Package: emacs;
Reported by: Keith David Bershatsky <esq <at> lawlist.com>
Date: Sat, 3 Jun 2017 17:51:02 UTC
Severity: wishlist
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From: Keith David Bershatsky <esq <at> lawlist.com> To: 27214 <at> debbugs.gnu.org Subject: bug#27214: truncate_undo_list in undo.c: exclusions, warnings, documentation. Date: Sat, 03 Jun 2017 10:49:36 -0700
In developing a fork of the popular undo-tree.el library (new features, enhancements, and bug-fixes), I found the following areas in `truncate_undo_list` that could use some improvements:
1. User-defined exclusions; e.g., a list of elements that will not be truncated unless the user has crossed the `undo-outer-limit' threshold.
EXAMPLE:
(defvar truncate-undo-list-exclusions '(undo-tree-canary)
"A list of user defined elements that will not be truncated during garbage
collection unless the user has reached the `undo-outer-limit`, in which
case ....")
2. User-enabled messages/warnings when truncation is occurring due to the `undo-limit`, or the `undo-strong-limit`, and some type of indication as to what was thrown out.
EXAMPLE:
(defvar truncate-undo-list-warnings t
"When non-nil, the internal function `truncate_undo_list' will generate
messages letting the user know that he/she has crossed the `undo-limit`
or `undo-strong-limit`, along with a shortened (redacted) list of
what is being truncated (mentioning the specific limit crossed).)
3. Some type of documentation system enabling a user to read about `truncate_undo_list'; e.g., consolidate the comments that are presently only visible if the user visits the source-code and add some additional information about the new features mentioned above.
BACKGROUND: Here is a reprint of the thread that I launched on emacs.stackexchange.com explaining my use-case and thoughts about a potential workaround:
https://emacs.stackexchange.com/q/33248/2287
**Q**: How to preserve the last entry in the `buffer-undo-list` when garbage collection occurs?
When using `undo-tree.el`, the library relies upon an `undo-tree-canary` being at the end of the `buffer-undo-list`. Emacs performs garbage collection **before** the Lisp code in `undo-tree` does its thing -- i.e.,`truncate_undo_list` in `undo.c` is activated and sometimes the `undo-tree-canary` is truncated. [A good example of this is where there is a programmatic `delete-region` followed by `insert` of significant amounts of different text, such as sorting certain sections of a buffer by `sort-reorder-buffer`, etc.] The default behavior of `undo-tree` is to begin a new `buffer-undo-tree` when a canary cannot be found -- i.e., the user loses all prior saved history. [See `undo-list-transfer-to-tree`.]
In looking at the C-source code in `truncate_undo_list` I see the following relevant section from a `while` loop that goes through the `buffer-undo-list` when figuring out whether to truncate before or after an undo-boundary (which is a `nil` entry):
/* When we get to a boundary, decide whether to truncate
either before or after it. The lower threshold, undo_limit,
tells us to truncate after it. If its size pushes past
the higher threshold undo_strong_limit, we truncate before it. */
if (NILP (elt))
{
if (size_so_far > undo_strong_limit)
break;
last_boundary = prev;
if (size_so_far > undo_limit)
break;
}
The relevant default values are as follows:
`undo-limit`: 80000
`undo-strong-limit`: 120000
`undo-outer-limit`: 12000000
It looks like I may be able to set `undo-limit` to *the same value* as `undo-strong-limit` and thereby force truncation to always occur *before* the undo-boundary, but I'm not 100% certain that is the case.
Additionally, I am concerned that if I set `undo-limit` to *the same value* as `undo-strong-limit`, that the earliest entries in the `buffer-undo-list` will always be truncated before subsequent entries. If that is the case, then this *may* be a bad thing ....
One drastic solution would be to modify `truncate_undo_list` to look for a `symbol` in the list and preserve it; however, that only benefits me if I run a custom version of Emacs. I'm working on developing a fork of `undo-tree.el`, and I'd like a solution that other people can use with the stock version of Emacs.
[*CAVEAT*: It is my assumption that the `buffer-undo-tree` that existed prior to garbage collection truncation as discussed above will still be usable after truncation occurs. I hope this is the case, but if that is a wrong assumption on my part, then please let me know. In my mind, I'm thinking of a major reorganization of the buffer where text is deleted and new text is inserted.]
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