nixos/filesystems: await builtin pstore module backend in mount-pstore

If the pstore module is builtin, it nonetheless can take considerable
time to register a backend despite /sys/fs/pstore already appearing
mounted, so the condition is moved into the main script to extend
waiting for the backend to this case.

1 files changed, 13 insertions, 8 deletions

diff --git a/nixos/modules/tasks/filesystems.nix b/nixos/modules/tasks/filesystems.nix
index 330facdca63..49c9a37a900 100644
--- a/nixos/modules/tasks/filesystems.nix
+++ b/nixos/modules/tasks/filesystems.nix
@@ -305,25 +305,30 @@ in
       in listToAttrs (map formatDevice (filter (fs: fs.autoFormat) fileSystems)) // {
     # Mount /sys/fs/pstore for evacuating panic logs and crashdumps from persistent storage onto the disk using systemd-pstore.
     # This cannot be done with the other special filesystems because the pstore module (which creates the mount point) is not loaded then.
-    # Since the pstore filesystem is usually empty right after mounting because the backend isn't registered yet, and a path unit cannot detect files inside of it, the same service waits for that to happen. systemd's restart mechanism can't be used here because the first failure also fails all dependent units.
         "mount-pstore" = {
           serviceConfig = {
             Type = "oneshot";
             # skip on kernels without the pstore module
             ExecCondition = "${pkgs.kmod}/bin/modprobe -b pstore";
-            ExecStart = "${pkgs.util-linux}/bin/mount -t pstore -o nosuid,noexec,nodev pstore /sys/fs/pstore";
-            ExecStartPost = pkgs.writeShellScript "wait-for-pstore.sh" ''
+            ExecStart = pkgs.writeShellScript "mount-pstore.sh" ''
               set -eu
-              TRIES=0
-              while [ $TRIES -lt 20 ] && [ "$(cat /sys/module/pstore/parameters/backend)" = "(null)" ]; do
-                sleep 0.1
-                TRIES=$((TRIES+1))
+              # if the pstore module is builtin it will have mounted the persistent store automatically. it may also be already mounted for other reasons.
+              ${pkgs.util-linux}/bin/mountpoint -q /sys/fs/pstore || ${pkgs.util-linux}/bin/mount -t pstore -o nosuid,noexec,nodev pstore /sys/fs/pstore
+              # wait up to five seconds (arbitrary, happened within one in testing) for the backend to be registered and the files to appear. a systemd path unit cannot detect this happening; and succeeding after a restart would not start dependent units.
+              TRIES=50
+              while [ "$(cat /sys/module/pstore/parameters/backend)" = "(null)" ]; do
+                if (( $TRIES )); then
+                  sleep 0.1
+                  TRIES=$((TRIES-1))
+                else
+                  echo "Persistent Storage backend was not registered in time." >&2
+                  exit 1
+                fi
               done
             '';
             RemainAfterExit = true;
           };
           unitConfig = {
-            ConditionPathIsMountPoint = "!/sys/fs/pstore";
             ConditionVirtualization = "!container";
             DefaultDependencies = false; # needed to prevent a cycle
           };