pre-commit check for pragma once and ///@file

This is in our style guide, we can cheaply enforce it, let's do it.

```
$ pre-commit
check-case-conflicts.....................................................Passed
check-executables-have-shebangs..........................................Passed
check-headers............................................................Failed
- hook id: check-headers
- exit code: 1

Missing pattern @file in file src/libexpr/value.hh

We found some header files that don't conform to the style guide.

The Lix style guide requests that header files:
- Begin with `#pragma once` so they only get parsed once
- Contain a doxygen comment (`/**` or `///`) containing `@file`, for
  example, `///@file`, which will make doxygen generate docs for them.

  When adding that, consider also adding a `@brief` with a sentence
  explaining what the header is for.

For more details: https://wiki.lix.systems/link/3#bkmrk-header-files

check-merge-conflicts....................................................Passed
check-shebang-scripts-are-executable.....................................Passed
check-symlinks.......................................(no files to check)Skipped
end-of-file-fixer........................................................Passed
mixed-line-endings.......................................................Passed
no-commit-to-branch......................................................Passed
release-notes........................................(no files to check)Skipped
treefmt..................................................................Passed
trim-trailing-whitespace.................................................Passed
```

Fixes: #233
Change-Id: I77150b9298c844ffedd0f85cc5250ae9208502e3

flake.nix

@@ -157,6 +157,7 @@
           nixUnstable = prev.nixUnstable;
 
           build-release-notes = final.buildPackages.callPackage ./maintainers/build-release-notes.nix { };
+          check-headers = final.buildPackages.callPackage ./maintainers/check-headers.nix { };
           clangbuildanalyzer = final.buildPackages.callPackage ./misc/clangbuildanalyzer.nix { };
 
           default-busybox-sandbox-shell = final.busybox.override {
@@ -353,6 +354,23 @@
                   ${lib.getExe pkgs.build-release-notes} doc/manual/rl-next doc/manual/rl-next-dev
                 '';
               };
+              check-headers = {
+                enable = true;
+                package = pkgs.check-headers;
+                files = "^src/";
+                types = [
+                  "c++"
+                  "file"
+                  "header"
+                ];
+                # generated files; these will never actually be seen by this
+                # check, and are left here as documentation
+                excludes = [
+                  "(parser|lexer)-tab\\.hh$"
+                  "\\.gen\\.hh$"
+                ];
+                entry = lib.getExe pkgs.check-headers;
+              };
               # TODO: Once the test suite is nicer, clean up and start
               # enforcing trailing whitespace on tests that don't explicitly
               # check for it.

maintainers/check-headers.nix

@@ -0,0 +1,7 @@
+{ writeShellApplication, gnugrep }:
+writeShellApplication {
+  name = "check-headers";
+
+  runtimeInputs = [ gnugrep ];
+  text = builtins.readFile ./check-headers.sh;
+}

maintainers/check-headers.sh

@@ -0,0 +1,47 @@
+#!/usr/bin/env bash
+
+set -eu
+
+# n.b. this might be printed multiple times if any violating header files are
+# in different parallelism groups inside pre-commit. We cannot do anything about
+# this.
+explanation=$(cat <<'EOF'
+
+We found some header files that don't conform to the style guide.
+
+The Lix style guide requests that header files:
+- Begin with `#pragma once` so they only get parsed once
+- Contain a doxygen comment (`/**` or `///`) containing `@file`, for
+  example, `///@file`, which will make doxygen generate docs for them.
+
+  When adding that, consider also adding a `@brief` with a sentence
+  explaining what the header is for.
+
+For more details: https://wiki.lix.systems/link/3#bkmrk-header-files
+EOF
+)
+
+check_file() {
+    grep -q "$1" "$2" || (echo "Missing pattern $1 in file $2" >&2; return 1)
+}
+
+patterns=(
+    # makes a file get included only once even if it is included multiple times
+    '^#pragma once$'
+    # as used in ///@file, makes the file appear to doxygen
+    '@file'
+)
+fail=0
+
+for pattern in "${patterns[@]}"; do
+    for file in "$@"; do
+        check_file "$pattern" "$file" || fail=1
+    done
+done
+
+if [[ $fail != 0 ]]; then
+    echo "$explanation" >&2
+    exit 1
+else
+    echo OK
+fi