[Buildroot] [PATCH 00/38] systemd support for various packages

[Alex Suykov]

Sun, May 24, 2015 at 01:21:58AM -0000, Cam Hutchison wrote:

> Type=simple is not necessarily the best way to do this. Handling errors
> is done differently by systemd with Type=simple vs Type=forking.

The difference in error handling is actually why I consider non-forking
services to be the only valid kind of services for anything more capable
that sysv initscripts.

> A recent blog post by Lucas Nussbaum has more details:
> http://www.lucas-nussbaum.net/blog/?p=877
> 
> With Type=simple, systemd considers the service started right after
> executing it. It will not flag errors on startup if the process exits
> soon after due to a config error, etc. With Type=forking, systemd will
> only consider the service started after the parent exits. As many
> daemons will check config files and command line args before forking,
> these errors will be detected and propagated to systemd.

Non-forking services signal initial configuration errors as well as any
errors past initial configuration phase by exiting with non-zero status.
Which systemd can take immediate action on, like restarting the service
or marking it as failed. It's mentioned in the blog post.

Forking daemons gain marginally better *initial* error reporting by losing
the ability to report errors at later stages. Detecting the state of a forked
daemon (as in running or dead), or killing it, is non-trivial and generally
involves time of check time of use race condition. With non-forking services,
both operations are trivial and reliable.

Does knowing the fine distiction between initial and non-initial failure
is worth the loss of control over the child process? I don't think so.

Reporting initial config status via parent process exit() is a specific
case of startup notification, the post mentions that too.
Any reliance on any kind of startup notification inherently results in
time of check time of use race conditon.