PopHead in TDynArray will try finalize an 'object' if is not null

dcoun · 2022-10-09 16:16:58

I was getting random exceptions from an ilist with objects
The exception is coming from mormot.core.data line 6988 (fInfo.ArrayRtti.ValueFinalize)

procedure TDynArray.ItemMoveTo(index: PtrInt; Dest: pointer);
var
  p: pointer;
begin
  p := ItemPtr(index);
  if (p = nil) or
     (Dest = nil) then
    exit;
  if (fInfo.ArrayRtti <> nil) and
     not fNoFinalize then
    fInfo.ArrayRtti.ValueFinalize(Dest); // also handle T*ObjArray
  MoveFast(p^, Dest^, fInfo.Cache.ItemSize);
  FillCharFast(p^, fInfo.Cache.ItemSize, 0);
end;

which is called by same unit, line 6858

function TDynArray.PopHead(var Dest): boolean;
begin
  result := GetCount <> 0;
  if result then
  begin
    ItemMoveTo(0, @Dest);
    Delete(0);
  end;
end;

The problem was that the object variable passed to pophead was not null and before poping the new object ItemMoveTo try to finalize it.
probably adding a Dest:=nil; somewhere before ItemMove in PopHead?
Or, probably adding this requirement (dest must be nil) in Pop's documentation?

Last edited by dcoun (2022-10-09 16:28:54)

ab · 2022-10-09 18:46:42

It seems to be a feature to me.
Forcing FillCharFast(Dest^, fInfo.Cache.ItemSize, 0) would leak memory in most use cases.

How to you use PopHead() ?
The Dest variable should indeed be valid (filled with zeros, or containing a valid instance).

dcoun · 2022-10-09 19:13:11

The definition is:

lowr:ilist<TepGenericSlave>;

The creation is:

lowr:=Collections.NewList<TepGenericSlave>;

The pop is used like the following and produces exceptions:

TepGenericSlave=class
....

obj:=TepGenericSlave.create;
...
repeat
...
lowr.Pop(obj,[popFromHead]);
...
until ....;

The pop is used like the following and does not produce exceptions:

TepGenericSlave=class
....

obj:=TepGenericSlave.create;
...
repeat
...
obj:=nil;
lowr.Pop(obj,[popFromHead]);
...
until ....;

Last edited by dcoun (2022-10-09 21:29:18)

ab · 2022-10-10 08:41:22

Pop() will extract the instance from the internal array storage.
It won't fill the destination properties, but copy the instance, after having released the existing instance.

I have added a test which shows how memory is managed, and that there is no leak:
https://github.com/synopse/mORMot2/commit/eadf3f20

If you got a GPF it is very likely than your "obj" instance life time was not correct.

dcoun · 2022-10-10 09:36:44

Last commit is perfect. Thank you a lot @ab

mORMot Open Source

#1 2022-10-09 16:16:58

PopHead in TDynArray will try finalize an 'object' if is not null

#2 2022-10-09 18:46:42

Re: PopHead in TDynArray will try finalize an 'object' if is not null

#3 2022-10-09 19:13:11

Re: PopHead in TDynArray will try finalize an 'object' if is not null

#4 2022-10-10 08:41:22

Re: PopHead in TDynArray will try finalize an 'object' if is not null

#5 2022-10-10 09:36:44

Re: PopHead in TDynArray will try finalize an 'object' if is not null

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