bug#18764: 24.4; electric-indent in *scratch* signals an error

[Noam Postavsky <npostavs@gmail.com>]

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Eli Zaretskii <eliz@gnu.org> writes:

>> We can be a little more discriminating than my initial blanket
>> `ignore-errors', see commit message in the attached patch:
>
> Fine with me, but I'd prefer a comment in the code explaining why we
> catch errors here.

I added some more explanation in the comment over the `catch', and
remembered to use condition-case-unless-debug.


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From 5daa803e6e00445570f66116f7ec4681cfdccb92 Mon Sep 17 00:00:00 2001
From: Noam Postavsky <npostavs@gmail.com>
Date: Wed, 20 Jun 2018 20:12:23 -0400
Subject: [PATCH v3] Suppress indent errors during electric indentation
 (Bug#18764)

* lisp/electric.el (electric-indent-post-self-insert-function):
Suppress errors from indent code, but don't suppress errors from
elsewhere in this function.  That way, if trouble is encountered with
electric indent "not working", the error should be reproducible by
calling indent directly (as is the case for Bug#18764), or else it's
from the electric indent code and will be reported normally.
---
 lisp/electric.el | 61 +++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 36 insertions(+), 25 deletions(-)

diff --git a/lisp/electric.el b/lisp/electric.el
index c00e7c00a5..a45faf2dbb 100644
--- a/lisp/electric.el
+++ b/lisp/electric.el
@@ -260,32 +260,43 @@ electric-indent-post-self-insert-function
                 (or (memq act '(nil no-indent))
                     ;; In a string or comment.
                     (unless (eq act 'do-indent) (nth 8 (syntax-ppss))))))))
-      ;; For newline, we want to reindent both lines and basically behave like
-      ;; reindent-then-newline-and-indent (whose code we hence copied).
-      (let ((at-newline (<= pos (line-beginning-position))))
-        (when at-newline
-          (let ((before (copy-marker (1- pos) t)))
-            (save-excursion
-              (unless (or (memq indent-line-function
-                                electric-indent-functions-without-reindent)
-                          electric-indent-inhibit)
-                ;; Don't reindent the previous line if the indentation function
-                ;; is not a real one.
+      ;; If we error during indent, silently give up since this is an
+      ;; automatic action that the user didn't explicitly request.
+      ;; But we don't want to suppress errors from elsewhere in *this*
+      ;; function, hence the `condition-case' and `throw' (Bug#18764).
+      (catch 'indent-error
+        ;; For newline, we want to reindent both lines and basically
+        ;; behave like reindent-then-newline-and-indent (whose code we
+        ;; hence copied).
+        (let ((at-newline (<= pos (line-beginning-position))))
+          (when at-newline
+            (let ((before (copy-marker (1- pos) t)))
+              (save-excursion
+                (unless (or (memq indent-line-function
+                                  electric-indent-functions-without-reindent)
+                            electric-indent-inhibit)
+                  ;; Don't reindent the previous line if the
+                  ;; indentation function is not a real one.
+                  (goto-char before)
+                  (condition-case-unless-debug ()
+                      (indent-according-to-mode)
+                    (error (throw 'indent-error nil))))
+                ;; We are at EOL before the call to
+                ;; `indent-according-to-mode', and after it we usually
+                ;; are as well, but not always.  We tried to address
+                ;; it with `save-excursion' but that uses a normal
+                ;; marker whereas we need `move after insertion', so
+                ;; we do the save/restore by hand.
                 (goto-char before)
-                (indent-according-to-mode))
-              ;; We are at EOL before the call to indent-according-to-mode, and
-              ;; after it we usually are as well, but not always.  We tried to
-              ;; address it with `save-excursion' but that uses a normal marker
-              ;; whereas we need `move after insertion', so we do the
-              ;; save/restore by hand.
-              (goto-char before)
-              (when (eolp)
-                ;; Remove the trailing whitespace after indentation because
-                ;; indentation may (re)introduce the whitespace.
-                (delete-horizontal-space t)))))
-        (unless (and electric-indent-inhibit
-                     (not at-newline))
-          (indent-according-to-mode))))))
+                (when (eolp)
+                  ;; Remove the trailing whitespace after indentation because
+                  ;; indentation may (re)introduce the whitespace.
+                  (delete-horizontal-space t)))))
+          (unless (and electric-indent-inhibit
+                       (not at-newline))
+            (condition-case-unless-debug ()
+                (indent-according-to-mode)
+              (error (throw 'indent-error nil)))))))))
 
 (put 'electric-indent-post-self-insert-function 'priority  60)
 
-- 
2.11.0