Currently with FileDescriptorStorePreserve=yes the FD store is kept around
regardless of what happens to a unit, which is useful in many cases. But in
some cases, for example when complex services crash horribly, it's hard to
reason about what was in the intermediate state, and it's better to start
fresh.
Add a new 'on-success' option for the FileDescriptorStorePreserve= setting
that keeps it around only for as long as the unit doesn't go to a persistently
failed state.
This is especially useful in combination with LUO, where we don't want to
keep around LUO sessions created by units that then proceeded to crash and
burn, and might be in a bad state afterwards.
LUO sessions cannot be nested under other sessions. This means we need
to handle them explicitly, and held them open in the shutdown binary
like we do with our own internal session, to allow services to create
their own.
The requirement to support third party sessions comes from VMMs that
wish to preserve VM(s) state(s) across kexec, as some file descriptors
(KVM's vmfd from the KVM_CREATE_VM ioctl) cannot be transfered between
processes via SCM_RIGHTS, so they cannot be stashed in the FD Store
directly. Also some file descriptors have to be handled all together or
not at all, again to do with KVM and devices that are all part of the
same vm.
In order to allow FD Stores of user units to survive a user
session restart, propagate FDs received via the protocol up one
level from user to system manager via sd_notify.
And the other way around, propagate them down via LISTEN_FDS
tagging them with the unit name so that the child manager can
inject them in the appropriate unit.
Ensure units that are dead or not loaded can get FDs added to
their stores, and that they are correctly propagated once the
unit is started or loaded. When the unit is not loaded we don't
know what the FD max limit is, so simply increase it for each FD
injected, and then when the unit is realised prune it down to
match the unit's now available config in case the limit is lower
than the number of FDs in the store.
Each FD sent up or down is assigned a monotonic index, and the manager
also sends a JSON map that associates the index with the original
unit and FDNAME:
{
"unit-name.service": [
{ "name": "fdname1", "index": 1 },
{ "name": "fdname2", "index": 2 }
],
...
}
This allows the manager to assign back the FDs to the appropriate
unit using the appropriate name, given the FDNAMEs are not unique.
Although extremely unlikely, there is a race present in solely checking the
$LISTEN_PID environment variable, due to PID recycling. Fix that by introducing
$LISTEN_PIDFDID, which contains the 64-bit ID of a pidfd for the child process
that is not subject to recycling.
This reverts commit 5e8ff010a1.
This broke all the URLs, we can't have that. (And actually, we probably don't
_want_ to make the change either. It's nicer to have all the pages in one
directory, so one doesn't have to figure out to which collection the page
belongs.)
ispell made some suggestions which I applied.
Addresses: https://github.com/systemd/systemd/pull/29209#pullrequestreview-1632623460
Also adds a brief paragraph about initrd transitions. (Plymouth really
should start using the fdstore for pinning DRM objects, and stop trying
to survive the initrd→host transition)
Yu Watanabe
Luca Boccassi
Daniel Foster
Nils K
Zbigniew Jędrzejewski-Szmek
hulkoba
Mike Yuan
Lennart Poettering
Joerg Behrmann