nixos: apply toposort to fileSystems to support bind and move mounts

And use new `config.system.build.fileSystems` property everywhere.

1 files changed, 23 insertions, 3 deletions

diff --git a/nixos/modules/tasks/filesystems.nix b/nixos/modules/tasks/filesystems.nix
index b0abf5eda871..78dca662dc9d 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: