path: root/HACKING

When creating servers that can lock up the computer in some matter, it is
important to set `server_characteristics.fork_for_safety = 1` and set clean
up code in `fork_cleanup`. This should also be done if there is a risk for
global memory leaks such as IPC semaphores that have not be removed. Everything
that must be cleaned up should be initialised before `server_initialised` is
called and cleanup properly if the server failes before `server_initialised`
has be successfulled invoked. After `server_initialised` has returned with a
zero value the cleanups should be done in `fork_cleanup`. Keep in mind that
`fork_cleanup` is only called if `server_characteristics.fork_for_safety == 1`.

Additionally it is good practice to have another computer which you can use to
SSH into the development computer and kill a server that is looking up the
computer. If a server has a child process that is the same program, the child
is probably the fork created by `server_initialised` when
`server_characteristics.fork_for_safety == 1`. If these cases, it is the child
process that should be killed and the parent process should be left alone. If
you do not have another computer you can use, you should use the `--alarm=`
option on the server to make it kill itself after a select time period. On
`server_initialised` the parent processes alarm will be transfered to the
child process. There is a race codition here, the alarm could be triggered
after the fork but before the child server has acknowledged this marked to
not do the cleanup, additional there is not specification on whether how
the return value of `alarm` is rounded, if an OS would return 0 if the
time left is for example 0,25 seconds, the alarm would be disabled. Therefore
do not make the value of the `--alarm=` option to small like for example
one second (that is `--alarm=1`). Note that the alarm is restricted by
`src/mds-base.c` to be at most one minute, any value larger than 60 will
be truncated down to 60.