overlayfs.bbclass: generate 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.

Signed-off-by: Vyacheslav Yurkov <uvv.mail@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

diff --git a/meta/classes/overlayfs.bbclass b/meta/classes/overlayfs.bbclass
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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