core: do not serialize mounts and automounts for switch-root

When e.g. tmp.mount is present in the initrd, and we serialize it, switch root,
and deserialize, the new systemd is confused because it thinks /tmp is mounted.
In general, it doesn't make sense to serialize anything that refers to paths in
the old root file system.

This fixes two errors for me:

1. tmp.mount was not mounted properly before local-fs.target. It would be
mounted as some point (I guess when we re-read /proc/self/mountinfo for some
other reason). In effect systemd-tmpfiles-setup.service would see one fs, and
some other units started later a different one. In particular gdm.service would
fail because the pre-created /tmp/.X11-unix with proper permissions would not
exist at time it was started.

2. # systemd[1]: proc-sys-fs-binfmt_misc.automount: Got hangup/error on autofs pipe from kernel. Likely our automount point has been unmounted by someone or something else?
   # systemd[1]: proc-sys-fs-binfmt_misc.automount: Failed with result 'unmounted'.
   # systemd[1]: Mounting proc-sys-fs-binfmt_misc.mount...
   # systemd[1]: Mounted proc-sys-fs-binfmt_misc.mount.
   # systemd[1]: Starting systemd-binfmt.service...
   # systemd[1]: Finished systemd-binfmt.service.
   # systemd[1]: proc-sys-fs-binfmt_misc.automount: Path /proc/sys/fs/binfmt_misc is already a mount point, refusing start.
   # systemd[1]: Failed to set up automount proc-sys-fs-binfmt_misc.automount.
   # systemd[1]: proc-sys-fs-binfmt_misc.automount: Path /proc/sys/fs/binfmt_misc is already a mount point, refusing start.
   # systemd[1]: Failed to set up automount proc-sys-fs-binfmt_misc.automount.
   # systemd[1]: proc-sys-fs-binfmt_misc.automount: Path /proc/sys/fs/binfmt_misc is already a mount point, refusing start.
   # systemd[1]: Failed to set up automount proc-sys-fs-binfmt_misc.automount.
   # systemd[1]: Stopping systemd-binfmt.service...
   # systemd[1]: systemd-binfmt.service: Deactivated successfully.
   # systemd[1]: Stopped systemd-binfmt.service.

I couldn't understand the error here, but in retrospect the first line is entirely
correct: "someone or something else" was the old systemd unmounting the old root.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/core/unit.h b/src/core/unit.h
index dddb419613..8818392731 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -668,6 +668,9 @@ typedef struct UnitVTable {
         /* True if units of this type shall be startable only once and then never again */
         bool once_only;
 
+        /* Do not serialize this unit when preparing for root switch */
+        bool exclude_from_switch_root_serialization;
+
         /* True if queued jobs of this type should be GC'ed if no other job needs them anymore */
         bool gc_jobs;