bug#22349: 24.5; Documentation of variable major-mode does not seem to tell the exactly true

[Rolf Ade]

The documentation of the variable `major-mode' reads as:

    Documentation:
    Symbol for current buffer's major mode.
    The default value (normally `fundamental-mode') affects new buffers.
    A value of nil means to use the current buffer's major mode, provided
    it is not marked as "special".

    When a mode is used by default, `find-file' switches to it before it
    reads the contents into the buffer and before it finishes setting up
    the buffer.  Thus, the mode and its hooks should not expect certain
    variables such as `buffer-read-only' and `buffer-file-coding-system'
    to be set up.

    You can customize this variable.

Emacs 25.1.50.2 still has excatly the same wording (just some `' changed
to ‘’, y'know).

Something in the second paragraph ("When a mode ... to be set up.")
doesn't seem to really describe, what happens. The text seem to suggest,
that in case the default major mode is used the mode hooks run before
the file content is available in the buffer.

(As a consequence, this seems to explain, why point movements in the
freshly opened buffer from within a mode hook doesn't work, see bug
report "24.5; Mode hooks doesn't run as expected if the major mode of a
new buffer is determined by the default major mode".)

But look at this:

- Prepare a file with a name/suffix, that isn't matched by anything in
  auto-mode-alist (and interpreter-mode-alist and magic-mode-alist),
  with some content in it. From what I see in my tests, the exact name
  and content doesn't matter (as long as it fullfills the above
  condition). To give an exact example recipe I see it with a file named
  test.abc with the content of (exclude the ""):

"
abc abc abc"

  (Nothing in the default value of auto-mode-alist matches this file
  name test.abc.)

- Start emacs -Q

- In the *scratch* buffer evaluate this lisp code:

(setq-default major-mode 'text-mode)
(add-hook 'text-mode-hook (lambda ()
                            (message "From text-mode hook: %s"
                                      (buffer-string))))

- Open the above prepared file test.abc (just a normal C-x C-f ...).

Emacs presents me a new buffer in major mode text-mode, filled with the
content of the file; everything so far expected. 

But, lesser expected, after reading the major-mode variable
documentation paragraph in question here, the echo area shows the output
of the hook, including the whole content of the buffer. (Or look at the
*Messages* buffer, to verify.)

How could it be possible, that the hook sees the content of the buffer
when - as the documentation say - "`find-file' switches to it [the
default major mode (and running its hooks)] before it reads the contents
into the buffer"?

So, at least this paragraph seems not be crystal clear in describing
what really happens.

But my main point isn't this nit about the wording. I should not
speculate inside a bug report (I know) but it seems to me, this
paragraph in question is just there to 'explain away' some confusing
behaviour as described and reported by me with the bug report "24.5;
Mode hooks doesn't run as expected if the major mode of a new buffer is
determined by the default major mode": 'Told you there is something
special and strange things may happen under this circumstances'.

The least surprising behaviour (at least for my eyes) would be:

If a new buffer (after a `file-find') gets the default major mode
because nothing in auto-mode-alist et. al. matches ensure, that the mode
hooks have the same result (work at the same state of things), as if the
major mode of the new buffer is determined by a match in
auto-mode-alist.

This is currently not so, please see my bug report "24.5; Mode hooks
doesn't run as expected if the major mode of a new buffer is determined
by the default major mode"

If such a fix isn't so easy feasible, for whatever internal reason of
the implementation please be more explicte what a hook could do and what
not (that it normally can) in this case ("certain variables such as
[...]" doesn't sound like a complete list, not to speak about other
subtleties, as the one described in the mentioned other bug report and
maybe other ones not known by me). But that would be only second-best.
(Just me, saying that.)




In GNU Emacs 24.5.1 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.10)
 of 2015-04-11 on linux-qg7d
Windowing system distributor `The X.Org Foundation', version 11.0.11203000
System Description:     openSUSE 12.2 (x86_64)

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=local
  locale-coding-system: utf-8-unix

Major mode: Message

Minor modes in effect:
  TeX-PDF-mode: t
  diff-auto-refine-mode: t
  shell-dirtrack-mode: t
  mml-mode: t
  desktop-save-mode: t
  show-paren-mode: t
  tooltip-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  auto-fill-function: message-do-auto-fill
  transient-mark-mode: t
  abbrev-mode: t

Recent messages:
Mark set
Saved text until "                    (buffer-string))))

"
Saving file /home/rolf/Mail/drafts/*message*-20160111-135955...
Wrote /home/rolf/Mail/drafts/*message*-20160111-135955
Mark set [2 times]
Desktop saved in ~/.emacs.d/
Making completion list...
Quit

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