Prevent the OEM mount point from showing up before Ignition mounts

Ignition may run a manual mount command for the OEM partition if the
user specifies it. This fails if the OEM mount unit is triggered early,
because it can't be mounted twice, thus we need to prevent that from
happening by unmounting the OEM partition (but not the tmpfs OEM mount
for PXE-provided OEM contents). To spare users from even dealing with
the OEM partition specification in Ignition we can ensure it is mounted
before Ignition writes files.

dracut/10diskless-generator/diskless-generator

@@ -129,6 +129,9 @@ if [[ "${copy_oem}" -eq 1 ]]; then
 # Automatically generated by diskless-generator
 
 [Unit]
+# This runs early for initrd-setup-root to populate it.
+# Ignition can mount something else on /usr/share/oem still
+# if the user wants so.
 Before=initrd-root-fs.target
 
 [Mount]

dracut/30ignition/ignition-files.service

@@ -13,6 +13,9 @@ After=ignition-setup.service ignition-disks.service
 # setup the root filesystem before we try do things like create users on it.
 Requires=initrd-setup-root.service
 After=initrd-setup-root.service
+# Already mount the OEM partition here so that it works to write files
+# without having to declare it as initrd mount in Ignition
+RequiresMountsFor=/sysroot/usr/ /sysroot/usr/share/oem/
 
 OnFailure=emergency.target
 OnFailureJobMode=isolate

dracut/30ignition/ignition-mount.service

@@ -35,5 +35,7 @@ After=ignition-setup.service
 Type=oneshot
 RemainAfterExit=yes
 EnvironmentFile=/run/ignition.env
+# Flatcar: Unmount any OEM mount in case the Ignition config has one defined
+ExecStartPre=/bin/bash -c 'if ! mount | grep -m 1 /sysroot/usr/share/oem | grep tmpfs; then umount /sysroot/usr/share/oem || true; fi'
 ExecStart=/usr/bin/ignition --root=/sysroot --platform=${PLATFORM_ID} --stage=mount --log-to-stdout
 ExecStop=/usr/bin/ignition --root=/sysroot --platform=${PLATFORM_ID} --stage=umount --log-to-stdout