Merge pull request #11484 from oxij/nixos-toposort-filesystems

lib: add toposort, nixos: use toposort for fileSystems to properly support bind and move mounts

lib/lists.nix

···
   reverseList = xs:
     let l = length xs; in genList (n: elemAt xs (l - n - 1)) l;
 
+  /* Depth-First Search (DFS) for lists `list != []`.
+
+     `before a b == true` means that `b` depends on `a` (there's an
+     edge from `b` to `a`).
+
+     Examples:
+
+         listDfs true hasPrefix [ "/home/user" "other" "/" "/home" ]
+           == { minimal = "/";                  # minimal element
+                visited = [ "/home/user" ];     # seen elements (in reverse order)
+                rest    = [ "/home" "other" ];  # everything else
+              }
+
+         listDfs true hasPrefix [ "/home/user" "other" "/" "/home" "/" ]
+           == { cycle   = "/";                  # cycle encountered at this element
+                loops   = [ "/" ];              # and continues to these elements
+                visited = [ "/" "/home/user" ]; # elements leading to the cycle (in reverse order)
+                rest    = [ "/home" "other" ];  # everything else
+
+   */
+
+  listDfs = stopOnCycles: before: list:
+    let
+      dfs' = us: visited: rest:
+        let
+          c = filter (x: before x us) visited;
+          b = partition (x: before x us) rest;
+        in if stopOnCycles && (length c > 0)
+           then { cycle = us; loops = c; inherit visited rest; }
+           else if length b.right == 0
+                then # nothing is before us
+                     { minimal = us; inherit visited rest; }
+                else # grab the first one before us and continue
+                     dfs' (head b.right)
+                          ([ us ] ++ visited)
+                          (tail b.right ++ b.wrong);
+    in dfs' (head list) [] (tail list);
+
+  /* Sort a list based on a partial ordering using DFS. This
+     implementation is O(N^2), if your ordering is linear, use `sort`
+     instead.
+
+     `before a b == true` means that `b` should be after `a`
+     in the result.
+
+     Examples:
+
+         toposort hasPrefix [ "/home/user" "other" "/" "/home" ]
+           == { result = [ "/" "/home" "/home/user" "other" ]; }
+
+         toposort hasPrefix [ "/home/user" "other" "/" "/home" "/" ]
+           == { cycle = [ "/home/user" "/" "/" ]; # path leading to a cycle
+                loops = [ "/" ]; }                # loops back to these elements
+
+         toposort hasPrefix [ "other" "/home/user" "/home" "/" ]
+           == { result = [ "other" "/" "/home" "/home/user" ]; }
+
+         toposort (a: b: a < b) [ 3 2 1 ] == { result = [ 1 2 3 ]; }
+
+   */
+
+  toposort = before: list:
+    let
+      dfsthis = listDfs true before list;
+      toporest = toposort before (dfsthis.visited ++ dfsthis.rest);
+    in
+      if length list < 2
+      then # finish
+           { result =  list; }
+      else if dfsthis ? "cycle"
+           then # there's a cycle, starting from the current vertex, return it
+                { cycle = reverseList ([ dfsthis.cycle ] ++ dfsthis.visited);
+                  inherit (dfsthis) loops; }
+           else if toporest ? "cycle"
+                then # there's a cycle somewhere else in the graph, return it
+                     toporest
+                # Slow, but short. Can be made a bit faster with an explicit stack.
+                else # there are no cycles
+                     { result = [ dfsthis.minimal ] ++ toporest.result; };
+
   /* Sort a list based on a comparator function which compares two
      elements and returns true if the first argument is strictly below
      the second argument.  The returned list is sorted in an increasing

nixos/lib/utils.nix

···
 
 rec {
 
+  # Check whenever fileSystem is needed for boot
+  fsNeededForBoot = fs: fs.neededForBoot
+                     || elem fs.mountPoint [ "/" "/nix" "/nix/store" "/var" "/var/log" "/var/lib" "/etc" ];
+
+  # Check whenever `b` depends on `a` as a fileSystem
+  # FIXME: it's incorrect to simply use hasPrefix here: "/dev/a" is not a parent of "/dev/ab"
+  fsBefore = a: b: ((any (x: elem x [ "bind" "move" ]) b.options) && (a.mountPoint == b.device))
+                || (hasPrefix a.mountPoint b.mountPoint);
+
   # Escape a path according to the systemd rules, e.g. /dev/xyzzy
   # becomes dev-xyzzy.  FIXME: slow.
   escapeSystemdPath = s:

nixos/modules/security/grsecurity.nix

···
     (fs: (fs.neededForBoot
           || elem fs.mountPoint [ "/" "/nix" "/nix/store" "/var" "/var/log" "/var/lib" "/etc" ])
           && fs.fsType == "zfs")
-    (attrValues config.fileSystems) != [];
+    config.system.build.fileSystems != [];
 
   # Ascertain whether NixOS container support is required
   containerSupportRequired =

nixos/modules/system/boot/stage-1.nix

···
 # the modules necessary to mount the root file system, then calls the
 # init in the root file system to start the second boot stage.
 
-{ config, lib, pkgs, ... }:
+{ config, lib, utils, pkgs, ... }:
 
 with lib;
 
···
   };
 
 
+  # The initrd only has to mount `/` or any FS marked as necessary for
+  # booting (such as the FS containing `/nix/store`, or an FS needed for
+  # mounting `/`, like `/` on a loopback).
+  fileSystems = filter utils.fsNeededForBoot config.system.build.fileSystems;
+
+
   # Some additional utilities needed in stage 1, like mount, lvm, fsck
   # etc.  We don't want to bring in all of those packages, so we just
   # copy what we need.  Instead of using statically linked binaries,
···
       ln -sf kmod $out/bin/modprobe
 
       # Copy resize2fs if needed.
-      ${optionalString (any (fs: fs.autoResize) (attrValues config.fileSystems)) ''
+      ${optionalString (any (fs: fs.autoResize) fileSystems) ''
         # We need mke2fs in the initrd.
         copy_bin_and_libs ${pkgs.e2fsprogs}/sbin/resize2fs
       ''}
···
 
       ${config.boot.initrd.extraUtilsCommandsTest}
     ''; # */
-
-
-  # The initrd only has to mount / or any FS marked as necessary for
-  # booting (such as the FS containing /nix/store, or an FS needed for
-  # mounting /, like / on a loopback).
-  #
-  # We need to guarantee that / is the first filesystem in the list so
-  # that if and when lustrateRoot is invoked, nothing else is mounted
-  fileSystems = let
-    filterNeeded = filter
-      (fs: fs.mountPoint != "/" && (fs.neededForBoot || elem fs.mountPoint [ "/nix" "/nix/store" "/var" "/var/log" "/var/lib" "/etc" ]));
-    filterRoot = filter
-      (fs: fs.mountPoint == "/");
-    allFileSystems = attrValues config.fileSystems;
-  in (filterRoot allFileSystems) ++ (filterNeeded allFileSystems);
 
 
   udevRules = pkgs.stdenv.mkDerivation {
···
   };
 
   config = mkIf (!config.boot.isContainer) {
-
     assertions = [
-      { assertion = any (fs: fs.mountPoint == "/") (attrValues config.fileSystems);
+      { assertion = any (fs: fs.mountPoint == "/") fileSystems;
         message = "The ‘fileSystems’ option does not specify your root file system.";
       }
       { assertion = let inherit (config.boot) resumeDevice; in

nixos/modules/tasks/encrypted-devices.nix

···
 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;

nixos/modules/tasks/filesystems.nix

···
 
 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}";
 
···
 
   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.
···
         # in your /etc/nixos/configuration.nix file.
 
         # 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}’.")
···
             + " " + (if skipCheck fs then "0" else
                      if fs.mountPoint == "/" then "1" else "2")
             + "\n"
-        )}
+        ) fileSystems}
 
         # Swap devices.
         ${flip concatMapStrings config.swapDevices (sw:
···
 
         formatDevice = fs:
           let
-            mountPoint' = escapeSystemdPath fs.mountPoint;
-            device' = escapeSystemdPath fs.device;
+            mountPoint' = "${escapeSystemdPath fs.mountPoint}.mount";
+            device'  = escapeSystemdPath fs.device;
+            device'' = "${device}.device";
           in nameValuePair "mkfs-${device'}"
           { description = "Initialisation of Filesystem ${fs.device}";
-            wantedBy = [ "${mountPoint'}.mount" ];
-            before = [ "${mountPoint'}.mount" "systemd-fsck@${device'}.service" ];
-            requires = [ "${device'}.device" ];
-            after = [ "${device'}.device" ];
+            wantedBy = [ mountPoint' ];
+            before = [ mountPoint' "systemd-fsck@${device'}.service" ];
+            requires = [ device'' ];
+            after = [ device'' ];
             path = [ pkgs.utillinux ] ++ config.system.fsPackages;
             script =
               ''

nixos/modules/tasks/filesystems/zfs.nix

···
 
   fsToPool = fs: datasetToPool fs.device;
 
-  zfsFilesystems = filter (x: x.fsType == "zfs") (attrValues config.fileSystems);
-
-  isRoot = fs: fs.neededForBoot || elem fs.mountPoint [ "/" "/nix" "/nix/store" "/var" "/var/log" "/var/lib" "/etc" ];
+  zfsFilesystems = filter (x: x.fsType == "zfs") config.system.build.fileSystems;
 
   allPools = unique ((map fsToPool zfsFilesystems) ++ cfgZfs.extraPools);
 
-  rootPools = unique (map fsToPool (filter isRoot zfsFilesystems));
+  rootPools = unique (map fsToPool (filter fsNeededForBoot zfsFilesystems));
 
   dataPools = unique (filter (pool: !(elem pool rootPools)) allPools);
 
···
 
       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