GitHub user changkun opened a pull request:
https://github.com/apache/guacamole-server/pull/179
GUACAMOLE-608: Fix connection write thread may encounter dead loop
guacd involves a `__write_all` function to write instruction as much as
possible. However, system call `write` is possible [return 0 and also set
errno](https://linux.die.net/man/2/write), which is not verified in the
function.
A possible case is `write` keeps return 0 (nothing writes to buffer).
Therefore the daemon process encounters a dead loop. Furthermore, it leads CPU
rate up to 99%.
The above phenomenon is observed from server:
```bash
$ top
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
15176 root 20 0 1988580 25720 3168 S 99.9 0.3 12176:50 guacd
25414 root 20 0 1726048 23324 3040 S 99.9 0.3 17227:15 guacd
...
$ sudo lsof -i | grep 15176 # no network connection
$ top -H -p 25414
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
25418 root 20 0 1726048 23324 3040 R 99.9 0.3 17230:06 guacd
25414 root 20 0 1726048 23324 3040 S 0.0 0.3 0:00.00 guacd
25420 root 20 0 1726048 23324 3040 S 0.0 0.3 0:06.16 guacd
```
This patch perhaps needs more investigation on when and why this is
happening, as well as wether should we record the failure to log, for instance:
```c
int written = write(fd, buffer, length);
if (written < 0)
return -1;
if (written == 0 || errno > 0) {
guacd_log(GUAC_LOG_ERROR, "Unable to write buffer to file descriptors:
%s", strerror(errno));
return -1;
}
```
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/changkun/guacamole-server jira/608
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/guacamole-server/pull/179.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #179
----
commit 5f094b32755a988c2ebece9bbdf659be56d2feb6
Author: Changkun Ou <hi@...>
Date: 2018-08-09T14:37:21Z
GUACAMOLE-608: Fix connection write thread may encounter dead loop.
----
---
