add support for systemd mount units

This is mainly useful for specifying mounts that depend on other
units. For example sshfs or davfs need network (and possibly
nameservices).

While systemd makes a distinction between local and remote
filesystems, this only works for in-kernel filesystems such as
nfs and cifs.

fuse-based filesystems (such as sshfs and davs) are classified as
local, so they fail without networking. By explicitly declaring these
mounts as full systemd units (as opposed to having systemd generate
them automatically from /etc/fstab), dependencies can be specified as
on every other unit.

In the future, we can probably port NixOS' filesystems handling to use
these native systemd.mount units and skip /etc/fstab altogether, but
this probably requires additional changes, such as starting systemd
even earlier during boot (stage 1).

modules/system/boot/systemd-unit-options.nix

@@ -202,4 +202,51 @@ rec {
 
   };
 
+  mountOptions = unitOptions // {
+
+    what = mkOption {
+      default = "";
+      example = "/dev/sda1";
+      type = types.uniq types.string;
+      description = "Absolute path of device node, file or other resource. (Mandatory)";
+    };
+
+    where = mkOption {
+      default = "";
+      example = "/mnt";
+      type = types.uniq types.string;
+      description = ''
+        Absolute path of a directory of the mount point.
+        Will be created if it doesn't exist. (Mandatory)
+      '';
+    };
+
+    type = mkOption {
+      default = "";
+      example = "ext4";
+      type = types.uniq types.string;
+      description = "File system type.";
+    };
+
+    options = mkOption {
+      default = "";
+      example = "noatime";
+      type = types.string;
+      merge = concatStringsSep ",";
+      description = "Options used to mount the file system.";
+    };
+
+    mountConfig = mkOption {
+      default = {};
+      example = { DirectoryMode = "0775"; };
+      type = types.attrs;
+      description = ''
+        Each attribute in this set specifies an option in the
+        <literal>[Mount]</literal> section of the unit.  See
+        <citerefentry><refentrytitle>systemd.mount</refentrytitle>
+        <manvolnum>5</manvolnum></citerefentry> for details.
+      '';
+    };
+  };
+
 }

modules/system/boot/systemd.nix

@@ -198,6 +198,19 @@ let
     };
   };
 
+  mountConfig = { name, config, ... }: {
+    config = {
+      mountConfig =
+        { What = config.what;
+          Where = config.where;
+        } // optionalAttrs (config.type != "") {
+          Type = config.type;
+        } // optionalAttrs (config.options != "") {
+          Options = config.options;
+        };
+    };
+  };
+
   toOption = x:
     if x == true then "true"
     else if x == false then "false"
@@ -277,6 +290,29 @@ let
         '';
     };
 
+  # this is by no means the full escaping-logic systemd uses
+  # so feel free to extend this further.
+  mountName = path:
+    let escaped = replaceChars [ "-"    " "    "/" ]
+                               [ "\x2d" "\x20" "-" ] (toString path);
+    in if (substring 0 1 escaped == "-")
+       then substring 1 (sub (stringLength escaped) 1) escaped
+       else escaped;
+
+  mountToUnit = name: def:
+    assert def.mountConfig.What != "";
+    assert def.mountConfig.Where != "";
+    { inherit (def) wantedBy enable;
+      text =
+        ''
+          [Unit]
+          ${attrsToSection def.unitConfig}
+
+          [Mount]
+          ${attrsToSection def.mountConfig}
+        '';
+    };
+
   nixosUnits = mapAttrsToList makeUnit cfg.units;
 
   units = pkgs.runCommand "units" { preferLocalBuild = true; }
@@ -387,6 +423,17 @@ in
       description = "Definition of systemd socket units.";
     };
 
+    boot.systemd.mounts = mkOption {
+      default = [];
+      type = types.listOf types.optionSet;
+      options = [ mountOptions unitConfig mountConfig ];
+      description = ''
+        Definition of systemd mount units.
+        This is a list instead of an attrSet, because systemd mandates the names to be derived from
+        the 'where' attribute.
+      '';
+    };
+
     boot.systemd.defaultUnit = mkOption {
       default = "multi-user.target";
       type = types.uniq types.string;
@@ -501,7 +548,10 @@ in
       { "rescue.service".text = rescueService; }
       // mapAttrs' (n: v: nameValuePair "${n}.target" (targetToUnit n v)) cfg.targets
       // mapAttrs' (n: v: nameValuePair "${n}.service" (serviceToUnit n v)) cfg.services
-      // mapAttrs' (n: v: nameValuePair "${n}.socket" (socketToUnit n v)) cfg.sockets;
+      // mapAttrs' (n: v: nameValuePair "${n}.socket" (socketToUnit n v)) cfg.sockets
+      // listToAttrs (map
+                   (v: let n = mountName v.where;
+                       in nameValuePair "${n}.mount" (mountToUnit n v)) cfg.mounts);
 
     system.requiredKernelConfig = map config.lib.kernelConfig.isEnabled [
       "CGROUPS" "AUTOFS4_FS" "DEVTMPFS"