Add packer fmt formatter (#169)

* Add packer formatter

that runs `packer fmt` on `*.pkr.hcl` and `*.pkrvars.hcl` files.

* Update README "Adding new formatters" instructions

to (a) remove mention of the deleted `./bors.toml.sh`, (b) consistently
render file names in monospace, (c) replace `nix develop -c treefmt`
with `nix fmt`, and (d) provide some additional guidance on how to test
the newly-introduced formatter.

---------

Co-authored-by: Jonas Chevalier <zimbatm@zimbatm.com>

README.md

@@ -219,6 +219,7 @@ This repo contains a top-level `default.nix` that returns the library helper fun
 * ocamlformat
 * opa
 * ormolu
+* packer
 * php-cs-fixer
 * prettier
 * protolint
@@ -278,14 +279,20 @@ PRs to add new formatters are welcome!
 
 In order to add a new formatter do the following things:
 
-1. Create a new entry in the ./programs/ folder.
-3. Run `./examples.sh` to update the ./examples folder.
-4. To test the program:
-    1. Extend the project's `./treefmt.nix` file (temporarily)
-    2. Add a bunch of sources in this repo
-    3. Run `nix fmt`
+1. Create a new entry in the `./programs/` folder.
+2. Run `./examples.sh` to update the `./examples` folder.
+3. To test the program:
+    1. Extend the project's `./treefmt.nix` file (temporarily) to enable the
+       new formatter and configure it in whatever manner is appropriate.
+    2. Add a bunch of pertinent sources in this repo -- for instance, if the
+       new formatter is meant to format `*.foo` files, add a number of `*.foo`
+       files, some well-formatted (and therefore expected to
+       be exempt from modification by `treefmt`) and some badly-formatted.
+    3. Run `nix fmt`.  Confirm that well-formatted files are unchanged and that
+       badly-formatted files are flagged as such.  Re-run `nix fmt` and confirm
+       that no additional changes were made.
     4. Once this is good, revert those changes.
-5. Submit the PR!
+4. Submit the PR!
 
 ## Commercial support
 

examples/formatter-packer.toml

@@ -0,0 +1,6 @@
+# Example generated by ../examples.sh
+[formatter.packer]
+command = "treefmt-nix-packer-fmt-wrapper"
+excludes = []
+includes = ["*.pkr.hcl", "*.pkrvars.hcl"]
+options = ["--"]

programs/packer.nix

@@ -0,0 +1,61 @@
+{ lib, pkgs, config, ... }:
+let
+  cfg = config.programs.packer;
+
+  # `packer fmt` accepts only a single template as an argument:
+  # > $ packer fmt -h |& head -n 1
+  # > Usage: packer fmt [options] [TEMPLATE]
+  # `packer fmt` supports recursively formatting all `*.pkr.hcl` and
+  # `*.pkrvars.hcl` files in a specified directory, but since we want to
+  # control the exact list of files-to-be-formatted, we instead have to invoke
+  # `packer fmt` once per selected file.  Consequently, we need a means of
+  # distinguishing which arguments are options (from
+  # `settings.formatter.packer.options`) and which are filenames; this is so
+  # that we can pass all options to each `packer fmt`.  The traditional `--`
+  # sequence suits this purpose.
+  command = pkgs.writers.writeBashBin "treefmt-nix-packer-fmt-wrapper" ''
+      set -eu
+
+      declare -a opts
+      saw_separator=0
+      while (( "$#" > 0 )); do
+        if [[ "$1" = -- ]]; then
+          saw_separator=1
+          shift
+          break
+        fi
+
+        opts+=("$1")
+        shift
+      done
+
+      if [[ "$saw_separator" = 0 ]]; then
+        echo 1>&2 "\
+    Error: ''${0##*/}: the end-of-options separator sequence is missing.
+    Please ensure that the final entry in 'settings.formatter.packer.options' is a bare double-dash ('--').
+    Otherwise, this program cannot determine which arguments are options to 'packer fmt' and which are filenames."
+        exit 1
+      fi
+
+      rc=0
+      for tmpl in "$@"; do
+        ${lib.escapeShellArg cfg.package}/bin/packer fmt "''${opts[@]}" "$tmpl" || rc="$?"
+      done
+
+      exit "''${rc:-0}"
+  '';
+in
+{
+  options.programs.packer = {
+    enable = lib.mkEnableOption "packer";
+    package = lib.mkPackageOption pkgs "packer" { };
+  };
+
+  config = lib.mkIf cfg.enable {
+    settings.formatter.packer = {
+      inherit command;
+      options = lib.mkAfter [ "--" ];
+      includes = [ "*.pkr.hcl" "*.pkrvars.hcl" ];
+    };
+  };
+}