bug#38694: 26.1; update-*-autoloads fails with local generated-autoload-file

["Braun Gábor" <braungb88@gmail.com>]

Let the current directory have only the following two files:

------------------ test-foo.el -------------------------
;;;###autoload
(defun test-foo-fun ()
  "FOO: a function just for testing."
  t)

;; Local Variables:
;; generated-autoload-file: "foo-autoloads.el"
;; End:
--------------------------------------------------------


------------------ test-bar.el -------------------------
;;;###autoload
(defun test-bar-fun ()
  "BAR: a noop test function."
  nil)
--------------------------------------------------------

1) Do the following:

emacs -Q test-foo.el

M-x update-file-autoloads RET
test-bar.el RET
test-autoloads.el RET


It produces an error message, which I copied from buffer *Messages*:

autoload-generate-file-autoloads: DIRECTORY/test-bar.el:0:0: error: 
wrong-type-argument: (stringp nil)

where in place of DIRECTORY there is the directory the files
above reside.

2) Similar as above but try generating autoloads for the whole
directory:

emacs -Q test-foo.el

M-x update-directory-autoloads RET RET
test-autoloads.el RET


This time the error message is (again from buffer *Messages*):

autoload-generate-file-autoloads: test-bar.el:0:0: error: wrong-type-
argument: (stringp nil)


IMHO, the root of the problem is that update-file-autoloads and
update-directory-autoloads bind generated-autoload-file buffer locally
(as it is a file local variable in test-foo.el) but intend to bind the
default value.  Note that the docstrings of both functions
mention the case of generated-autoload-file being buffer-local, so
they ought to support this case.

As the documented behaviour of these functions do not
depend on the current buffer, I propose to simply switch to a
temporary buffer so that the local settings don't interfere.
This is a preemptive measure of other similar latent problems.

Admittedly, creating a temporary buffer just to bind variables'
default value seems to be wasteful, and I am very interested in
learning a better, more idiomatic approach.

All in all, the patch is the following, adding wrappers
`(with-temp-buffer ...)'.  (The code has not been reindented, 
reformatted
solely to keep the patch readable.)

--- lisp/emacs-lisp/autoload.el
+++ lisp/emacs-lisp/autoload.el
@@ -916,6 +916,9 @@ update-file-autoloads
   (interactive (list (read-file-name "Update autoloads for file: ")
                     current-prefix-arg
                     (read-file-name "Write autoload definitions to 
file: ")))
+  ;; The temporary buffer is only to bind the default value of e.g.,
+  ;; `generated-autoload-file'.  The current buffer is irrelevant.
+  (with-temp-buffer
   (let* ((generated-autoload-file (or outfile generated-autoload-file))
         (autoload-modified-buffers nil)
         ;; We need this only if the output file handles more than one 
input.
@@ -927,6 +930,7 @@ update-file-autoloads
       (if (called-interactively-p 'interactive)
           (message "Autoload section for %s is up to date." file)))
     (if no-autoloads file)))
+  )

 (defun autoload-find-destination (file load-name)
   "Find the destination point of the current buffer's autoloads.
@@ -1028,6 +1032,9 @@ update-directory-autoloads
 file binds `generated-autoload-file' as a file-local variable,
 write its autoloads into the specified file instead."
   (interactive "DUpdate autoloads from directory: ")
+  ;; The temporary buffer is only to bind default value of e.g.,
+  ;; `generated-autoload-file'.  The current buffer is irrelevant.
+  (with-temp-buffer
   (let* ((files-re (let ((tmp nil))
                     (dolist (suf (get-load-suffixes))
                        ;; We don't use module-file-suffix below because
@@ -1142,6 +1149,7 @@ update-directory-autoloads
       ;; In case autoload entries were added to other files because of
       ;; file-local autoload-generated-file settings.
       (autoload-save-buffers))))
+  )

 (define-obsolete-function-alias 'update-autoloads-from-directories
     'update-directory-autoloads "22.1")



Best wishes,

     Gábor


In GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.5)
 of 2019-09-23, modified by Debian built on x86-grnet-01
Windowing system distributor 'The X.Org Foundation', version 
11.0.12004000
System Description:	Debian GNU/Linux 10 (buster)

Recent messages:
Generating autoloads for ~/.emacs.d/autoload-test/test-foo.el...
(New file)
Generating autoloads for ~/.emacs.d/autoload-test/test-foo.el...done
Quit
Generating autoloads for ~/.emacs.d/autoload-test/test-bar.el...
(New file)
Generating autoloads for ~/.emacs.d/autoload-test/test-bar.el...done
scroll-down-command: Beginning of buffer
C-c M-w is undefined
scroll-up-command: End of buffer

Configured using:
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Memory information:
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