bug#39137: 26.3; ispell-really-hunspell sometimes fails in init

[Bob Rogers <rogers@modulargenetics.com>]

   Spell checking can die with "(wrong-type-argument stringp nil)" when
ispell-really-hunspell is true.  To reproduce (if it reproduces at all
on your system), just do "emacs -Q" with Hunspell installed and M-$ on
any of the words in the startup screen.

   On some of the systems I use, it fails reliably when initializing; on
others, it works reliably.  All systems are running the openSUSE Leap
15.1 release of GNU/Linux, fully updated, with the identical locally-
built RPM version of Emacs 26.3 (with no local patches) and the distro
Hunspell 1.6.2.  The key seems to be whether ispell-dictionary-alist is
initialized before ispell-start-process is called; just a warning to
anyone who fails to reproduce this.  But since it fails in "emacs -Q" on
systems with apparently identical software, I am at a complete loss to
figure this out.

When it fails, the backtrace looks like this:

	Debugger entered--Lisp error: (wrong-type-argument stringp nil)
	  string-match("," nil 0)
	  split-string(nil "," t)
	  ispell-parse-hunspell-affix-file(nil)
	  ispell-hunspell-fill-dictionary-entry(nil)
	  ispell-start-process()
	  ispell-init-process()
	  ispell-buffer-local-words()
	  ispell-accept-buffer-local-defs()
	  ispell-word(nil nil nil t)
	  funcall-interactively(ispell-word nil nil nil t)
	  call-interactively(ispell-word nil nil)
	  command-execute(ispell-word)

The problem seems to be related to this code in the start of
ispell-start-process:

  ;; Parse hunspell affix file if using hunspell and entry is uninitialized.
  (if ispell-really-hunspell
      (or (cadr (assoc ispell-current-dictionary ispell-dictionary-alist))
	  (ispell-hunspell-fill-dictionary-entry ispell-current-dictionary)))

It appears that "nil" is valid for ispell-current-dictionary and means
"default", but ispell-hunspell-fill-dictionary-entry expects only a
string.  On that basis, I would claim that there is a bug that has
perhaps been hidden by being on a rare execution path.  But if
ispell-dictionary-alist is already initialized with a "nil" entry, then
ispell-hunspell-fill-dictionary-entry never gets called, leading to the
"reproducible irreproducibility" of this bug.  And if
ispell-dictionary-alist should *always* be initialized and contain the
current entry, then this is dead code.

   I don't know where or how ispell-current-dictionary is getting set up
on some systems but not on others, nor how it ought to be set up, so I
am unable to take this any further.


					-- Bob Rogers
					   Modular Genetics Inc.
					   http://www.modulargenetics.com/

------------------------------------------------------------------------
In GNU Emacs 26.3 (build 1, x86_64-suse-linux-gnu, GTK+ Version 3.22.30)
 of 2019-11-21 built on yuggoth
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Entering debugger...

Configured using:
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