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