[Audacity-devel] 'Paste-over' crash in 1.3.3 Beta - Progress Report.

[James C. <cr...@in...>]

There's a repeatable crash in 1.3.3, also CVS HEAD.
Windows XP. From the symptoms/progress so far, it's very
likely to be present on all platforms.


To reproduce:
- In mono, project at 22050 Hz, generate tone, 30 seconds.
- Select a small part, about 500ms, and copy it.
- Paste it repeatedly, each time pasting over the
right hand boundary of the previously pasted section.

I get the crash usually after five pastes.
It has taken up to nine pastes, never more.

The crash is related to the 'editing a clip may move
other clips' preference. When that's disabled (it's now
enabled by default) I don't get the crash. However, it's
probable that it's just tickling a bug in the code for clips
- since enabling this option leads to many adjacent clips
with the wide-black indicators for adjacent clips showing.

------

The bug results in the following of a NULL pointer.

WaveTrack::Paste() can either be pasting
'into' i.e. inserting, or it can be pasting
'over' where it first removes a section, and then
recursively calls itself to paste into. The pasting
over route has the problem. WaveTrack()::Paste does a
WaveTrack::Cut() and the problem arises when the
WaveTrack::Cut() returns false. In that case no new
copy has been made, and a temporary Track pointer
called tmp is NULL.

If we detect the 'NULL' at this point and stop the
process, we don't crash any more. But we don't paste
in the new clip either!

I THINK the problem that needs fixing is that when
WaveClip::CreateFromCopy() was added, the meaning of
a true and false return wasn't really taken into account.
When Sequence::Copy() returns true, CreateFromCopy() returns
true, and everything works fine. However in
WaveTrack::Copy() when WaveClip::CreateFromCopy()
returns false, as it can do, WaveTrack::Copy() now returns
straight away without doing anything with its 'Track **dest'
parameter.

I'm not sure what WaveTrack::Copy should be doing with its
**dest parameter, but it surely shouldn't be leaving it NULL?

So, does anyone know how WaveTrack::Copy should be setting
**dest in this case? If so, we can fix the problem.

--James.

A second point, quite aside from the crash. I find
enabling 'editing a clip may move other clips' makes for more
cumbersome editing for my editing pattern. I do a lot of 'pasting over',
and it means that I now get a lot of black lines at the boundaries where
I've pasted. That means two extra clicks after each
paste, to get rid of lines I don't want.

Can we keep the previous functionality and disable this option
by default? As an alternative, can it not operate when pasting
over? Also worth considering as well is making the black lines
only one pixel wide, as they currently obscure samples close
to the joins.

--James.