[systemd-devel] Changes to dependency graph during boot

[Conrad Hoffmann]

Thank you for that thorough explanation, much appreciated!

Cheers,
Conrad

On 6/25/19 11:14 AM, Lennart Poettering wrote:
> On Mo, 24.06.19 16:41, Conrad Hoffmann (ch at bitfehler.net) wrote:
> 
>> Hi,
>>
>> TL;DR: I was wondering what happens if a unit executed early during the
>> boot process changes the current dependency graph by either enabling or
>> even starting another unit that was previously disabled. Is this defined
>> behaviour, and if so what are the rules?
> 
> So, this is usually safe, but there are some issues with this, and I
> would not recommend reloading the dep tree during a regular, clean
> boot. It's not designed for that.
> 
> In some cases reloading the unit files during operation is not
> safe. Specifically, consider a unit file with contains something like
> that:
> 
> ```
> [Sevice]
> Type=oneshot
> ExecStart=/usr/bin/something with-a-param
> ExecStart=/usr/bin/something-else also-a-parm
> ExecStart=/usr/bin/something-third third-param
> ```
> 
> Now your service is already at the execution of the middle program,
> and now you reload the whole thing and make a modification of the unit
> file:
> 
> ```
> [Sevice]
> Type=oneshot
> ExecStart=/usr/bin/something with-a-param
> ExecStart=/usr/bin/something-else changed-param
> ExecStart=/usr/bin/something-third third-param
> ```
> 
> Now the question is where execution should continue: the middle line
> was changed, and we can easily see that as humans, but it's not clear
> from systemd's pov. It could also be that the old line was removed and
> a new line inserted instead. Depending how you see that after the
> reload it could mean to easier continue execution with the second line
> or with the third line, and it's really not clear semantically what is
> meant here.
> 
> systemd employs some heuristics and tries to do some reasonable smart
> choice, but it's black magic and it might be wrong. But this means:
> reloading the whole daemon during clean code paths is
> questionnable. it's fine doing that when you install or remove
> packages, i.e. not in common codepaths but only in an upgrade
> codepaths, but doing that on every single boot, as part of a clean
> start-up is something i would advocate like that.
> 
> Hence: changing units and issuing daemon reloads during boot-up is
> ugly. Don't do it. Besides these issues pointed out above, it's also
> slow.
> 
> Noe that if you have all units in place already and just want to
> enqueue them, that's entirely safe, the common case and
> recommended. The suggestions above are just about of modifying units,
> creating them, removing them and then reloading systemd. Don't do that.
> 
> In systemd, we have the "generator" concept, that is our recommended
> way to extend the unit dependency tree with foreign and automatically
> generated units. Generators run very early during boot, before the
> initial transaction is generated, which means everything they do is
> already complete by the time systemd calculates the initial
> transaction, and thus while that transaction and all its jobs are
> executed unit files are not changed anymore. Generally I'd strongly
> recommend using generators over manually modifying/creating units
> during boot combined with a daemon reload.
> 
> For more information about generators see:
> 
> https://www.freedesktop.org/software/systemd/man/systemd.generator.html
> 
> Lennart
> 
> --
> Lennart Poettering, Berlin
>