[systemd-devel] [PATCH 3/3] Use a stamp file to avoid running systemd-fsck-root.service twice

[Zbigniew Jędrzejewski-Szmek]

In the initramfs, we run systemd-fsck@<sysroot-device>.service.
In the real system we run systemd-fsck-root.service. It is hard
to pass the information that the latter should not run if the first
succeeded using unit state only.

- in the real system, we need a synchronization point between the fsck
  for root and other fscks, to express the dependency to run this
  systemd-fsck at .service "before all other systemd-fsck@ units". We
  cannot express it directly, because there are no wildcard
  dependencies. We could use a target as a sychronization point, but
  then we would have to provide drop-ins to order
  systemd-fsck at -.service before the target, and all others after it,
  which becomes messy. The currently used alternative of having a
  special unit (systemd-fsck-root.service) makes it easy to express
  this dependency, and seems to be the best solution.

- we cannot use systemd-fsck-root.service in the initramfs, because
  other fsck units should not be ordered after it. In the real system,
  the root device is always checked and mounted before other filesystems,
  but in the initramfs this doesn't have to be true: /sysroot might be
  stacked on other filesystems and devices.

- the name of the root device can legitimately be specified in a
  different way in the initramfs (on the kernel command line, or
  automatically discovered through GPT), and in the real fs (in /etc/fstab).
  Even if we didn't need systemd-fsck-root.service as a synchronization
  point, it would be hard to ensure the same instance parameter is
  provided for systemd-fsck at .service in the initrams and the real
  system.

Let's use a "side channel" to pass this information.
/run/systemd/fsck-root-done is touched after fsck in the initramfs
succeeds, through an ExecStartPost line in a drop-in for
systemd-fsck at sysroot.service.

https://bugzilla.redhat.com/show_bug.cgi?id=1201979
---
 src/shared/generator.c             | 7 +++++++
 units/systemd-fsck-root.service.in | 1 +
 2 files changed, 8 insertions(+)

diff --git a/src/shared/generator.c b/src/shared/generator.c
index 7b2f846175..a71222d1cb 100644
--- a/src/shared/generator.c
+++ b/src/shared/generator.c
@@ -78,6 +78,13 @@ int generator_write_fsck_deps(
                         "RequiresOverridable=%1$s\n"
                         "After=%1$s\n",
                         fsck);
+
+                if (in_initrd() && path_equal(where, "/sysroot"))
+                        return write_drop_in_format(dir, fsck, 50, "stamp",
+                                                    "# Automatically generated by %s\n\n"
+                                                    "[Service]\n"
+                                                    "ExecStartPost=-/bin/touch /run/systemd/fsck-root-done\n",
+                                                    program_invocation_short_name);
         }
 
         return 0;
diff --git a/units/systemd-fsck-root.service.in b/units/systemd-fsck-root.service.in
index 3617abf04a..48dacc841c 100644
--- a/units/systemd-fsck-root.service.in
+++ b/units/systemd-fsck-root.service.in
@@ -11,6 +11,7 @@ Documentation=man:systemd-fsck-root.service(8)
 DefaultDependencies=no
 Before=local-fs.target shutdown.target
 ConditionPathIsReadWrite=!/
+ConditionPathExists=!/run/systemd/fsck-root-done
 
 [Service]
 Type=oneshot
-- 
2.3.5