bug#16286: 24.3.50; insert-file-contents may bring invisible garbage

[Andrey Kotlarski]

In trunk inserting few bytes from file may sometimes result in nothing
visible in the buffer while invisible artifacts are present and may
affect subsequent operations.  Moreover, there doesn't seem to be way to
recover from this.  Here's example session with emacs -Q:

(let ((file "test.txt"))
  (unless (file-exists-p file)
    (find-file file)
    (insert "абв")                      ;Cyrillic letters
    (save-buffer)
    (kill-buffer))

  (let ((buf (generate-new-buffer "test")))
    (switch-to-buffer buf)
    (insert-file-contents file nil 0 2) ;inserts а
    (goto-char (point-max))
    (insert-file-contents file nil 2 3) ;returns 0 bytes inserted, nothing 
visible in the buffer
                                        ;but actually there is
    (erase-buffer)                      ;and still is
    (insert-file-contents file nil 2 4) ;should insert б, instead let: Wrong 
type argument: inserted-chars, 1
    (message "%S" (buffer-string)) ;"бЀ" while buffer is visibly empty
    ))

Trying to insert multibyte characters now brings content length issues,
garbage inserted and at some point Emacs crashes.

In release 24.3 and earlier insert-file-contents seems to always insert
something, be it wrongly decoded or raw eight-bit characters.  But it is
visible and easy to deal with.  The above example works fine there.
This is useful for the vlf package (https://github.com/m00natic/vlfi) as
a way to detect insufficient amount of bytes requested and allows
further adjustment.


In GNU Emacs 24.3.50.1 (x86_64-pc-linux-gnu)
 of 2013-12-29 on andrexhe
Bzr revision: 115803 eggert@cs.ucla.edu-20131229075253-hmeofd1oihd5n3rk
Windowing system distributor `The X.Org Foundation', version 11.0.11405000
Configured using:
 `configure --build=x86_64-pc-linux-gnu --enable-link-time-optimization
 --with-x-toolkit=no --with-wide-int --without-toolkit-scroll-bars
 --without-xaw3d --without-gpm --without-gconf --without-gsettings
 build_alias=x86_64-pc-linux-gnu 'CFLAGS=-march=native -mtune=native -O2
 -pipe''

Important settings:
  value of $LC_COLLATE: C
  value of $LANG: bg_BG.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Fundamental

Minor modes in effect:
  tooltip-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
M-x r e p o - e m - b u <tab> <return>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
let: Wrong type argument: inserted-chars, 1

Load-path shadows:
None found.

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