Handle lists being modified by use of ~

Adjust how lists are built when ~ is used, so we do not crash
even if the list is modified as it is being constructed.

This also makes the behaviour of GAP consistent between bytecode
and immediate code.

src/exprs.c

@@ -919,9 +919,18 @@ Obj EvalPermExpr (
 **
 ** 'EvalListExpr' just calls 'ListExpr1' and 'ListExpr2' to evaluate the
 ** list expression.
+**
+** The 'safe' argument to ListExpr1 and ListExpr2 handle code which abuses ~
+** access and change the partially built list.
+**
+** When 'safe' is 1, we use a slower code path which ensures the list is
+** in a valid state after each element is added, and not crash if the
+** size or TNUM of the list is changed during the construction of elements
+** in the list.
+**
 */
-Obj ListExpr1 ( Expr expr );
-void ListExpr2 ( Obj list, Expr expr );
+Obj ListExpr1 ( Expr expr, Int safe );
+void ListExpr2 ( Obj list, Expr expr, Int safe );
 Obj RecExpr1 ( Expr expr );
 void RecExpr2 ( Obj rec, Expr expr );
 
@@ -931,8 +940,8 @@ Obj EvalListExpr (
  Obj list; /* list value, result */
 
  /* evaluate the list expression */
- list = ListExpr1( expr );
- ListExpr2( list, expr );
+ list = ListExpr1( expr, 0 );
+ ListExpr2( list, expr, 0 );
 
  /* return the result */
  return list;
@@ -964,13 +973,13 @@ Obj EvalListTildeExpr (
  tilde = STATE( Tilde );
 
  /* create the list value */
- list = ListExpr1( expr );
+ list = ListExpr1( expr, 1 );
 
  /* assign the list to '~' */
  STATE(Tilde) = list;
 
  /* evaluate the subexpressions into the list value */
- ListExpr2( list, expr );
+ ListExpr2( list, expr, 1 );
 
  /* restore old value of '~' */
  STATE(Tilde) = tilde;
@@ -999,8 +1008,9 @@ Obj EvalListTildeExpr (
 ** '[ [1], ~[1] ]' requires that the value of one subexpression is entered
 ** into the list value before the next subexpression is evaluated.
 */
-Obj ListExpr1 (
- Expr expr )
+ALWAYS_INLINE Obj ListExpr1 (
+ Expr expr,
+ Int safe )
 {
  Obj list; /* list value, result */
  Int len; /* logical length of the list */
@@ -1015,15 +1025,21 @@ Obj ListExpr1 (
  else {
  list = NEW_PLIST( T_PLIST, len );
  }
- SET_LEN_PLIST( list, len );
+ if(safe) {
+ SET_LEN_PLIST( list, 0 );
+ }
+ else {
+ SET_LEN_PLIST( list, len );
+ }
 
  /* return the list */
  return list;
 }
 
-void ListExpr2 (
+ALWAYS_INLINE void ListExpr2 (
  Obj list,
- Expr expr )
+ Expr expr,
+ Int safe )
 {
  Obj sub; /* value of a subexpression */
  Int len; /* logical length of the list */
@@ -1053,17 +1069,25 @@ void ListExpr2 (
  {
  if (posshole == 1)
  {
- SET_FILT_LIST(list, FN_IS_NDENSE);
+ if(!safe || TNUM_OBJ(list) == T_PLIST) {
+ SET_FILT_LIST(list, FN_IS_NDENSE);
+ }
  posshole = 2;
  }
  sub = EVAL_EXPR( ADDR_EXPR(expr)[i-1] );
- SET_ELM_PLIST( list, i, sub );
+ if(safe) {
+ ASS_LIST( list, i, sub );
+ }
+ else {
+ SET_ELM_PLIST( list, i, sub );
+ }
  CHANGED_BAG( list );
  }
 
  }
- if (!posshole)
+ if (!posshole && !safe) {
  SET_FILT_LIST(list, FN_IS_DENSE);
+ }
 
 }
 

tst/testinstall/tilde.tst

@@ -85,4 +85,15 @@ true
 gap> i := InputTextString( "local a; a := 10; return rec(a := a*10, b := ~);" );;
 gap> r := rec(a := ~, b := ReadAsFunction(i)(), c := ~.b.a);
 rec( a := ~, b := rec( a := 100, b := ~.b ), c := 100 )
+gap> [Length(~),Length(~),Length(~)];
+[ 0, 1, 2 ]
+gap> (function() return [Length(~),Length(~),Length(~)]; end)();
+[ 0, 1, 2 ]
+gap> rem := function(l) Remove(l); return 1; end;;
+gap> [2,rem(~),rem(~),rem(~)];
+[ ,,, 1 ]
+gap> [2,rem(~),3,4,rem(~),5,6,rem(~)];
+[ , 1, 3,, 1, 5,, 1 ]
+gap> (function() return [2,rem(~),3,4,rem(~),5,6,rem(~)]; end)();
+[ , 1, 3,, 1, 5,, 1 ]
 gap> STOP_TEST( "tilde.tst", 1);