bug#72511: 30.0.50; prefix-completions is always nil in help--symbol-completion-table

bug#72511: 30.0.50; prefix-completions is always nil in help--symbol-completion-table

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

On lines 209-214 inside the function `help--symbol-complete-table' in
help-fns.el there is a following block of code:

(when help-enable-completion-autoload
  (let ((prefixes (radix-tree-prefixes (help-definition-prefixes) string)))
    (help--load-prefixes prefixes)))
(let ((prefix-completions
       (and help-enable-completion-autoload
            (mapcar #'intern (all-completions string definition-prefixes)))))

By default, `help-enable-completion-autoload' is t, so this code should
normally run. The perceived problem here is that the definition of
`help-definition-prefixes' function is as follows:

(defun help-definition-prefixes ()
  "Return the up-to-date radix-tree form of `definition-prefixes'."
  (when (> (hash-table-count definition-prefixes) 0)
    (maphash (lambda (prefix files)
               (let ((old (radix-tree-lookup help-definition-prefixes prefix)))
                 (setq help-definition-prefixes
                       (radix-tree-insert help-definition-prefixes
                                          prefix (append old files)))))
             definition-prefixes)
    (clrhash definition-prefixes))
  help-definition-prefixes)

Because of the `clrhash', `definition-prefixes' will always be empty
after the function call to `help-definition-prefixes'. So in practice,
`definition-prefixes' will always have zero elements when we're trying
to calculate `prefix-completions', which makes the calls to
`all-completions' and `mapcar' basically no-ops. The end result is that
`prefix-completions' will always be NIL.

This can be verified by evaluating the following code in emacs -Q:

(require 'radix-tree)
(require 'help-fns)

(let ((string "string-remove-prefix"))
  (format "Definition prefixes count before: %d\n%sDefinition prefixes count after: %d\nPrefix completions: %s\n"
          (hash-table-count definition-prefixes)
          (or
           (when help-enable-completion-autoload
             (let ((prefixes (radix-tree-prefixes (help-definition-prefixes) string)))
               (help--load-prefixes prefixes)))
           "")
          (hash-table-count definition-prefixes)
          (let ((prefix-completions
                 (and help-enable-completion-autoload
                      (mapcar #'intern (all-completions string definition-prefixes)))))
            prefix-completions)))

This has been first introduced in commit fd8084a (Automatically find
vars and functions via definition-prefixes)

I'm not sure if this was the intended behavior or not, but it seemed
suspicious to me, so I decided it's better to report it.

It might also be useful to note that `prefix-completion' is used in the
following way:

(complete-with-action action obarray string
                      (if pred (lambda (sym)
                                 (or (funcall pred sym)
                                     (memq sym prefix-completions)))))

I'm not sure what `pred' will be in this context, or what impact
`prefix-completions' being always NIL will have, if any. But I believe
that at the very least the code pertaining to `prefix-completions' could
concievably be removed.

Also, was clearing out the entirety of `definition-prefixes' on completion
really the intended behavior? As it stands, just C-h f s <TAB> is enough
to completely truncate it.

In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.33, cairo version 1.16.0) of 2024-01-09 built on cdr
Repository revision: 774c8ec74c98d69d56b2511a613145f2b69fb2eb
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version
11.0.12101004 System Description: Linux Mint 21.2

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG
LIBSELINUX LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG SECCOMP SOUND
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bug#72511: 30.0.50; prefix-completions is always nil in help--symbol-completion-table

[Eli Zaretskii]

> Date: Wed, 7 Aug 2024 13:53:05 +0200
> From:  Chris Roberts via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> On lines 209-214 inside the function `help--symbol-complete-table' in
> help-fns.el there is a following block of code:
> 
> (when help-enable-completion-autoload
>   (let ((prefixes (radix-tree-prefixes (help-definition-prefixes) string)))
>     (help--load-prefixes prefixes)))
> (let ((prefix-completions
>        (and help-enable-completion-autoload
>             (mapcar #'intern (all-completions string definition-prefixes)))))
> 
> By default, `help-enable-completion-autoload' is t, so this code should
> normally run. The perceived problem here is that the definition of
> `help-definition-prefixes' function is as follows:
> 
> (defun help-definition-prefixes ()
>   "Return the up-to-date radix-tree form of `definition-prefixes'."
>   (when (> (hash-table-count definition-prefixes) 0)
>     (maphash (lambda (prefix files)
>                (let ((old (radix-tree-lookup help-definition-prefixes prefix)))
>                  (setq help-definition-prefixes
>                        (radix-tree-insert help-definition-prefixes
>                                           prefix (append old files)))))
>              definition-prefixes)
>     (clrhash definition-prefixes))
>   help-definition-prefixes)
> 
> Because of the `clrhash', `definition-prefixes' will always be empty
> after the function call to `help-definition-prefixes'. So in practice,
> `definition-prefixes' will always have zero elements when we're trying
> to calculate `prefix-completions', which makes the calls to
> `all-completions' and `mapcar' basically no-ops. The end result is that
> `prefix-completions' will always be NIL.
> 
> This can be verified by evaluating the following code in emacs -Q:
> 
> (require 'radix-tree)
> (require 'help-fns)
> 
> (let ((string "string-remove-prefix"))
>   (format "Definition prefixes count before: %d\n%sDefinition prefixes count after: %d\nPrefix completions: %s\n"
>           (hash-table-count definition-prefixes)
>           (or
>            (when help-enable-completion-autoload
>              (let ((prefixes (radix-tree-prefixes (help-definition-prefixes) string)))
>                (help--load-prefixes prefixes)))
>            "")
>           (hash-table-count definition-prefixes)
>           (let ((prefix-completions
>                  (and help-enable-completion-autoload
>                       (mapcar #'intern (all-completions string definition-prefixes)))))
>             prefix-completions)))
> 
> This has been first introduced in commit fd8084a (Automatically find
> vars and functions via definition-prefixes)
> 
> I'm not sure if this was the intended behavior or not, but it seemed
> suspicious to me, so I decided it's better to report it.
> 
> It might also be useful to note that `prefix-completion' is used in the
> following way:
> 
> (complete-with-action action obarray string
>                       (if pred (lambda (sym)
>                                  (or (funcall pred sym)
>                                      (memq sym prefix-completions)))))
> 
> I'm not sure what `pred' will be in this context, or what impact
> `prefix-completions' being always NIL will have, if any. But I believe
> that at the very least the code pertaining to `prefix-completions' could
> concievably be removed.
> 
> Also, was clearing out the entirety of `definition-prefixes' on completion
> really the intended behavior? As it stands, just C-h f s <TAB> is enough
> to completely truncate it.

Thanks, I'm adding Stefan, who wrote this code, to the discussion.

bug#72511: 30.0.50; prefix-completions is always nil in help--symbol-completion-table

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

> (when help-enable-completion-autoload
>   (let ((prefixes (radix-tree-prefixes (help-definition-prefixes) string)))
>     (help--load-prefixes prefixes)))
> (let ((prefix-completions
>        (and help-enable-completion-autoload
>             (mapcar #'intern (all-completions string definition-prefixes)))))
>
> By default, `help-enable-completion-autoload' is t, so this code should
> normally run. The perceived problem here is that the definition of
> `help-definition-prefixes' function is as follows:
>
> (defun help-definition-prefixes ()
>   "Return the up-to-date radix-tree form of `definition-prefixes'."
>   (when (> (hash-table-count definition-prefixes) 0)
>     (maphash (lambda (prefix files)
>                (let ((old (radix-tree-lookup help-definition-prefixes prefix)))
>                  (setq help-definition-prefixes
>                        (radix-tree-insert help-definition-prefixes
>                                           prefix (append old files)))))
>              definition-prefixes)
>     (clrhash definition-prefixes))
>   help-definition-prefixes)
>
> Because of the `clrhash', `definition-prefixes' will always be empty
> after the function call to `help-definition-prefixes'.

Duh, indeed.
IIRC it's a leftover from some older version of the code.
I think the patch below is in order.

> Also, was clearing out the entirety of `definition-prefixes' on completion
> really the intended behavior?

Originally, yes (after which we'd just use the `help-definition-prefixes`
radix-tree instead), but then other places appeared where using the
radix-tree was not convenient.


        Stefan


diff --git a/lisp/help-fns.el b/lisp/help-fns.el
index 0a469a1fd6d..e3dc23036db 100644
--- a/lisp/help-fns.el
+++ b/lisp/help-fns.el
@@ -86,14 +86,14 @@ help-definition-prefixes
 
 (defun help-definition-prefixes ()
   "Return the up-to-date radix-tree form of `definition-prefixes'."
-  (when (> (hash-table-count definition-prefixes) 0)
+  (when (and (null help-definition-prefixes)
+             (> (hash-table-count definition-prefixes) 0))
     (maphash (lambda (prefix files)
                (let ((old (radix-tree-lookup help-definition-prefixes prefix)))
                  (setq help-definition-prefixes
                        (radix-tree-insert help-definition-prefixes
                                           prefix (append old files)))))
-             definition-prefixes)
-    (clrhash definition-prefixes))
+             definition-prefixes))
   help-definition-prefixes)
 
 (defun help--loaded-p (file)

bug#72511: 30.0.50; prefix-completions is always nil in help--symbol-completion-table

[Eli Zaretskii]

> Cc: 72511@debbugs.gnu.org
> Date: Wed, 14 Aug 2024 11:50:47 -0400
> From:  Stefan Monnier via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> > (when help-enable-completion-autoload
> >   (let ((prefixes (radix-tree-prefixes (help-definition-prefixes) string)))
> >     (help--load-prefixes prefixes)))
> > (let ((prefix-completions
> >        (and help-enable-completion-autoload
> >             (mapcar #'intern (all-completions string definition-prefixes)))))
> >
> > By default, `help-enable-completion-autoload' is t, so this code should
> > normally run. The perceived problem here is that the definition of
> > `help-definition-prefixes' function is as follows:
> >
> > (defun help-definition-prefixes ()
> >   "Return the up-to-date radix-tree form of `definition-prefixes'."
> >   (when (> (hash-table-count definition-prefixes) 0)
> >     (maphash (lambda (prefix files)
> >                (let ((old (radix-tree-lookup help-definition-prefixes prefix)))
> >                  (setq help-definition-prefixes
> >                        (radix-tree-insert help-definition-prefixes
> >                                           prefix (append old files)))))
> >              definition-prefixes)
> >     (clrhash definition-prefixes))
> >   help-definition-prefixes)
> >
> > Because of the `clrhash', `definition-prefixes' will always be empty
> > after the function call to `help-definition-prefixes'.
> 
> Duh, indeed.
> IIRC it's a leftover from some older version of the code.
> I think the patch below is in order.

Thanks, please install on the emacs-30 branch.

bug#72511: 30.0.50; prefix-completions is always nil in help--symbol-completion-table

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

>> IIRC it's a leftover from some older version of the code.
>> I think the patch below is in order.
>
> Thanks, please install on the emacs-30 branch.

Done, thanks,


        Stefan