From 870fa77ff6eeba7eff8596869b50c4c475033c03 Mon Sep 17 00:00:00 2001
From: hyperfekt <git@hyperfekt.net>
Date: Wed, 2 Dec 2020 00:52:54 +0100
Subject: [PATCH] nixos/filesystems: mount persistent storage to /sys/fs/pstore

---
 nixos/modules/tasks/filesystems.nix | 34 +++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/nixos/modules/tasks/filesystems.nix b/nixos/modules/tasks/filesystems.nix
index e468cb88003..2aeb8598ada 100644
--- a/nixos/modules/tasks/filesystems.nix
+++ b/nixos/modules/tasks/filesystems.nix
@@ -272,10 +272,10 @@ in
         wants = [ "local-fs.target" "remote-fs.target" ];
       };
 
-    # Emit systemd services to format requested filesystems.
     systemd.services =
-      let
 
+    # Emit systemd services to format requested filesystems.
+      let
         formatDevice = fs:
           let
             mountPoint' = "${escapeSystemdPath fs.mountPoint}.mount";
@@ -302,8 +302,34 @@ in
             unitConfig.DefaultDependencies = false; # needed to prevent a cycle
             serviceConfig.Type = "oneshot";
           };
-
-      in listToAttrs (map formatDevice (filter (fs: fs.autoFormat) fileSystems));
+      in listToAttrs (map formatDevice (filter (fs: fs.autoFormat) fileSystems)) // {
+    # Mount /sys/fs/pstore for evacuating panic logs and crashdumps from persistent storage onto the disk using systemd-pstore.
+    # This cannot be done with the other special filesystems because the pstore module (which creates the mount point) is not loaded then.
+    # Since the pstore filesystem is usually empty right after mounting because the backend isn't registered yet, and a path unit cannot detect files inside of it, the same service waits for that to happen. systemd's restart mechanism can't be used here because the first failure also fails all dependent units.
+        "mount-pstore" = {
+          serviceConfig = {
+            Type = "oneshot";
+            ExecStart = "${pkgs.utillinux}/bin/mount -t pstore -o nosuid,noexec,nodev pstore /sys/fs/pstore";
+            ExecStartPost = pkgs.writeShellScript "wait-for-pstore.sh" ''
+              set -eu
+              TRIES=0
+              while [ $TRIES -lt 20 ] && [ "$(cat /sys/module/pstore/parameters/backend)" = "(null)" ]; do
+                sleep 0.1
+                TRIES=$((TRIES+1))
+              done
+            '';
+            RemainAfterExit = true;
+          };
+          unitConfig = {
+            ConditionVirtualization = "!container";
+            DefaultDependencies = false; # needed to prevent a cycle
+          };
+          after = [ "modprobe@pstore.service" ];
+          requires = [ "modprobe@pstore.service" ];
+          before = [ "systemd-pstore.service" ];
+          wantedBy = [ "systemd-pstore.service" ];
+        };
+      };
 
     systemd.tmpfiles.rules = [
       "d /run/keys 0750 root ${toString config.ids.gids.keys}"