fix(programs): prevent yamlfmt from modifying unchanged files

examples/formatter-yamlfmt.toml

@@ -1,6 +1,6 @@
 # Example generated by ../examples.sh
 [formatter.yamlfmt]
-command = "yamlfmt"
+command = "yamlfmt-forall"
 excludes = []
 includes = ["*.yaml", "*.yml"]
 options = []

programs/yamlfmt.nix

@@ -1,6 +1,19 @@
 { lib, pkgs, config, ... }:
 let
   cfg = config.programs.yamlfmt;
+  # Run the given program with the given argument (filepath), while expecting
+  # the program to *always* write back to the filepath. The program takes
+  # -in for alternative input.
+  #
+  # The resultant script behaves the same except for avoiding writing to file if
+  # the output hasn't changed.
+  modify-unless-unchanged = pkgs.writeShellScriptBin "modify-unless-unchanged"
+    ''
+      PROG=$1
+      FILEPATH=$2
+      OUTPUT=$($PROG -in < "$FILEPATH")
+      echo "$OUTPUT" | diff -q "$FILEPATH" - > /dev/null || echo "$OUTPUT" > "$FILEPATH"
+    '';
 in
 {
   options.programs.yamlfmt = {
@@ -10,7 +23,20 @@ in
 
   config = lib.mkIf cfg.enable {
     settings.formatter.yamlfmt = {
-      command = cfg.package;
+      command = pkgs.writeShellApplication {
+        name = "yamlfmt-forall";
+        runtimeInputs = [
+          cfg.package
+          modify-unless-unchanged
+        ];
+        text = ''
+          for file in "$@"; do
+            # Use the modify-unless-unchanged hack until this is implemented:
+            # https://github.com/google/yamlfmt/issues/163
+            modify-unless-unchanged yamlfmt "$file"
+          done
+        '';
+      };
       includes = [ "*.yaml" "*.yml" ];
     };
   };