bug#58367: `package-install-file' rejects some .tar files (tentative patch)

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> 2) A package where all files are in a single directory vs having
> child directories.  A package with child directories currently only
> installs successfully if it's already extracted; the manual also says
> they are allowed.

I'm pretty sure I've successfully installed the Hyperbole package
(which includes subdirectories) directly from GNU ELPA and I haven't
heard complaints about the Proof-General (M|NonGNU)ELPA package (which
relies even more heavily on subdirectories) either, so the problem is
likely linked to some other "detail".

[..time passes..]

Thanks for the tarballs for the tests, they were very helpful.
The patch also pointed in the right direction.

I installed the patch below (plus regression tests) into `master`.

[ Whether the name comparisons should be done on names that have gone
  through `expand-file-name` or not is a good question.
  Efficiency/simplicity suggests not to do that, and the code's history
  doesn't explain why it's done, but I have the vague recollection that
  it tries to handle some cases of funny file names, e.g. with .. in
  them.
  In the end I kept the `expand-file-name` version mostly so as to
  reduce the size of the change (and associated risk of regression).  ]


        Stefan


diff --git a/lisp/emacs-lisp/package.el b/lisp/emacs-lisp/package.el
index 4268f7d27a7..d619142d64c 100644
--- a/lisp/emacs-lisp/package.el
+++ b/lisp/emacs-lisp/package.el
@@ -930,7 +930,7 @@ package-untar-buffer
         (or (string-match regexp name)
             ;; Tarballs created by some utilities don't list
             ;; directories with a trailing slash (Bug#13136).
-            (and (string-equal dir name)
+            (and (string-equal (expand-file-name dir) name)
                  (eq (tar-header-link-type tar-data) 5))
             (error "Package does not untar cleanly into directory %s/" dir)))))
   (tar-untar-buffer))
@@ -1192,8 +1192,12 @@ package-tar-file-info
   "Find package information for a tar file.
 The return result is a `package-desc'."
   (cl-assert (derived-mode-p 'tar-mode))
-  (let* ((dir-name (file-name-directory
-                    (tar-header-name (car tar-parse-info))))
+  (let* ((dir-name (named-let loop
+                       ((filename (tar-header-name (car tar-parse-info))))
+                     (let ((dirname (file-name-directory filename)))
+                       ;; The first file can be in a subdir: look for the top.
+                       (if dirname (loop (directory-file-name dirname))
+                         (file-name-as-directory filename)))))
          (desc-file (package--description-file dir-name))
          (tar-desc (tar-get-file-descriptor (concat dir-name desc-file))))
     (unless tar-desc