bug#30891: 26.0.91; indent of malformed lisp

[Noam Postavsky <npostavs@gmail.com>]

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I recently got a private report about a problem with the new lisp indent
functions: they don't work well when the buffer contains syntax errors.
For example, indent-region in a buffer containing:

)
(+ 2
   3)

gives

Debugger entered--Lisp error: (wrong-type-argument consp nil)
  lisp-indent-calc-next(#s(lisp-indent-state :stack (nil) :ppss (0 nil nil nil nil nil 0 nil nil nil nil) :ppss-point 146))
  lisp-indent-region(146 158)
  indent-region(146 158 nil)
  funcall-interactively(indent-region 146 158 nil)
  call-interactively(indent-region nil nil)
  command-execute(indent-region)

The error can be fixed with the patch below, I think it's safe for
emacs-26.  But, it still gives wrong indentation if you do indent-region
on the latter two lines.  That could be fixed with

    (advice-add 'lisp-ppss :override
                #'lisp-ppss-open-paren-in-column-0-is-defun-start)

The lisp-ppss-open-paren-in-column-0-is-defun-start function is included
(but uncalled) in the patch.  Using it in the override suggested above
will bring back Bug#27920 though.  We could include the function and
suggest to use it (with the aforementioned caveat) in etc/NEWS or
PROBLEMS perhaps?



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From da0916b591a983c9b4ee9c8201600a77ba506b92 Mon Sep 17 00:00:00 2001
From: Noam Postavsky <npostavs@gmail.com>
Date: Sat, 17 Mar 2018 21:14:11 -0400
Subject: [PATCH v1] Handle indentation of malformed Lisp

* lisp/emacs-lisp/lisp-mode.el
(lisp-ppss-open-paren-in-column-0-is-defun-start): New function.
* lisp/emacs-lisp/lisp-mode.el (lisp-indent-calc-next): If we run out
of indent stack, reset the parse state.
---
 lisp/emacs-lisp/lisp-mode.el | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/lisp/emacs-lisp/lisp-mode.el b/lisp/emacs-lisp/lisp-mode.el
index f082983d48..58710e9876 100644
--- a/lisp/emacs-lisp/lisp-mode.el
+++ b/lisp/emacs-lisp/lisp-mode.el
@@ -794,6 +794,21 @@ lisp-ppss
           (parse-partial-sexp sexp-start pos nil nil (syntax-ppss sexp-start)))
       pss)))
 
+(defun lisp-ppss-open-paren-in-column-0-is-defun-start (&optional pos)
+ (save-excursion
+   (if pos
+       (goto-char pos)
+     (setq pos (point)))
+   (or (looking-at-p "^(")
+       (re-search-backward "^(" nil t)
+       (goto-char (point-min)))
+   (let ((ppss (parse-partial-sexp (point) pos)))
+     (if (< (car ppss) 0)
+         ;; Too many close parens, probably syntax error.  Give a
+         ;; fresh state.
+         (parse-partial-sexp (point) (point))
+       ppss))))
+
 (cl-defstruct (lisp-indent-state
                (:constructor nil)
                (:constructor lisp-indent-initial-state
@@ -844,6 +859,10 @@ lisp-indent-calc-next
     (prog1
         (let (indent)
           (cond ((= (forward-line 1) 1) nil)
+                ;; Negative depth, probably some kind of syntax error.
+                ((null indent-stack)
+                 ;; Reset state.
+                 (setq ppss (parse-partial-sexp (point) (point))))
                 ((car indent-stack))
                 ((integerp (setq indent (calculate-lisp-indent ppss)))
                  (setf (car indent-stack) indent))
-- 
2.11.0