3 files changed, 26 insertions, 6 deletions
diff --git a/nixos/modules/tasks/encrypted-devices.nix b/nixos/modules/tasks/encrypted-devices.nix
index 457b86e95ab..b1a7711ddcb 100644
--- a/nixos/modules/tasks/encrypted-devices.nix
+++ b/nixos/modules/tasks/encrypted-devices.nix
@@ -3,7 +3,7 @@
 with lib;
 
 let
-  fileSystems = attrValues config.fileSystems ++ config.swapDevices;
+  fileSystems = config.system.build.fileSystems ++ config.swapDevices;
   encDevs = filter (dev: dev.encrypted.enable) fileSystems;
   keyedEncDevs = filter (dev: dev.encrypted.keyFile != null) encDevs;
   keylessEncDevs = filter (dev: dev.encrypted.keyFile == null) encDevs;
diff --git a/nixos/modules/tasks/filesystems.nix b/nixos/modules/tasks/filesystems.nix
index b0abf5eda87..78dca662dc9 100644
--- a/nixos/modules/tasks/filesystems.nix
+++ b/nixos/modules/tasks/filesystems.nix
@@ -5,7 +5,16 @@ with utils;
 
 let
 
-  fileSystems = attrValues config.fileSystems;
+  fileSystems' = toposort fsBefore (attrValues config.fileSystems);
+
+  fileSystems = if fileSystems' ? "result"
+                then # use topologically sorted fileSystems everywhere
+                     fileSystems'.result
+                else # the assertion below will catch this,
+                     # but we fall back to the original order
+                     # anyway so that other modules could check
+                     # their assertions too
+                     (attrValues config.fileSystems);
 
   prioOption = prio: optionalString (prio != null) " pri=${toString prio}";
 
@@ -162,6 +171,17 @@ in
 
   config = {
 
+    assertions = let
+      ls = sep: concatMapStringsSep sep (x: x.mountPoint);
+    in [
+      { assertion = ! (fileSystems' ? "cycle");
+        message = "The ‘fileSystems’ option can't be topologically sorted: mountpoint dependency path ${ls " -> " fileSystems'.cycle} loops to ${ls ", " fileSystems'.loops}";
+      }
+    ];
+
+    # Export for use in other modules
+    system.build.fileSystems = fileSystems;
+
     boot.supportedFilesystems = map (fs: fs.fsType) fileSystems;
 
     # Add the mount helpers to the system path so that `mount' can find them.
@@ -177,7 +197,7 @@ in
         # This is a generated file.  Do not edit!
 
         # Filesystems.
-        ${flip concatMapStrings fileSystems (fs:
+        ${concatMapStrings (fs:
             (if fs.device != null then fs.device
              else if fs.label != null then "/dev/disk/by-label/${fs.label}"
              else throw "No device specified for mount point ‘${fs.mountPoint}’.")
@@ -188,7 +208,7 @@ in
             + " " + (if skipCheck fs then "0" else
                      if fs.mountPoint == "/" then "1" else "2")
             + "\n"
-        )}
+        ) fileSystems}
 
         # Swap devices.
         ${flip concatMapStrings config.swapDevices (sw:
diff --git a/nixos/modules/tasks/filesystems/zfs.nix b/nixos/modules/tasks/filesystems/zfs.nix
index 4ff3ffc74b1..77059fa43ff 100644
--- a/nixos/modules/tasks/filesystems/zfs.nix
+++ b/nixos/modules/tasks/filesystems/zfs.nix
@@ -36,7 +36,7 @@ let
 
   fsToPool = fs: datasetToPool fs.device;
 
-  zfsFilesystems = filter (x: x.fsType == "zfs") (attrValues config.fileSystems);
+  zfsFilesystems = filter (x: x.fsType == "zfs") config.system.build.fileSystems;
 
   isRoot = fs: fs.neededForBoot || elem fs.mountPoint [ "/" "/nix" "/nix/store" "/var" "/var/log" "/var/lib" "/etc" ];
 
@@ -277,7 +277,7 @@ in
 
       systemd.services = let
         getPoolFilesystems = pool:
-          filter (x: x.fsType == "zfs" && (fsToPool x) == pool) (attrValues config.fileSystems);
+          filter (x: x.fsType == "zfs" && (fsToPool x) == pool) config.system.build.fileSystems;
 
         getPoolMounts = pool:
           let