -634,6 +634,7 @@ static void handle_session(ssh_event event, ssh_session session)
 	int node_started = 0;
 	int stdoutfd;
 	int is_sftp = 0;
+	int res;
 	long int timeout; /* in seconds */
 	/* We set the user when we have access to the session userdata,
 	 * but we need to attach it the node when we have access to the
@@ -893,9 +894,45 @@ static void handle_session(ssh_event event, ssh_session session)
 		bbs_node_exit(cdata.node);
 	}
 
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations" /* ssh_channel_get_exit_status, string_len, string_data */
 	/* Goodbye */
-	ssh_channel_send_eof(sdata.channel);
-	ssh_channel_close(sdata.channel);
+	ssh_channel_set_blocking(sdata.channel, 0); /* Set nonblocking, to avoid ssh_channel_close hanging */
+	/* Try to cleanly end the connection */
+	res = ssh_channel_send_eof(sdata.channel);
+	if (res == SSH_ERROR) {
+		int code = ssh_channel_get_exit_status(sdata.channel);
+		bbs_error("SSH session ended uncleanly with code %d\n", code);
+	}
+	if (ssh_channel_is_open(sdata.channel) && !ssh_channel_is_eof(sdata.channel)) {
+		/* Try to end the connection cleanly, if we can, but don't wait forever for that.
+		 * Sometimes SSH sessions can also be stale, in which case we won't get an EOF
+		 * (or anythiing else) from that client when we force disconnect it.
+		 * Setting SO_KEEPALIVE could help slightly but doesn't eliminate this possibility. */
+		char buf[512];
+		int eof_waitcount = 0;
+		bbs_debug(3, "Channel not EOF yet\n");
+		do {
+			res = ssh_channel_read(sdata.channel, buf, sizeof(buf), 0);
+			if (res == SSH_ERROR) {
+				int code = ssh_channel_get_exit_status(sdata.channel);
+				bbs_error("SSH session ended uncleanly with code %d\n", code);
+			} else if (res == SSH_EOF) {
+				bbs_debug(3, "Received EOF\n");
+				break;
+			} else if (res) {
+				bbs_debug(3, "Received %d byte%s\n", res, ESS(res));
+			} else {
+				bbs_debug(5, "Channel still not EOF yet\n");
+			}
+			if (++eof_waitcount > 200) {
+				bbs_warning("SSH client still hasn't disconnected cleanly, forcibly disconnecting...\n");
+				break;
+			}
+			usleep(250000); /* Avoid tight loop */
+		} while (ssh_channel_is_open(sdata.channel) && !ssh_channel_is_eof(sdata.channel));
+	}
+	ssh_channel_close(sdata.channel); /* In some cases, this may be the 2nd time calling this, but shouldn't hurt */
+	ssh_channel_free(sdata.channel);
 }
 
 /* === SFTP functions === */
@@ -1173,7 +1210,6 @@ static int handle_read(sftp_client_message msg)
 	return 0;
 }
 
-#pragma GCC diagnostic ignored "-Wdeprecated-declarations" /* string_len and string_data */
 static int handle_write(sftp_client_message msg)
 {
 	size_t len;