b9c1883a9c
Oftentimes it is useful to allow the per-service fd store to survive longer than for a restart. This is useful in various scenarios: 1. An fd to some security relevant object needs to be stashed somewhere, that should not be cleaned automatically, because the security enforcement would be dropped then. 2. A user namespace fd should be allocated on first invocation and be kept around until the user logs out (i.e. systemd --user ends), á la #16328 (This does not implement what #16318 asks for, but should solve the use-case discussed there.) 3. There's interest in allow a concept of "userspace reboots" where the kernel stays running, and userspace is swapped out (i.e. all services exit, and the rootfs transitioned into a new version of it) while keeping some select resources pinned, very similar to how we implement a switch root. Thus it is useful to allow services to exit, while leaving their fds around till the very end. This is exposed through a new FileDescriptorStorePreserve= setting that is closely modelled after RuntimeDirectoryPreserve= (in fact it reused the same internal type), since we want similar behaviour in the end, and quite often they probably want to be used together.
System and Service Manager
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