[Buildroot] [PATCH 2/4] package/skeleton-systemd: systemd-ify mounting /var tmpfs with ro rootfs

[Yann E. MORIN]

To mount our /var tmpfs when the rootfs is mounted read-only (really,
not remounted reqd-write), we use an entry in fstab.

However, /etc could also be a tmpfs (for full state-less systems, or
easy factory-reset, see [0]).

So, we register /var as a systemd mount unit, so that we can also have
the /var factory populated and functional even when /etc is empty. The
var.mount unit is heavily modelled after systemd's own tmp.mount one, so
we carry the same license for that file (in case that may apply). We add
an explicit reverse dependency to systemd-tmpfiles-setup.service, to
ensure /var is mounted before we try to populate it.

[0] http://0pointer.de/blog/projects/stateless.html

Signed-off-by: Yann E. MORIN <yann.morin at orange.com>
Cc: Norbert Lange <nolange79 at gmail.com>
Cc: Romain Naour <romain.naour at smile.fr>
Cc: Jérémy Rosen <jeremy.rosen at smile.fr>
[yann.morin.1998 at free.fr:
  - split original patch in two
  - this one only handles converting /var mounting into a systemd unit
  - adapt commit log accordingly
]
Signed-off-by: Yann E. MORIN <yann.morin.1998 at free.fr>
---
 .../skeleton-init-systemd.mk                   |  3 ++-
 package/skeleton-init-systemd/var.mount        | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 package/skeleton-init-systemd/var.mount

diff --git a/package/skeleton-init-systemd/skeleton-init-systemd.mk b/package/skeleton-init-systemd/skeleton-init-systemd.mk
index 7b66732ef4..970951d553 100644
--- a/package/skeleton-init-systemd/skeleton-init-systemd.mk
+++ b/package/skeleton-init-systemd/skeleton-init-systemd.mk
@@ -30,7 +30,6 @@ else
 # back there by the tmpfiles.d mechanism.
 define SKELETON_INIT_SYSTEMD_ROOT_RO_OR_RW
 	echo "/dev/root / auto ro 0 1" >$(TARGET_DIR)/etc/fstab
-	echo "tmpfs /var tmpfs mode=1777 0 0" >>$(TARGET_DIR)/etc/fstab
 endef
 
 define SKELETON_INIT_SYSTEMD_PRE_ROOTFS_VAR
@@ -52,6 +51,8 @@ define SKELETON_INIT_SYSTEMD_PRE_ROOTFS_VAR
 			|| exit 1; \
 		fi; \
 	done >$(TARGET_DIR)/usr/lib/tmpfiles.d/buildroot-factory.conf
+	$(INSTALL) -D -m 0644 $(SKELETON_INIT_SYSTEMD_PKGDIR)/var.mount \
+		$(TARGET_DIR)/usr/lib/systemd/system/var.mount
 endef
 SKELETON_INIT_SYSTEMD_ROOTFS_PRE_CMD_HOOKS += SKELETON_INIT_SYSTEMD_PRE_ROOTFS_VAR
 
diff --git a/package/skeleton-init-systemd/var.mount b/package/skeleton-init-systemd/var.mount
new file mode 100644
index 0000000000..6b165dff6d
--- /dev/null
+++ b/package/skeleton-init-systemd/var.mount
@@ -0,0 +1,18 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# Modelled after systemd's tmp.mount
+
+[Unit]
+Description=Buildroot /var tmpfs
+DefaultDependencies=no
+Conflicts=umount.target
+Before=basic.target local-fs.target umount.target systemd-tmpfiles-setup.service
+After=swap.target
+
+[Mount]
+What=tmpfs
+Where=/var
+Type=tmpfs
+Options=mode=1777,strictatime,nosuid,nodev,size=50%%,nr_inodes=1m
+
+[Install]
+WantedBy=basic.target
-- 
2.25.1