bug#74718: 29.4; Huge metadata with flex completion style

bug#74718: 29.4; Huge metadata with flex completion style

[Thierry Volpiatto]

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The flex completion style uses a function
(completion--flex-adjust-metadata) to create its display-sort-function.
completion-all-completions calls completion--nth-completion which then
uses setcdr on metadata to setup sort-function with this function.  At
each call of completion-all-completions the metadata is growing quickly
and become huge (see attached file).  In addition to the
display-sort-function becoming huge, the objects are duplicated at each
turn because there is no measure to prevent duplicates.

Here the offending code in minibuffer.el (in
completion--nth-completion):

--8<---------------cut here---------------start------------->8---
   (adjust-fn (get (cdr result-and-style) 'completion--adjust-metadata)))
    (when (and adjust-fn metadata)
      (setcdr metadata (cdr (funcall adjust-fn metadata))))
--8<---------------cut here---------------end--------------->8---

the bug is reproductible easily with helm which allows examining the
object while the session is running, here how to reproduce:

For example we will use `switch-to-buffer' (C-x b) with helm-mode enabled:

1) Once helm is installed enable helm-mode:

--8<---------------cut here---------------start------------->8---
    (helm-mode 1)
--8<---------------cut here---------------end--------------->8---


2) Then configure switch-to-buffer so that it uses flex style:

--8<---------------cut here---------------start------------->8---
    (add-to-list 'helm-completion-styles-alist '(switch-to-buffer . (emacs helm flex)))
--8<---------------cut here---------------end--------------->8---


3) Open some buffers, for example all helm files
4) C-x b to call switch-to-buffer and type a flex pattern like "hlmfi" to narrow down to "helm-fi..."
5) Insist typing "hlm" "hlmfi" "hlm" etc... (not too much because the metadata is growing incredibily quickly)
6) Now consult the log with C-h d (see the second section, the helm source)



In GNU Emacs 29.4 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.33,
 cairo version 1.16.0) of 2024-08-24 built on IPad-S340
Windowing system distributor 'The X.Org Foundation', version 11.0.12101004
System Description: Linux Mint 21.3

Configured using:
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-- 
Thierry

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bug#74718: 29.4; Huge metadata with flex completion style

[Dmitry Gutov]

Hi Thierry,

On 07/12/2024 10:06, Thierry Volpiatto wrote:
> 
> The flex completion style uses a function
> (completion--flex-adjust-metadata) to create its display-sort-function.
> completion-all-completions calls completion--nth-completion which then
> uses setcdr on metadata to setup sort-function with this function.  At
> each call of completion-all-completions the metadata is growing quickly
> and become huge (see attached file).  In addition to the
> display-sort-function becoming huge, the objects are duplicated at each
> turn because there is no measure to prevent duplicates.

Thanks for the report.

> Here the offending code in minibuffer.el (in
> completion--nth-completion):
> 
> --8<---------------cut here---------------start------------->8---
>     (adjust-fn (get (cdr result-and-style) 'completion--adjust-metadata)))
>      (when (and adjust-fn metadata)
>        (setcdr metadata (cdr (funcall adjust-fn metadata))))
> --8<---------------cut here---------------end--------------->8---
> 
> the bug is reproductible easily with helm which allows examining the
> object while the session is running, here how to reproduce:
> 
> For example we will use `switch-to-buffer' (C-x b) with helm-mode enabled:
> 
> 1) Once helm is installed enable helm-mode:
> ...

Can you also reproduce it without Helm?

I've tried to do that with the default UI for a bit, and also with 
company-mode enabled (which also reuses this code). No luck triggering 
this so far.

To test this, I added a printing statement and re-evaluated the defun:

diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
index 405ee21cdb2..6ae3c7b5ee3 100644
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -1310,6 +1310,7 @@ completion--nth-completion
           (adjust-fn (get (cdr result-and-style) 
'completion--adjust-metadata)))
      (when (and adjust-fn metadata)
        (setcdr metadata (cdr (funcall adjust-fn metadata))))
+    (message "%S" metadata)
      (if requote
          (funcall requote (car result-and-style) n)
        (car result-and-style))))

bug#74718: 29.4; Huge metadata with flex completion style

[Thierry Volpiatto]

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Hi Dmitry,

Dmitry Gutov <dmitry@gutov.dev> writes:

> Can you also reproduce it without Helm?

Here a small recipe you can play with:

From emacs -Q:

1) Open some buffers
2) eval this in scratch:

(setq completion-styles '(flex))
(let* ((collection (mapcar #'buffer-name (buffer-list)))
       (metadata (completion-metadata "" collection nil))
       (completion-function (lambda (str _pred _action)
                              (let* ((comps (completion-all-completions str collection nil (length str) metadata))
				     (sort-fn (completion-metadata-get metadata 'display-sort-function))
				     (last (last comps)))
				(when (cdr last)
				  (setcdr last nil))
				(message "%S" metadata)
				(if (and sort-fn (> (length str) 0)) (funcall sort-fn comps) comps)))))
  (completing-read "test: " completion-function))

Try to complete with some flex patterns e.g. "hlm" for "helm" and
consult *messages* buffer.

-- 
Thierry

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bug#74718: 29.4; Huge metadata with flex completion style

[Daniel Mendler via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Thierry Volpiatto <thievol@posteo.net> writes:
> From emacs -Q:
>
> 1) Open some buffers
> 2) eval this in scratch:
>
> (setq completion-styles '(flex))
> (let* ((collection (mapcar #'buffer-name (buffer-list)))
>        (metadata (completion-metadata "" collection nil))
>        (completion-function (lambda (str _pred _action)
>                               (let* ((comps (completion-all-completions str collection nil (length str) metadata))
> 				     (sort-fn (completion-metadata-get metadata 'display-sort-function))
> 				     (last (last comps)))
> 				(when (cdr last)
> 				  (setcdr last nil))
> 				(message "%S" metadata)
> 				(if (and sort-fn (> (length str) 0)) (funcall sort-fn comps) comps)))))
>   (completing-read "test: " completion-function))

Hello Thierry,

`completion-all-completions' should not be called inside completion
tables. Instead they should use `all-completions' to perform filtering
instead. `completion-all-completions' is the "frontend" API, which uses
completion styles, which then call the completion table backend.

The prototypical programmable completion table has the following form,
where `complete-with-action' provides the default implementation for the
ACTION argument:

(let ((candidates '("list" "of" "candidates")))
  (lambda (str pred action)
    (complete-with-action action candidates str pred)))

Depending on your use case, you may want to implement some ACTIONs
yourself, e.g., `metadata':

(let ((candidates '("list" "of" "candidates")))
  (lambda (str pred action)
    (if (eq action 'metadata)
        `(metadata (category . my-candidate-category))
      (complete-with-action action candidates str pred))))

Furthermore candidates can be computed dynamically, see the completion
tables `completion-table-dynamic' or `completion-table-with-cache'.

Daniel

bug#74718: 29.4; Huge metadata with flex completion style

[Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors]

> Here the offending code in minibuffer.el (in
> completion--nth-completion):
>
> --8<---------------cut here---------------start------------->8---
>    (adjust-fn (get (cdr result-and-style) 'completion--adjust-metadata)))
>     (when (and adjust-fn metadata)
>       (setcdr metadata (cdr (funcall adjust-fn metadata))))
> --8<---------------cut here---------------end--------------->8---

Thanks Thierry.
The above code is indeed a hack we should try get rid of.
I suspect it remained unnoticed until now because most UIs get a "fresh"
new metadata before calling `completion-try/all-completions`, but we
shouldn't rely on such a property.

I think I vaguely remember when this code was added that we discussed
whether it's OK to do that (for all I know, I may even have suggested
this hack), so replacing it with a more robust solution may be
not straightforward.

In the mean time, maybe a patch like the one below can temporarily paper
over the underlying problem?


        Stefan


diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
index 405ee21cdb2..d258085c778 100644
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -1307,9 +1307,15 @@ completion--nth-completion
                                string table pred point)))
                    (and probe (cons probe style))))))
            (completion--styles md)))
-         (adjust-fn (get (cdr result-and-style) 'completion--adjust-metadata)))
-    (when (and adjust-fn metadata)
-      (setcdr metadata (cdr (funcall adjust-fn metadata))))
+         (adjust-fn (get (cdr result-and-style) 'completion--adjust-metadata))
+         (adjusted (completion-metadata-get
+                    metadata 'completion--adjusted-metadata)))
+    (when (and adjust-fn metadata
+               ;; Avoid re-applying the same adjustment (bug#74718).
+               (not (memq (cdr result-and-style) adjusted)))
+      (setcdr metadata `((completion--adjusted-metadata
+                          ,(cdr result-and-style) . ,adjusted)
+                         . ,(cdr (funcall adjust-fn metadata)))))
     (if requote
         (funcall requote (car result-and-style) n)
       (car result-and-style))))

bug#74718: 29.4; Huge metadata with flex completion style

[Thierry Volpiatto]

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Hello Stefan,

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> Here the offending code in minibuffer.el (in
>> completion--nth-completion):
>>
>> --8<---------------cut here---------------start------------->8---
>>    (adjust-fn (get (cdr result-and-style) 'completion--adjust-metadata)))
>>     (when (and adjust-fn metadata)
>>       (setcdr metadata (cdr (funcall adjust-fn metadata))))
>> --8<---------------cut here---------------end--------------->8---
>
> Thanks Thierry.
> The above code is indeed a hack we should try get rid of.
> I suspect it remained unnoticed until now because most UIs get a "fresh"
> new metadata before calling `completion-try/all-completions`, but we
> shouldn't rely on such a property.
>
> I think I vaguely remember when this code was added that we discussed
> whether it's OK to do that (for all I know, I may even have suggested
> this hack), so replacing it with a more robust solution may be
> not straightforward.
>
> In the mean time, maybe a patch like the one below can temporarily paper
> over the underlying problem?

Yes your patch fixed it, it avoids the duplicates as well.

Thanks.

>
>         Stefan
>
>
> diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
> index 405ee21cdb2..d258085c778 100644
> --- a/lisp/minibuffer.el
> +++ b/lisp/minibuffer.el
> @@ -1307,9 +1307,15 @@ completion--nth-completion
>                                 string table pred point)))
>                     (and probe (cons probe style))))))
>             (completion--styles md)))
> -         (adjust-fn (get (cdr result-and-style) 'completion--adjust-metadata)))
> -    (when (and adjust-fn metadata)
> -      (setcdr metadata (cdr (funcall adjust-fn metadata))))
> +         (adjust-fn (get (cdr result-and-style) 'completion--adjust-metadata))
> +         (adjusted (completion-metadata-get
> +                    metadata 'completion--adjusted-metadata)))
> +    (when (and adjust-fn metadata
> +               ;; Avoid re-applying the same adjustment (bug#74718).
> +               (not (memq (cdr result-and-style) adjusted)))
> +      (setcdr metadata `((completion--adjusted-metadata
> +                          ,(cdr result-and-style) . ,adjusted)
> +                         . ,(cdr (funcall adjust-fn metadata)))))
>      (if requote
>          (funcall requote (car result-and-style) n)
>        (car result-and-style))))

-- 
Thierry

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bug#74718: 29.4; Huge metadata with flex completion style

[Thierry Volpiatto]

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Hello Daniel,

Daniel Mendler <mail@daniel-mendler.de> writes:

> `completion-all-completions' should not be called inside completion
> tables.

Yes it "should not" but it "can be" as well.  I have still some code
doing this, though not used by default.

Thanks.

-- 
Thierry

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