bug#48331: [PATCH draft] build-system: emacs: Generate -pkg.el file in case it is missing

[Andrew Tropin <andrew@trop.in>]

---
 Thank you for the patches, tested, works for me!  The solution is much more
 precise than mutating package-directory-list variable, good job.  I don't see
 any major problems in the implementation (but I'm not very fluent elisp dev
 and maybe missing something).


 I drafted a simple build phase, which generates -pkg.el in case it is missing.

 There are at least a few problems with this implementation:

 1. There is no information about package record available during build, which
 makes it hard to get package name and package version.  I can't use any
 regexp to obtain this information from name or elpa-name-ver, because
 package name and version can have arbitrary form: comment-dwim-2-1.0,
 cyberpunk-2019-theme-20191008-alpha or something like that.
 2. It's also not so easy to extract description of the package from
 somewhere, the first option is to pass package record to build phases somehow,
 another is to parse PACKAGE-NAME.el file comments section.
 3. This one I consider as a minor flaw: there is no generic solution for
 packages built with build systems other than emacs-build-system.

 So, this patch is very dirty and I publish it only for future reference.

 The intuition says that we should split name and version in build phase
 arguments, also it seems that it will be useful to provide other information
 about package during build time for cases like this one.  I'll learn this
 area a bit more and probably will make another thread someday.

 guix/build/emacs-build-system.scm | 60 ++++++++++++++++++++++++++++++-
 1 file changed, 59 insertions(+), 1 deletion(-)

diff --git a/guix/build/emacs-build-system.scm
b/guix/build/emacs-build-system.scm
index f13162d6c4..2bb102b4be 100644
--- a/guix/build/emacs-build-system.scm
+++ b/guix/build/emacs-build-system.scm
@@ -116,6 +116,63 @@ environment variable\n" source-directory))
     (parameterize ((%emacs emacs))
       (emacs-byte-compile-directory (elpa-directory out)))))

+
+(define* (add-pkg-file-if-missing #:key name outputs #:allow-other-keys
+                                  #:rest args)
+  "Generate simple -pkg.el in case package doesn't have it in source code."
+  (define (file-contains-nul-char? file)
+    (call-with-input-file file
+      (lambda (in)
+        (let loop ((line (read-line in 'concat)))
+          (cond
+           ((eof-object? line) #f)
+           ((string-index line #\nul) #t)
+           (else (loop (read-line in 'concat))))))
+      #:binary #t))
+
+  (let* ((out (assoc-ref outputs "out"))
+         (el-dir (elpa-directory out))
+         (elpa-name-ver (store-directory->elpa-name-version out))
+         (el-files (remove file-contains-nul-char?
+                           (find-files (getcwd) "\\.el$")))
+         (el-names (map (lambda (x) (basename x ".el")) el-files))
+
+         (possible-names
+          (fold (lambda (x acc)
+                  (cons
+                   (string-append
+                    (if (not (null? acc)) (string-append (first acc) "-") "")
+                    x)
+                   acc))
+                '()
+                (string-split elpa-name-ver #\-)))
+
+         (package-names (append-map
+                         (lambda (name)
+                           (let ((m (member name el-names)))
+                             (if m (list (car m)) '())))
+                         possible-names))
+
+         (package-name (if (null? package-names) "" (car package-names)))
+         (package-version (string-drop elpa-name-ver
+                                       (1+ (string-length package-name))))
+         (package-description "description should be here")
+         (pkg-file (string-append el-dir "/" package-name "-pkg.el")))
+
+    (when (not (file-exists? pkg-file))
+      (with-output-to-file pkg-file
+        (lambda ()
+          (format
+           #t
+           "\
+(define-package
+  ~s
+  ~s
+  ~s
+  nil)"
+           package-name package-version package-description))))
+    #t))
+
 (define* (patch-el-files #:key outputs #:allow-other-keys)
   "Substitute the absolute \"/bin/\" directory with the right location in the
 store in '.el' files."
@@ -293,8 +350,9 @@ for libraries following the ELPA convention."
     (add-after 'make-autoloads 'enable-autoloads-compilation
       enable-autoloads-compilation)
     (add-after 'enable-autoloads-compilation 'patch-el-files patch-el-files)
+    (add-after 'patch-el-files 'add-pkg-file-if-missing
add-pkg-file-if-missing)
     ;; The .el files are byte compiled directly in the store.
-    (add-after 'patch-el-files 'build build)
+    (add-after 'add-pkg-file-if-missing 'build build)
     (add-after 'build 'validate-compiled-autoloads validate-compiled-autoloads)
     (add-after 'validate-compiled-autoloads 'move-doc move-doc)))

-- 
2.31.1