[bug#55420] [PATCH v2 2/2] guix: emacs-utils: Add emacs-header-parse.

[Fredrik Salomonsson <plattfot@posteo.net>]

* guix/build/emacs-utils.scm (emacs-header-parse): New procedure.

* tests/build-emacs-utils.scm ("emacs-header-parse: fetch version",
  "emacs-header-parse: fetch keywords", "emacs-header-parse: fetch
  nonexistent author"): New tests.
---
 guix/build/emacs-utils.scm  |  9 +++++++++
 tests/build-emacs-utils.scm | 30 ++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/guix/build/emacs-utils.scm b/guix/build/emacs-utils.scm
index 1684bf3262..8ee547f2b3 100644
--- a/guix/build/emacs-utils.scm
+++ b/guix/build/emacs-utils.scm
@@ -38,6 +38,7 @@ (define-module (guix build emacs-utils)
 
             emacs-generate-autoloads
             emacs-byte-compile-directory
+            emacs-header-parse
 
             as-display
             emacs-substitute-sexps
@@ -114,6 +115,14 @@ (define* (emacs-byte-compile-directory dir)
                 (byte-recompile-directory (file-name-as-directory ,dir) 0 1))))
     (emacs-batch-eval expr)))
 
+(define (emacs-header-parse section file)
+  "Parse the header SECTION in FILE and return it as a string."
+  (emacs-batch-script
+   `(progn
+     (require 'lisp-mnt)
+     (find-file ,file)
+     (princ (lm-header ,section)))))
+
 (define as-display         ;syntactic keyword for 'emacs-substitute-sexps'
   '(as display))
 
diff --git a/tests/build-emacs-utils.scm b/tests/build-emacs-utils.scm
index 03b73b1fed..57647caaa2 100644
--- a/tests/build-emacs-utils.scm
+++ b/tests/build-emacs-utils.scm
@@ -38,4 +38,34 @@ (define-module (test build-emacs-utils)
                               "Lisp error: (wrong-type-argument numberp \"three\")")))
     (emacs-batch-script '(mapcar 'number-to-string (list 1 2 "three")))))
 
+(define (test-emacs-header-parse section)
+  (call-with-temporary-directory
+   (lambda (directory)
+     (let ((mock-elisp-file (string-append directory "/foo.el")))
+       (call-with-output-file mock-elisp-file
+         (lambda (port)
+           (display ";;; foo --- mock emacs package -*- lexical-binding: t -*-
+
+;; Created: 4 Jun 2022
+;; Keywords: lisp test
+;; Version: 1.0.0
+;;; Commentary:
+;;; Code:
+;;; foo.el ends here
+"
+                    port)))
+       (emacs-header-parse section mock-elisp-file)))))
+
+(test-equal "emacs-header-parse: fetch version"
+    "1.0.0"
+  (test-emacs-header-parse "version"))
+
+(test-equal "emacs-header-parse: fetch keywords"
+    "lisp test"
+  (test-emacs-header-parse "keywords"))
+
+(test-equal "emacs-header-parse: fetch nonexistent author"
+    "nil"
+  (test-emacs-header-parse "author"))
+
 (test-end "build-emacs-utils")
-- 
2.36.1