1 files changed, 189 insertions, 24 deletions

diff --git a/src/data.c b/src/data.c
index 61b5da8b5b6..eee2a52a37a 100644
--- a/src/data.c
+++ b/src/data.c
@@ -1225,7 +1225,7 @@ DEFUN ("set", Fset, Sset, 2, 2, 0,
        doc: /* Set SYMBOL's value to NEWVAL, and return NEWVAL.  */)
   (register Lisp_Object symbol, Lisp_Object newval)
 {
-  set_internal (symbol, newval, Qnil, 0);
+  set_internal (symbol, newval, Qnil, SET_INTERNAL_SET);
   return newval;
 }
 
@@ -1233,13 +1233,14 @@ DEFUN ("set", Fset, Sset, 2, 2, 0,
    If buffer/frame-locality is an issue, WHERE specifies which context to use.
    (nil stands for the current buffer/frame).
 
-   If BINDFLAG is false, then if this symbol is supposed to become
-   local in every buffer where it is set, then we make it local.
-   If BINDFLAG is true, we don't do that.  */
+   If BINDFLAG is SET_INTERNAL_SET, then if this symbol is supposed to
+   become local in every buffer where it is set, then we make it
+   local.  If BINDFLAG is SET_INTERNAL_BIND or SET_INTERNAL_UNBIND, we
+   don't do that.  */
 
 void
 set_internal (Lisp_Object symbol, Lisp_Object newval, Lisp_Object where,
-              bool bindflag)
+              enum Set_Internal_Bind bindflag)
 {
   bool voide = EQ (newval, Qunbound);
   struct Lisp_Symbol *sym;
@@ -1250,18 +1251,29 @@ set_internal (Lisp_Object symbol, Lisp_Object newval, Lisp_Object where,
       return; */
 
   CHECK_SYMBOL (symbol);
-  if (SYMBOL_CONSTANT_P (symbol))
+  sym = XSYMBOL (symbol);
+  switch (sym->trapped_write)
     {
+    case SYMBOL_NOWRITE:
       if (NILP (Fkeywordp (symbol))
-          || !EQ (newval, Fsymbol_value (symbol)))
-        xsignal1 (Qsetting_constant, symbol);
+          || !EQ (newval, Fsymbol_value (symbol)))
+        xsignal1 (Qsetting_constant, symbol);
       else
-        /* Allow setting keywords to their own value.  */
-        return;
-    }
+        /* Allow setting keywords to their own value.  */
+        return;
+
+    case SYMBOL_TRAPPED_WRITE:
+      notify_variable_watchers (symbol, voide? Qnil : newval,
+                                (bindflag == SET_INTERNAL_BIND? Qlet :
+                                 bindflag == SET_INTERNAL_UNBIND? Qunlet :
+                                 voide? Qmakunbound : Qset),
+                                where);
+      /* FALLTHROUGH!  */
+    case SYMBOL_UNTRAPPED_WRITE:
+        break;
 
-  maybe_set_redisplay (symbol);
-  sym = XSYMBOL (symbol);
+    default: emacs_abort ();
+    }
 
  start:
   switch (sym->redirect)
@@ -1385,6 +1397,130 @@ set_internal (Lisp_Object symbol, Lisp_Object newval, Lisp_Object where,
     }
   return;
 }
+
+static void
+set_symbol_trapped_write (Lisp_Object symbol, enum symbol_trapped_write trap)
+{
+  struct Lisp_Symbol* sym = XSYMBOL (symbol);
+  if (sym->trapped_write == SYMBOL_NOWRITE)
+    xsignal1 (Qtrapping_constant, symbol);
+  else if (sym->redirect == SYMBOL_LOCALIZED
+           && SYMBOL_BLV (sym)->frame_local)
+    xsignal1 (Qtrapping_frame_local, symbol);
+  sym->trapped_write = trap;
+}
+
+static void
+restore_symbol_trapped_write (Lisp_Object symbol)
+{
+  set_symbol_trapped_write (symbol, SYMBOL_TRAPPED_WRITE);
+}
+
+static void
+harmonize_variable_watchers (Lisp_Object alias, Lisp_Object base_variable)
+{
+  if (!EQ (base_variable, alias)
+      && EQ (base_variable, Findirect_variable (alias)))
+    set_symbol_trapped_write
+      (alias, XSYMBOL (base_variable)->trapped_write);
+}
+
+DEFUN ("add-variable-watcher", Fadd_variable_watcher, Sadd_variable_watcher,
+       2, 2, 0,
+       doc: /* Cause WATCH-FUNCTION to be called when SYMBOL is set.
+
+It will be called with 4 arguments: (SYMBOL NEWVAL OPERATION WHERE).
+SYMBOL is the variable being changed.
+NEWVAL is the value it will be changed to.
+OPERATION is a symbol representing the kind of change, one of: `set',
+`let', `unlet', `makunbound', and `defvaralias'.
+WHERE is a buffer if the buffer-local value of the variable being
+changed, nil otherwise.
+
+All writes to aliases of SYMBOL will call WATCH-FUNCTION too.  */)
+  (Lisp_Object symbol, Lisp_Object watch_function)
+{
+  symbol = Findirect_variable (symbol);
+  set_symbol_trapped_write (symbol, SYMBOL_TRAPPED_WRITE);
+  map_obarray (Vobarray, harmonize_variable_watchers, symbol);
+
+  Lisp_Object watchers = Fget (symbol, Qwatchers);
+  Lisp_Object member = Fmember (watch_function, watchers);
+  if (NILP (member))
+    Fput (symbol, Qwatchers, Fcons (watch_function, watchers));
+  return Qnil;
+}
+
+DEFUN ("remove-variable-watcher", Fremove_variable_watcher, Sremove_variable_watcher,
+       2, 2, 0,
+       doc: /* Undo the effect of `add-variable-watcher'.
+Remove WATCH-FUNCTION from the list of functions to be called when
+SYMBOL (or its aliases) are set.  */)
+  (Lisp_Object symbol, Lisp_Object watch_function)
+{
+  symbol = Findirect_variable (symbol);
+  Lisp_Object watchers = Fget (symbol, Qwatchers);
+  watchers = Fdelete (watch_function, watchers);
+  if (NILP (watchers))
+    {
+      set_symbol_trapped_write (symbol, SYMBOL_UNTRAPPED_WRITE);
+      map_obarray (Vobarray, harmonize_variable_watchers, symbol);
+    }
+  Fput (symbol, Qwatchers, watchers);
+  return Qnil;
+}
+
+DEFUN ("get-variable-watchers", Fget_variable_watchers, Sget_variable_watchers,
+       1, 1, 0,
+       doc: /* Return a list of SYMBOL's active watchers.  */)
+  (Lisp_Object symbol)
+{
+  return (SYMBOL_TRAPPED_WRITE_P (symbol) == SYMBOL_TRAPPED_WRITE)
+    ? Fget (Findirect_variable (symbol), Qwatchers)
+    : Qnil;
+}
+
+void
+notify_variable_watchers (Lisp_Object symbol,
+                          Lisp_Object newval,
+                          Lisp_Object operation,
+                          Lisp_Object where)
+{
+  symbol = Findirect_variable (symbol);
+
+  ptrdiff_t count = SPECPDL_INDEX ();
+  record_unwind_protect (restore_symbol_trapped_write, symbol);
+  /* Avoid recursion.  */
+  set_symbol_trapped_write (symbol, SYMBOL_UNTRAPPED_WRITE);
+
+  if (NILP (where)
+      && !EQ (operation, Qset_default) && !EQ (operation, Qmakunbound)
+      && !NILP (Flocal_variable_if_set_p (symbol, Fcurrent_buffer ())))
+    {
+      XSETBUFFER (where, current_buffer);
+    }
+
+  if (EQ (operation, Qset_default))
+    operation = Qset;
+
+  for (Lisp_Object watchers = Fget (symbol, Qwatchers);
+       CONSP (watchers);
+       watchers = XCDR (watchers))
+    {
+      Lisp_Object watcher = XCAR (watchers);
+      /* Call subr directly to avoid gc.  */
+      if (SUBRP (watcher))
+        {
+          Lisp_Object args[] = { symbol, newval, operation, where };
+          funcall_subr (XSUBR (watcher), ARRAYELTS (args), args);
+        }
+      else
+        CALLN (Ffuncall, watcher, symbol, newval, operation, where);
+    }
+
+  unbind_to (count, Qnil);
+}
+
 
 /* Access or set a buffer-local symbol's default value.  */
 
@@ -1471,16 +1607,27 @@ for this variable.  */)
   struct Lisp_Symbol *sym;
 
   CHECK_SYMBOL (symbol);
-  if (SYMBOL_CONSTANT_P (symbol))
+  sym = XSYMBOL (symbol);
+  switch (sym->trapped_write)
     {
+    case SYMBOL_NOWRITE:
       if (NILP (Fkeywordp (symbol))
-          || !EQ (value, Fdefault_value (symbol)))
-        xsignal1 (Qsetting_constant, symbol);
+          || !EQ (value, Fsymbol_value (symbol)))
+        xsignal1 (Qsetting_constant, symbol);
       else
-        /* Allow setting keywords to their own value.  */
-        return value;
+        /* Allow setting keywords to their own value.  */
+        return value;
+
+    case SYMBOL_TRAPPED_WRITE:
+      /* Don't notify here if we're going to call Fset anyway.  */
+      if (sym->redirect != SYMBOL_PLAINVAL)
+        notify_variable_watchers (symbol, value, Qset_default, Qnil);
+      /* FALLTHROUGH!  */
+    case SYMBOL_UNTRAPPED_WRITE:
+        break;
+
+    default: emacs_abort ();
     }
-  sym = XSYMBOL (symbol);
 
  start:
   switch (sym->redirect)
@@ -1651,7 +1798,7 @@ The function `default-value' gets the default value and `set-default' sets it.
     default: emacs_abort ();
     }
 
-  if (sym->constant)
+  if (SYMBOL_CONSTANT_P (variable))
     error ("Symbol %s may not be buffer-local", SDATA (SYMBOL_NAME (variable)));
 
   if (!blv)
@@ -1726,7 +1873,7 @@ Instead, use `add-hook' and specify t for the LOCAL argument.  */)
     default: emacs_abort ();
     }
 
-  if (sym->constant)
+  if (sym->trapped_write == SYMBOL_NOWRITE)
     error ("Symbol %s may not be buffer-local",
            SDATA (SYMBOL_NAME (variable)));
 
@@ -1838,6 +1985,9 @@ From now on the default value will apply in this buffer.  Return VARIABLE.  */)
     default: emacs_abort ();
     }
 
+  if (sym->trapped_write == SYMBOL_TRAPPED_WRITE)
+    notify_variable_watchers (variable, Qnil, Qmakunbound, Fcurrent_buffer ());
+
   /* Get rid of this buffer's alist element, if any.  */
   XSETSYMBOL (variable, sym);   /* Propagate variable indirection.  */
   tem = Fassq (variable, BVAR (current_buffer, local_var_alist));
@@ -1920,7 +2070,7 @@ frame-local bindings).  */)
     default: emacs_abort ();
     }
 
-  if (sym->constant)
+  if (SYMBOL_TRAPPED_WRITE_P (variable))
     error ("Symbol %s may not be frame-local", SDATA (SYMBOL_NAME (variable)));
 
   blv = make_blv (sym, forwarded, valcontents);
@@ -3465,6 +3615,8 @@ syms_of_data (void)
   DEFSYM (Qcyclic_variable_indirection, "cyclic-variable-indirection");
   DEFSYM (Qvoid_variable, "void-variable");
   DEFSYM (Qsetting_constant, "setting-constant");
+  DEFSYM (Qtrapping_constant, "trapping-constant");
+  DEFSYM (Qtrapping_frame_local, "trapping-frame-local");
   DEFSYM (Qinvalid_read_syntax, "invalid-read-syntax");
 
   DEFSYM (Qinvalid_function, "invalid-function");
@@ -3543,6 +3695,10 @@ syms_of_data (void)
   PUT_ERROR (Qvoid_variable, error_tail, "Symbol's value as variable is void");
   PUT_ERROR (Qsetting_constant, error_tail,
              "Attempt to set a constant symbol");
+  PUT_ERROR (Qtrapping_constant, error_tail,
+             "Attempt to trap writes to a constant symbol");
+  PUT_ERROR (Qtrapping_frame_local, error_tail,
+             "Attempt to trap writes to a frame local variable");
   PUT_ERROR (Qinvalid_read_syntax, error_tail, "Invalid read syntax");
   PUT_ERROR (Qinvalid_function, error_tail, "Invalid function");
   PUT_ERROR (Qwrong_number_of_arguments, error_tail,
@@ -3721,10 +3877,19 @@ syms_of_data (void)
   DEFVAR_LISP ("most-positive-fixnum", Vmost_positive_fixnum,
                doc: /* The largest value that is representable in a Lisp integer.  */);
   Vmost_positive_fixnum = make_number (MOST_POSITIVE_FIXNUM);
-  XSYMBOL (intern_c_string ("most-positive-fixnum"))->constant = 1;
+  make_symbol_constant (intern_c_string ("most-positive-fixnum"));
 
   DEFVAR_LISP ("most-negative-fixnum", Vmost_negative_fixnum,
                doc: /* The smallest value that is representable in a Lisp integer.  */);
   Vmost_negative_fixnum = make_number (MOST_NEGATIVE_FIXNUM);
-  XSYMBOL (intern_c_string ("most-negative-fixnum"))->constant = 1;
+  make_symbol_constant (intern_c_string ("most-negative-fixnum"));
+
+  DEFSYM (Qwatchers, "watchers");
+  DEFSYM (Qmakunbound, "makunbound");
+  DEFSYM (Qunlet, "unlet");
+  DEFSYM (Qset, "set");
+  DEFSYM (Qset_default, "set-default");
+  defsubr (&Sadd_variable_watcher);
+  defsubr (&Sremove_variable_watcher);
+  defsubr (&Sget_variable_watchers);
 }