flake: add formatter (#3620)

* format: improve argument handling

For now, fail if the user tries to format a specific file/directory,
or runs the formatter from within a subdirectory.

Handling these situations is slightly tricky because `find -path` is
not very flexible.

* flake: add formatter

This allows running the formatter with `nix fmt`, added in Nix 2.8.

* format: use git ls-files

This is cleaner than `find` and allows us to restrict formatting to
particular files or subdirectories.

flake.nix

@@ -93,6 +93,13 @@
           tests = import ./tests { inherit pkgs; };
         in tests.run);
 
+      formatter = forAllSystems (system:
+        let pkgs = nixpkgs.legacyPackages.${system};
+        in pkgs.linkFarm "format" [{
+          name = "bin/format";
+          path = ./format;
+        }]);
+
       packages = forAllSystems (system:
         let
           pkgs = nixpkgs.legacyPackages.${system};

format

@@ -1,27 +1,51 @@
 #! /usr/bin/env nix-shell
-#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/6616de389ed55fba6eeba60377fc04732d5a207c.tar.gz -i bash -p findutils nixfmt
+#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/6616de389ed55fba6eeba60377fc04732d5a207c.tar.gz -i bash -p git gnugrep gnused findutils nixfmt
 
-CHECK_ARG=
+nixfmt_args=()
+files=()
 
-case $1 in
+for arg do
+    case $arg in
         -h)
             echo "$0 [-c]"
+            exit
             ;;
         -c)
-        CHECK_ARG=-c
+            nixfmt_args+=("$arg")
+            ;;
+        -*)
+            echo "unrecognised flag: $arg" >&2
+            exit 1
+            ;;
+        *)
+            files+=("$arg")
             ;;
     esac
+done
 
 # The excludes are for files touched by open pull requests and we want
 # to avoid merge conflicts.
-find . -name '*.nix' \
-  ! -path ./modules/default.nix \
-  ! -path ./modules/files.nix \
-  ! -path ./modules/home-environment.nix \
-  ! -path ./modules/lib/default.nix \
-  ! -path ./modules/lib/file-type.nix \
-  ! -path ./modules/misc/news.nix \
-  ! -path ./modules/programs/ssh.nix \
-  ! -path ./modules/programs/zsh.nix \
-  ! -path ./tests/default.nix \
-  -exec nixfmt $CHECK_ARG {} +
+excludes=(
+    modules/default.nix
+    modules/files.nix
+    modules/home-environment.nix
+    modules/lib/default.nix
+    modules/lib/file-type.nix
+    modules/misc/news.nix
+    modules/programs/ssh.nix
+    modules/programs/zsh.nix
+    tests/default.nix
+)
+
+exclude_args=()
+for e in "${excludes[@]}"; do
+    exclude_args+=(-e "$e")
+done
+
+git_root=$(git rev-parse --show-toplevel)
+
+git ls-files -z --cached --others --full-name -- "${files[@]}" |
+    grep -z '\.nix$' |
+    grep -z -v "${exclude_args[@]}" |
+    sed -z "s|^|$git_root/|" |
+    xargs -0 nixfmt "${nixfmt_args[@]}"