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Re: Question about defcustom and load-history


From: Stefan Monnier
Subject: Re: Question about defcustom and load-history
Date: Mon, 06 May 2019 12:37:39 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

Mauro Aranda <address@hidden> writes:
>>> In the falsy case, i.e., when the variable has an initial default value,
>>> a `defvar' form is evaled resulting in one entry, and then the symbol is
>>> pushed again to `current-load-list' from where it probably gets to
>>> `load-history'.
>>
>> Indeed the manual handling of current-load-list should likely be moved
>> to the true branch of the `if`.
>
> Hello Tassilo and Stefan,
>
> Thank you both for your answers.
>
> After reading Tassilo explanation, I went on to have a look at
> 'custom.el' and found out that if a custom variable has
> 'custom-initialize-default' as the :initialize function, a 'defvar' is
> evaled too.  So, those custom variables appear thrice in 'load-history'.
>
> For example, loading 'ansi-color', you'll see
> 'ansi-color-faces-vector' and 'ansi-color-names-vector' triplicated.
>
> This doesn't look intentional to me.  Should I file a bug report?

I haven't seen a corresponding bug-report, so I'll reply here.
I installed the patch below into `master` which should fix this problem
for good.


        Stefan


    * lisp/custom.el: Avoid adding vars to load-history multiple times
    
    Avoid the abuse of (eval `(defvar ...)) which tends to end up
    adding redundant entries in `load-history`, as discussed in
    https://lists.gnu.org/r/help-gnu-emacs/2019-03/msg00237.html
    
    (custom-initialize-default): Don't add to load-history.
    (custom-declare-variable): Use internal--define-uninitialized-variable
    and only add the var to load-history once.  Do it before calling
    `initialize` so the special-variable-p flag is set.
    
    * src/eval.c (Finternal__define_uninitialized_variable): New function.
    (Fdefvar, Fdefconst): Use it.
    (syms_of_eval): Defsubr' it.


diff --git a/lisp/custom.el b/lisp/custom.el
index 53b8045f05..29bf9e570a 100644
--- a/lisp/custom.el
+++ b/lisp/custom.el
@@ -56,8 +56,14 @@ custom-initialize-default
 the car of that and use it as the default binding for symbol.
 Otherwise, EXP will be evaluated and used as the default binding for
 symbol."
-  (eval `(defvar ,symbol ,(let ((sv (get symbol 'saved-value)))
-                            (if sv (car sv) exp)))))
+  (condition-case nil
+      (default-toplevel-value symbol)   ;Test presence of default value.
+    (void-variable
+     ;; The var is not initialized yet.
+     (set-default-toplevel-value
+      symbol (eval (let ((sv (get symbol 'saved-value)))
+                     (if sv (car sv) exp))
+                   t)))))
 
 (defun custom-initialize-set (symbol exp)
   "Initialize SYMBOL based on EXP.
@@ -188,18 +194,13 @@ custom-declare-variable
                (t
                 (custom-handle-keyword symbol keyword value
                                        'custom-variable))))))
+    ;; Set the docstring, record the var on load-history, as well
+    ;; as set the special-variable-p flag.
+    (internal--define-uninitialized-variable symbol doc)
     (put symbol 'custom-requests requests)
     ;; Do the actual initialization.
     (unless custom-dont-initialize
       (funcall initialize symbol default)))
-  ;; Use defvar to set the docstring as well as the special-variable-p flag.
-  ;; FIXME: We should reproduce more of `defvar's behavior, such as the warning
-  ;; when the var is currently let-bound.
-  (if (not (default-boundp symbol))
-      ;; Don't use defvar to avoid setting a default-value when undesired.
-      (when doc (put symbol 'variable-documentation doc))
-    (eval `(defvar ,symbol nil ,@(when doc (list doc)))))
-  (push symbol current-load-list)
   (run-hooks 'custom-define-hook)
   symbol)
 
diff --git a/src/eval.c b/src/eval.c
index 3fd9a40a3a..567c32e0d7 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -715,6 +715,25 @@ DEFUN ("set-default-toplevel-value", 
Fset_default_toplevel_value,
   return Qnil;
 }
 
+DEFUN ("internal--define-uninitialized-variable",
+       Finternal__define_uninitialized_variable,
+       Sinternal__define_uninitialized_variable, 1, 2, 0,
+       doc: /* Define SYMBOL as a variable, with DOC as its docstring.
+This is like `defvar' and `defconst' but without affecting the variable's
+value.  */)
+  (Lisp_Object symbol, Lisp_Object doc)
+{
+  XSYMBOL (symbol)->u.s.declared_special = true;
+  if (!NILP (doc))
+    {
+      if (!NILP (Vpurify_flag))
+       doc = Fpurecopy (doc);
+      Fput (symbol, Qvariable_documentation, doc);
+    }
+  LOADHIST_ATTACH (symbol);
+  return Qnil;
+}
+
 DEFUN ("defvar", Fdefvar, Sdefvar, 1, UNEVALLED, 0,
        doc: /* Define SYMBOL as a variable, and return SYMBOL.
 You are not required to define a variable in order to use it, but
@@ -754,32 +773,25 @@ usage: (defvar SYMBOL &optional INITVALUE DOCSTRING)  */)
     {
       if (!NILP (XCDR (tail)) && !NILP (XCDR (XCDR (tail))))
        error ("Too many arguments");
+      Lisp_Object exp = XCAR (tail);
 
       tem = Fdefault_boundp (sym);
+      tail = XCDR (tail);
 
       /* Do it before evaluating the initial value, for self-references.  */
-      XSYMBOL (sym)->u.s.declared_special = true;
+      Finternal__define_uninitialized_variable (sym, CAR (tail));
 
       if (NILP (tem))
-       Fset_default (sym, eval_sub (XCAR (tail)));
+       Fset_default (sym, eval_sub (exp));
       else
        { /* Check if there is really a global binding rather than just a let
             binding that shadows the global unboundness of the var.  */
          union specbinding *binding = default_toplevel_binding (sym);
          if (binding && EQ (specpdl_old_value (binding), Qunbound))
            {
-             set_specpdl_old_value (binding, eval_sub (XCAR (tail)));
+             set_specpdl_old_value (binding, eval_sub (exp));
            }
        }
-      tail = XCDR (tail);
-      tem = Fcar (tail);
-      if (!NILP (tem))
-       {
-         if (!NILP (Vpurify_flag))
-           tem = Fpurecopy (tem);
-         Fput (sym, Qvariable_documentation, tem);
-       }
-      LOADHIST_ATTACH (sym);
     }
   else if (!NILP (Vinternal_interpreter_environment)
           && (SYMBOLP (sym) && !XSYMBOL (sym)->u.s.declared_special))
@@ -827,19 +839,12 @@ usage: (defconst SYMBOL INITVALUE [DOCSTRING])  */)
       docstring = XCAR (XCDR (XCDR (args)));
     }
 
+  Finternal__define_uninitialized_variable (sym, docstring);
   tem = eval_sub (XCAR (XCDR (args)));
   if (!NILP (Vpurify_flag))
     tem = Fpurecopy (tem);
-  Fset_default (sym, tem);
-  XSYMBOL (sym)->u.s.declared_special = true;
-  if (!NILP (docstring))
-    {
-      if (!NILP (Vpurify_flag))
-       docstring = Fpurecopy (docstring);
-      Fput (sym, Qvariable_documentation, docstring);
-    }
-  Fput (sym, Qrisky_local_variable, Qt);
-  LOADHIST_ATTACH (sym);
+  Fset_default (sym, tem);      /* FIXME: set-default-toplevel-value? */
+  Fput (sym, Qrisky_local_variable, Qt); /* FIXME: Why?  */
   return sym;
 }
 
@@ -4198,6 +4203,7 @@ alist of active lexical bindings.  */);
   defsubr (&Sdefvaralias);
   DEFSYM (Qdefvaralias, "defvaralias");
   defsubr (&Sdefconst);
+  defsubr (&Sinternal__define_uninitialized_variable);
   defsubr (&Smake_var_non_special);
   defsubr (&Slet);
   defsubr (&SletX);




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