system.file: init

modules/module-list.nix

@@ -36,6 +36,7 @@
   ./system/defaults/ActivityMonitor.nix
   ./system/defaults/WindowManager.nix
   ./system/etc.nix
+  ./system/files
   ./system/keyboard.nix
   ./system/launchd.nix
   ./system/nvram.nix

modules/system/activation-scripts.nix

@@ -55,6 +55,7 @@ in
       # We run `etcChecks` again just in case someone runs `activate`
       # directly without `activate-user`.
       ${cfg.activationScripts.etcChecks.text}
+      ${cfg.activationScripts.filesChecks.text}
       ${cfg.activationScripts.extraActivation.text}
       ${cfg.activationScripts.groups.text}
       ${cfg.activationScripts.users.text}
@@ -71,6 +72,7 @@ in
       ${cfg.activationScripts.keyboard.text}
       ${cfg.activationScripts.fonts.text}
       ${cfg.activationScripts.nvram.text}
+      ${cfg.activationScripts.files.text}
 
       ${cfg.activationScripts.postActivation.text}
 

modules/system/default.nix

@@ -114,6 +114,7 @@ in
 
         ln -s ${cfg.build.patches}/patches $out/patches
         ln -s ${cfg.build.etc}/etc $out/etc
+        ln -s ${cfg.build.files} $out/links.json
         ln -s ${cfg.path} $out/sw
 
         mkdir -p $out/Library

modules/system/files/default.nix

@@ -0,0 +1,65 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  text = import ./write-text.nix {
+    inherit lib;
+    mkTextDerivation = name: text: pkgs.writeText "system-file-${name}" text;
+  };
+
+  rawFiles = filterAttrs (n: v: v.enable) config.system.file;
+
+  files = mapAttrs' (name: value: nameValuePair value.target {
+    type = "link";
+    inherit (value) source;
+  }) rawFiles;
+
+  linksJSON = pkgs.writeText "system-files.json" (builtins.toJSON {
+    version = 1;
+    inherit files;
+  });
+
+  emptyJSON = pkgs.writeText "empty-files.json" (builtins.toJSON {
+    version = 1;
+    files = {};
+  });
+
+  python = lib.getExe pkgs.python3;
+  linker = ./linker.py;
+in
+
+{
+  options = {
+    system.file = mkOption {
+      type = types.attrsOf (types.submodule text);
+      default = {};
+      description = ''
+        Set of files that have to be linked/copied out of the Nix store.
+      '';
+    };
+  };
+
+  config = {
+    system.build.files = linksJSON;
+
+    system.activationScripts.filesChecks.text = ''
+      echo "checking for systemwide file collisions..." >&2
+      OLD=/run/current-system/links.json
+      if [ ! -e "$OLD" ]; then
+        OLD=${emptyJSON}
+      fi
+      CHECK_ONLY=1 ${python} ${linker} "$OLD" "$systemConfig"/links.json
+    '';
+
+    system.activationScripts.files.text = ''
+      echo "setting up files systemwide..." >&2
+      OLD=/run/current-system/links.json
+      if [ ! -e "$OLD" ]; then
+        OLD=${emptyJSON}
+      fi
+      ${python} ${linker} "$OLD" "$systemConfig"/links.json
+    '';
+  };
+}

modules/system/files/linker.py

@@ -0,0 +1,185 @@
+from dataclasses import dataclass
+from sys import argv, exit
+import json
+import os
+from pathlib import Path
+
+if len(argv) != 3:
+    print(f"Usage: {argv[0]} <old_system_links.json> <new_system_links.json>")
+    exit(1)
+
+class FileInfo:
+    source: Path
+    type: str
+
+    def __init__(self, d):
+        self.source = Path(d['source'])
+        self.type = d['type']
+
+with open(argv[1], "r") as file:
+    data = json.load(file)
+    if data['version'] != 1:
+        print(f"Unknown schema version in {argv[1]}")
+        exit(1)
+    old_files: dict[Path, FileInfo] = {
+        Path(k): FileInfo(v)
+        for
+        (k,v)
+        in
+        data['files'].items()
+    }
+
+with open(argv[2], "r") as file:
+    data = json.load(file)
+    if data['version'] != 1:
+        print(f"Unknown schema version in {argv[2]}")
+        exit(1)
+    new_files: dict[Path, FileInfo] = {
+        Path(k): FileInfo(v)
+        for
+        (k,v)
+        in
+        data['files'].items()
+    }
+
+DRY_RUN = 'DRY_RUN' in os.environ.keys()
+CHECK_ONLY = 'CHECK_ONLY' in os.environ.keys()
+
+@dataclass
+class Transaction:
+    source: Path
+    destination: Path
+    type: str
+transactions: list[Transaction] = []
+problems: list[Path] = []
+
+path: Path
+# Go through all files in the new generation
+for path in new_files:
+    new_file: FileInfo = new_files[path]
+    if path.exists(follow_symlinks=False):
+        # There is a file at this path
+        # It could be a regular file or a symlink (including broken symlinks)
+
+        if path.is_symlink():
+            # The file is a symlink
+
+            if path in old_files:
+                # The old generation had a file at this path
+                if old_files[path].type == "link":
+                    # The old generation's file was a link
+                    link_target = path.readlink()
+                    # This handles both relative and absolute symlinks
+                    #   If the link is relative, we need to prepend the parent
+                    #   If the link is absolute, the prepended parent is ignored
+                    if path.parent / link_target == old_files[path].source:
+                        # The link has not changed since last system activation, so we can overwrite it
+                        transactions.append(Transaction(new_file.source, path, 'link'))
+                    elif path.parent / link_target == new_file.source:
+                        # The link already points to the new target
+                        continue
+                    else:
+                        # The link is to somewhere else
+                        problems.append(path)
+                else:
+                    # The old generation's file was not a link.
+                    # Because we know that the file on disk is a link,
+                    # we know that we can't overwrite this file
+                    problems.append(path)
+            else:
+                # The old generation did not have a file at this path,
+                # and we never overwrite links that weren't created by us
+                problems.append(path)
+        else:
+            # The file is a regular file
+            problems.append(path)
+    else:
+        # There is no file at this path
+        transactions.append(Transaction(new_file.source, path, new_file.type))
+
+# Check problems
+for problem in problems:
+    print(f"Existing file at path {problem}")
+
+if len(problems) > 0:
+    print("Aborting")
+    exit(1)
+
+if CHECK_ONLY:
+    # We don't perform any checks when planning removal of old files, so we can exit here
+    exit(0)
+
+# Remove all remaining files from the old generation that aren't in the new generation
+path: Path
+for path in old_files:
+    old_file: FileInfo = old_files[path]
+    if path in new_files:
+        # Already handled when we iterated through new_files above
+        continue
+    if path.exists(follow_symlinks=False):
+        # There is a file at this path
+        # It could be a regular file or a symlink (including broken symlinks)
+
+        if path.is_symlink():
+            # The file is a symlink
+
+            if old_file.type != "link":
+                # This files wasn't a link at last activation, which means that the user changed it
+                # Therefore we don't touch it
+                continue
+
+            # Check that its destination remains the same
+            link_target = path.readlink()
+            if path.parent / link_target == old_file.source:
+                # The link has not changed since last system activation, so we can overwrite it
+                transactions.append(Transaction(path, path, "remove"))
+            else:
+                # The link is to somewhere else, so leave it alone
+                continue
+        else:
+            # The file is a regular file
+            continue
+    else:
+        # There's no file at this path anymore, so we have nothing to do anyway
+        continue
+
+enclosingDirectories: list[Path] = []
+# Perform all transactions
+for t in transactions:
+    # NOTE: the naming scheme for transaction properties is confusing
+    # We are **NOT** using the same scheme as symlinks when we talk about
+    # source/destination. The way we are using these names, `source` is a path
+    # in the Nix store, and `destination` is the path in the system where the source
+    # should be linked or copied to.
+    # In the special case of removing files, `destination` can be ignored
+    if DRY_RUN:
+        match t.type:
+            case "link":
+                print(f"ln -s {t.source} {t.destination}")
+            case "remove":
+                print(f"rm {t.source}")
+            case _:
+                print(f"Unknown transaction type {t.type}")
+    else:
+        match t.type:
+            case "link":
+                # Ensure parent directory exists
+                path.parent.mkdir(parents=True,exist_ok=True)
+                # Remove the file if it exists (we should only get to this case if it's an old symlink we're replacing)
+                # This does not properly handle race conditions, but I think we'd fail in the checking stage
+                #   if your config has a race condition
+                t.destination.unlink(missing_ok=True)
+                # Link the file into place
+                t.destination.symlink_to(t.source)
+            case "remove":
+                enclosingDirectories.append(t.source.parent)
+                t.source.unlink()
+            case _:
+                print(f"Unknown transaction type {t.type}")
+
+for directory in enclosingDirectories:
+    if not directory.is_dir():
+        continue
+    if any(directory.iterdir()):
+        continue
+    print(f"The directory {directory} has been emptied; you may want to remove it")

modules/system/files/write-text.nix

@@ -0,0 +1,52 @@
+{ lib, mkTextDerivation }:
+
+{ config, name, ... }:
+
+with lib;
+
+let
+  fileName = file: last (splitString "/" file);
+  mkDefaultIf = cond: value: mkIf cond (mkDefault value);
+
+  drv = mkTextDerivation (fileName name) config.text;
+in
+
+{
+  options = {
+    enable = mkOption {
+      type = types.bool;
+      default = true;
+      description = ''
+        Whether this file should be generated.
+        This option allows specific files to be disabled.
+      '';
+    };
+
+    text = mkOption {
+      type = types.lines;
+      default = "";
+      description = ''
+        Text of the file.
+      '';
+    };
+
+    target = mkOption {
+      type = types.str;
+      default = "/${name}";
+      description = ''
+        Name of symlink.  Defaults to the attribute name preceded by a slash (the root directory).
+      '';
+    };
+
+    source = mkOption {
+      type = types.path;
+      description = ''
+        Path of the source file.
+      '';
+    };
+  };
+
+  config = {
+    source = mkDefault drv;
+  };
+}

release.nix

@@ -122,6 +122,7 @@ in {
   tests.system-defaults-write = makeTest ./tests/system-defaults-write.nix;
   tests.system-environment = makeTest ./tests/system-environment.nix;
   tests.system-keyboard-mapping = makeTest ./tests/system-keyboard-mapping.nix;
+  tests.system-files = makeTest ./tests/system-files.nix;
   tests.system-packages = makeTest ./tests/system-packages.nix;
   tests.system-path = makeTest ./tests/system-path.nix;
   tests.system-shells = makeTest ./tests/system-shells.nix;

tests/system-files.nix

@@ -0,0 +1,32 @@
+{ config, pkgs, ... }:
+
+let
+  contents = "Hello, world!";
+  jq = pkgs.lib.getExe pkgs.jq;
+in
+
+{
+  system.file."tmp/hello.txt".text = contents;
+
+  test = ''
+    echo 'checking that system links file exists' >&2
+    test -e ${config.out}/links.json
+
+    echo 'checking links file version' >&2
+    test "$(${jq} .version ${config.out}/links.json)" = "1"
+
+    echo 'checking that test file is in links.json' >&2
+    test ! "$(${jq} '.files."/tmp/hello.txt"' ${config.out}/links.json)" = "null"
+
+    echo 'checking that test file is a link' >&2
+    test "$(${jq} '.files."/tmp/hello.txt".type == "link"' ${config.out}/links.json)" = "true"
+
+    echo 'checking that the link source is correct' >&2
+    diff <(${jq} -r '.files."/tmp/hello.txt".source' ${config.out}/links.json) <(echo ${config.system.file."tmp/hello.txt".source})
+
+    # I wanted to check the contents of the file as well, but I am prevented from doing so, I think by the sandbox
+    # echo 'checking that the link source has the correct contents' >&2
+    # diff "$(${jq} '.files."/tmp/hello.txt".source' ${config.out}/links.json)" <(echo ${contents}) # >/dev/null
+    # # grep '${contents}' "$(${jq} '.files."/tmp/hello.txt".source' ${config.out}/links.json)"
+  '';
+}