Michal Schmidt
3a11183858
There are a lot of forking daemons that do not exactly follow the initialization steps as described in daemon(7). It is common that they do not bother waiting in the parent process for the child to write the PID file before exiting. The daemons' developers often do not perceive this as a bug and they're unwilling to change. Currently systemd warns about the missing PID file and falls back to guessing the main PID. Being not quite deterministic, the guess can be wrong with bad consequences. If the guessing is disabled, determinism is achieved at the cost of losing the ability of noticing when the main process of the service dies. As long as it does not negatively affect properly written services, systemd should strive for compatibility even with services with racy daemonization. It is possible to provide determinism _and_ main process supervision to them. If the PID file is not there, rather than guessing and considering the service running immediately after getting the SIGCHLD from the ExecStart (or ExecStartPost) process, we can keep the service in the activating state for a bit longer. We can use inotify to wait for the PID file to appear. Only when it finally does appear and we read a valid PID from it, we'll move the service to the running state. If the PID file never appears, the usual timeout kicks in and the service fails.
systemd System and Session Manager
DETAILS:
http://0pointer.de/blog/projects/systemd.html
WEB SITE:
http://www.freedesktop.org/wiki/Software/systemd
GIT:
git://anongit.freedesktop.org/systemd
ssh://git.freedesktop.org/git/systemd
GITWEB:
http://cgit.freedesktop.org/systemd/
MAILING LIST:
http://lists.freedesktop.org/mailman/listinfo/systemd-devel
http://lists.freedesktop.org/mailman/listinfo/systemd-commits
IRC:
#systemd on irc.freenode.org
BUG REPORTS:
https://bugs.freedesktop.org/enter_bug.cgi?product=systemd
AUTHOR:
Lennart Poettering with major support from Kay Sievers
LICENSE:
GPLv2+ for all code, except sd-daemon.[ch] which is MIT
REQUIREMENTS:
Linux kernel >= 2.6.39
with devtmpfs
with cgroups (but it's OK to disable all controllers)
optional but strongly recommended: autofs4, ipv6
libudev >= 172
dbus >= 1.4.0
libcap
gtk+ >= 2.20 (optional)
PAM >= 1.1.2 (optional)
libcryptsetup (optional)
libaudit (optional)
libselinux (optional)
tcpwrappers (optional)
libnotify (optional)
When you build from git you need the following additional dependencies:
vala >= 0.10
docbook-xsl
xsltproc
automake
autoconf
libtool
gperf
make, gcc, and similar tools
During runtime you need the following dependencies:
util-linux > v2.18 (requires fsck -l, agetty -s)
sulogin (from sysvinit-tools, optional but recommended)
plymouth (optional)
dracut (optional)
When systemd-hostnamed is used it is strongly recommended to
install nss-myhostname to ensure that in a world of
dynamically changing hostnames the hostname stays resolveable
under all circumstances. In fact, systemd-hostnamed will warn
if nss-myhostname is not installed. Packagers are encouraged to
add a dependency on nss-myhostname to the package that
includes systemd-hostnamed.
Note that D-Bus can link against libsystemd-login.so, which
results in a cyclic build dependency. To accomodate for this
please build D-Bus without systemd first, then build systemd,
then rebuild D-Bus with systemd support.
WARNINGS:
systemd will warn you during boot if /etc/mtab is not a
symlink to /proc/mounts. Please ensure that /etc/mtab is a
proper symlink.
systemd will warn you during boot if /usr is on a different
file system than /. While in systemd itself very little will
break if /usr is on a separate partition many of its
dependencies very likely will break sooner or later in one
form or another. For example udev rules tend to refer to
binaries in /usr, binaries that link to libraries in /usr or
binaries that refer to data files in /usr. Since these
breakages are not always directly visible systemd will warn
about this, since this kind of file system setup is not really
supported anymore by the basic set of Linux OS components.
For more information on this issue consult
http://freedesktop.org/wiki/Software/systemd/separate-usr-is-broken
ENGINEERING AND CONSULTING SERVICES:
ProFUSION <http://profusion.mobi> offers professional
engineering and consulting services for systemd for embedded
and other use. Please contact Gustavo Barbieri
<barbieri@profusion.mobi> for more information.
Disclaimer: This notice is not a recommendation or official
endorsement. However, ProFUSION's upstream work has been very
beneficial for the systemd project.