bug#19865: tar-untar-buffer: should honor default-directory

[Ivan Shmakov <ivan@siamics.net>]

>>>>> Eli Zaretskii <eliz@gnu.org> writes:
>>>>> From: Stefan Monnier  Date: Mon, 16 Feb 2015 14:34:02 -0500

[…]

 >> I agree that having to be careful in which buffer we are when we
 >> read a given variable because it might be buffer-local is a source
 >> of maintenance headaches, but we have that all over the place in
 >> Elisp, and we don't really have any "better solution".

 > I don't really see a problem here that needs a solution.

	Yet at least two other Emacs users do.

 > A year from now no one will remember or understand why we use
 > with-current-buffer in that place.  Doing so for such a weak reason
 > is unwise, and no amount of cruft we have elsewhere can justify
 > adding to that.

	We’re using with-current-buffer there for the sole reason that
	write-region operates strictly on the current buffer.  It is so
	without this change, and it remains so with it; the change
	maintains this status quo just perfectly.  And as long as such
	use /is/ acceptable in the current code, I fail to see why it
	wouldn’t be in the replacement being discussed.

	What /is/ changed is that with-current-buffer will now go
	immediately before write-region:

   (with-current-buffer data-buffer
     (write-region …))

	Cf. the former:

   (with-current-buffer data-buffer
     … lots of code to make sure the reader’s lost…
             (write-region …))

	And at the same time:

   (let ((name (expand-file-name (the-name-of-the-archive-member))))
     …)

	Versus the current:

   (let ((name (the-name-of-the-archive-member)))
     …)

	Please also note that the use of expand-file-name will still be
	necessary if we decide to add an extra optional target-directory
	argument to the function later, – and the /new/ code could
	easily be changed to that effect, like:

   (unless target-directory
     (setq target-directory default-directory))
   (let ((name (expand-file-name (the-name-of-the-archive-member)
                                 target-directory)))
     …)

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