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# Class for generation of overlayfs mount units
#
# It's often desired in Embedded System design to have a read-only rootfs.
# But a lot of different applications might want to have a read-write access to
# some parts of a filesystem. It can be especially useful when your update mechanism
# overwrites the whole rootfs, but you want your application data to be preserved
# between updates. This class provides a way to achieve that by means
# of overlayfs and at the same time keeping the base rootfs read-only.
#
# Usage example.
#
# Set a mount point for a partition overlayfs is going to use as upper layer
# in your machine configuration. Underlying file system can be anything that
# is supported by overlayfs. This has to be done in your machine configuration.
# QA check fails to catch file existence if you redefine this variable in your recipe!
#
# OVERLAYFS_MOUNT_POINT[data] ?= "/data"
#
# The class assumes you have a data.mount systemd unit defined in your
# systemd-machine-units recipe and installed to the image.
#
# Then you can specify writable directories on a recipe base
#
# OVERLAYFS_WRITABLE_PATHS[data] = "/usr/share/my-custom-application"
#
# To support several mount points you can use a different variable flag. Assume we
# want to have a writable location on the file system, but not interested where the data
# survive a reboot. Then we could have a mnt-overlay.mount unit for a tmpfs file system:
#
# OVERLAYFS_MOUNT_POINT[mnt-overlay] = "/mnt/overlay"
# OVERLAYFS_WRITABLE_PATHS[mnt-overlay] = "/usr/share/another-application"
#
# Note: the class does not support /etc directory itself, because systemd depends on it
REQUIRED_DISTRO_FEATURES += "systemd overlayfs"
inherit systemd features_check
python do_create_overlayfs_units() {
CreateDirsUnitTemplate = """[Unit]
Description=Overlayfs directories setup
Requires={DATA_MOUNT_UNIT}
After={DATA_MOUNT_UNIT}
DefaultDependencies=no
[Service]
Type=oneshot
ExecStart=mkdir -p {DATA_MOUNT_POINT}/workdir{LOWERDIR} && mkdir -p {DATA_MOUNT_POINT}/upper{LOWERDIR}
RemainAfterExit=true
StandardOutput=journal
[Install]
WantedBy=multi-user.target
"""
MountUnitTemplate = """[Unit]
Description=Overlayfs mount unit
Requires={CREATE_DIRS_SERVICE}
After={CREATE_DIRS_SERVICE}
[Mount]
What=overlay
Where={LOWERDIR}
Type=overlay
Options=lowerdir={LOWERDIR},upperdir={DATA_MOUNT_POINT}/upper{LOWERDIR},workdir={DATA_MOUNT_POINT}/workdir{LOWERDIR}
[Install]
WantedBy=multi-user.target
"""
def prepareUnits(data, lower):
from oe.overlayfs import mountUnitName, helperUnitName
args = {
'DATA_MOUNT_POINT': data,
'DATA_MOUNT_UNIT': mountUnitName(data),
'CREATE_DIRS_SERVICE': helperUnitName(lower),
'LOWERDIR': lower,
}
with open(os.path.join(d.getVar('WORKDIR'), mountUnitName(lower)), 'w') as f:
f.write(MountUnitTemplate.format(**args))
with open(os.path.join(d.getVar('WORKDIR'), helperUnitName(lower)), 'w') as f:
f.write(CreateDirsUnitTemplate.format(**args))
overlayMountPoints = d.getVarFlags("OVERLAYFS_MOUNT_POINT")
for mountPoint in overlayMountPoints:
for lower in d.getVarFlag('OVERLAYFS_WRITABLE_PATHS', mountPoint).split():
prepareUnits(d.getVarFlag('OVERLAYFS_MOUNT_POINT', mountPoint), lower)
}
# we need to generate file names early during parsing stage
python () {
from oe.overlayfs import strForBash, unitFileList
unitList = unitFileList(d)
for unit in unitList:
d.appendVar('SYSTEMD_SERVICE:' + d.getVar('PN'), ' ' + unit);
d.appendVar('FILES:' + d.getVar('PN'), ' ' + strForBash(unit))
d.setVar('OVERLAYFS_UNIT_LIST', ' '.join([strForBash(s) for s in unitList]))
}
do_install:append() {
install -d ${D}${systemd_system_unitdir}
for unit in ${OVERLAYFS_UNIT_LIST}; do
install -m 0444 ${WORKDIR}/${unit} ${D}${systemd_system_unitdir}
done
}
addtask create_overlayfs_units before do_install
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