Fix denote: link previews during Org mode startup in silos

[lewang]

Follow-up to the :preview support from #317. With
org-startup-with-link-previews enabled, previews of denote: links
fail in two ways when the notes live in a silo (a dir-local
denote-directory).

Problem

• Major mode bodies run before directory-local variables are applied:
  run-mode-hooks calls hack-local-variables only after the mode
  hooks. Org renders startup link previews inside the org-mode body,
  so denote-directories resolves identifiers against the global
  denote-directory instead of the silo's dir-local value.
• When an identifier does not resolve, denote-link-preview-file
  passes nil to org-link-preview-file, which signals
  wrong-type-argument. During startup previews this aborts the
  entire org-mode setup, leaving the buffer half-initialized with
  directory-local variables never applied.

Changes

• denote-directories now honors a dir-local denote-directory that
  has not yet been applied to the buffer. A new helper,
  denote-directories--dir-local-value, reads the value from the
  directory-local variables data on demand.
• denote-link-preview-file skips the preview when the target does
  not resolve, instead of erroring.

Technical Details

• The on-demand read uses hack-dir-local--get-variables, available
  in Emacs 30 or higher; the call is fboundp-guarded, so older
  versions behave exactly as before.
• A buffer-local value or an explicit dynamic binding of
  denote-directory still takes precedence, preserving the documented
  let-binding override of denote-directories.
• The value is read without the safe-local-variable filter, which is
  equivalent for this variable: its safe-local-variable predicate
  accepts any string or list value.

Testing

• New ERT tests in tests/denote-test.el: the unapplied dir-local
  value is honored; a dynamic binding and a buffer-local value each
  take precedence; an unresolvable preview target returns nil without
  error.
• Verified interactively on Emacs 31 with Org 9.8.7: a silo buffer
  with org-startup-with-link-previews now initializes fully and
  renders denote: image previews.

[protesilaos]

Thank you @lewang for doing this! For some reason I did not receive the notification and am only noticing this now.

Major mode bodies run before directory-local variables are applied: run-mode-hooks calls hack-local-variables only after the mode hooks. Org renders startup link previews inside the org-mode body, so denote-directories resolves identifiers against the global denote-directory instead of the silo's dir-local value.

I remember having such a problem one time, which had to do with my dired-preview package---I still need to fix that there. But otherwise I cannot reproduce this problem. Does it happen consistently on your end? Maybe you can show me a recipe to reproduce this inside of emacs -Q?

Otherwise, yes, I am keen on fixing this problem.

[lewang]

No trouble at all about the notification. Thank you for the
encouragement, and for Denote itself -- the silo design is what made my
notes tractable in the first place.

Just a note that the repro below is generated with AI assistance, but the script
is verified by me (human).

Yes, it happens consistently: the unpatched build failed on every run and
never once came up clean.

Two things hide it:

• It leaves almost no trace. normal-mode wraps its set-auto-mode
  call in its own condition-case, so nothing reaches the debugger and
  there is no backtrace -- just one echo-area line, File mode specification error: (wrong-type-argument stringp nil). The buffer
  still ends up in org-mode and still fontifies, so nothing looks
  broken. The damage is silent: the error aborts the org-mode body
  before run-mode-hooks, so hack-local-variables never runs and the
  silo's directory-local variables are never applied. What a user
  notices later is Denote quietly operating on the global
  denote-directory instead of the silo's.
• It needs a graphical frame. org-link-preview-file opens with
  (when (display-graphic-p) ...), so under --batch the preview is
  never attempted and the run looks perfectly clean.

It also needs Org >= 9.8, where the :preview parameter shipped.
Released Emacs still bundles 9.7.x, in which denote-link-preview-file is
never called at all.

Recipe

Hands-off version, which pins Org from GNU ELPA into a throwaway init
directory and touches nothing under $HOME
(gist):

sh repro-715.sh /path/to/denote

By hand:

mkdir -p /tmp/silo /tmp/elsewhere
printf '((nil . ((denote-directory . "/tmp/silo"))))\n' > /tmp/silo/.dir-locals.el
cp any-image.png /tmp/silo/20260101T000001--pic__test.png

cat > /tmp/silo/20260101T000000--note__test.org <<'EOF'
#+title:      note
#+identifier: 20260101T000000

[[denote:20260101T000001]]
EOF

emacs -Q -L /path/to/denote

M-: (progn (setq-default denote-directory "/tmp/elsewhere")
           (require 'denote)
           (setq org-startup-with-link-previews t))

C-x C-f /tmp/silo/20260101T000000--note__test.org

Then: the echo area flashes the error (C-h e if it has scrolled past),
C-h v denote-directory still reads /tmp/elsewhere, and no preview
appears. The global value only has to be somewhere that does not contain
the identifier; the empty scratch directory is just so nothing can be
created under $HOME.

Cause

denote-link-preview-file                 target "20260101T000001"
denote-link--ol-resolve-link-to-target => (nil "20260101T000001" nil)
org-link-preview-file                    path=nil => wrong-type-argument

denote-directories is consulted from inside the org-mode body, before
the dir-local denote-directory has been applied, so the identifier is
looked up in the global directory and does not resolve.

The two commits

Tested separately, since either alone leaves something broken:

denote	error	dir-locals applied	preview
upstream 1004f73	yes	no	no
+ d9d1257	no	yes	no
+ 2fe3bcd	no	yes	yes
both, i.e. this PR	no	yes	yes

d9d1257 alone stops the abort, so org-mode finishes and the dir-locals
do get applied, but the preview still silently does not render. 2fe3bcd
is what makes previews work inside a silo.