Re: [Audacity-devel] bug 1808 sbsms

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On Sat, Jan 6, 2018 at 2:20 PM, Henric Jungheim <sof...@he...> wrote:

>
> Wouldn't that mean writing N bytes of zeros every time N
> bytes are read from the ring buffer?
>

You mean at most N.   "grow" might better have been called "reserve."  It
means, guarantee at least so many places at write_pos.  The assumption in
the memmove is that there are.

Yes, some zeroes, but I have enough other glimmerings of understanding now
to think it's harmless.  Pay attention to the difference between big N (a
member of the class) and little n (what's really getting consumed).

The object has a "length" field.  It keeps 2 * length elements allocated.
It has a read_pos always not more than write_pos.  When elements are
consumed past the mid-point, this routine shifts data left and adjusts the
offsets.

The complication is that there may also be partially ready data at and
after write_pos to be preserved too.  See void SampleBuf :: write(grain *g,
int h) in buffer.cpp.  It may advance write_pos a nonzero amount h, less
than the growth parameter N.  I think this computes batches of transformed
samples with tails, of total length N >= h, and does overlap-add.  I
confirmed that this routine is hit before the crash.

Note though that the condition guaranteed by grow(N) at the top of that is
NO LONGER guaranteed after write_pos advances by h, and I think that's why
advance() can then go wrong, and needs grow(N) to repair the condition
again.

So I think N is meant to be the *upper* *bound* of the size of the partly
computed stuff still in the buffer.  To fix advance, either maintain an
exact bound (hard) or do what I did (easy but a little wasteful).

Or maybe instead, grow(N + h) in SampleBuf::write would also work.

PRL

>
> BTW, the effect of running the test here was an exception on
> line 1348 of ProgressDialog.cpp:
>    Exception thrown: read access violation.
>    this->mGauge->**** was 0xFDFDFDFD. occurred
>
> Note that the fdfdfdfd pattern is used by VC++'s debug
> malloc() to mark the guards before and after heap
> allocations.
>    https://en.wikipedia.org/wiki/Magic_number_(programming)
>
> That could, of course, be the result of a wild memmove()
> clobbering things elsewhere after reading outside an
> allocated buffer.
>
> On Sat, Jan 06, 2018 at 02:00:20PM -0500, Paul Licameli wrote:
> > EUREKA!
> >
> > I think I figured out a one-line fix in the library, without needing to
> > understand all of it.  Just examining the ring buffer classes to find the
> > error in memory management.
> >
> > The fix is this the grow(N) line:
> >
> > template<class T>
> > void ArrayRingBuffer<T> :: advance(long n) {
> >  * grow(N);*
> >   memset(buf+readPos,0,n*sizeof(T));
> >   readPos += n;
> >   if(readPos >= length) {
> >     long endPos;
> >     endPos = writePos+N;
> >     memmove(buf,buf+readPos,(endPos-readPos)*sizeof(T));
> >     memset(buf+readPos,0,((length<<1)-readPos)*sizeof(T));
> >     writePos -= readPos;
> >     readPos = 0;
> >   }
> > }
> >
> > I can now Undo and Repeat Last Effect many times in the originally
> reported
> > use case.
> >
> > Please try the others again on your system.
> >
> > PRL
> >
> >
> >
> > On Sat, Jan 6, 2018 at 1:50 PM, Paul Licameli <pau...@gm...>
> > wrote:
> >
> > >
> > >
> > > On Sat, Jan 6, 2018 at 1:35 PM, Steve the Fiddle <
> ste...@gm...
> > > > wrote:
> > >
> > >> These steps give a repeatable freeze:
> > >>
> > >> 1) Launch Audacity
> > >> 2) Set Project rate to 11025
> > >> 3) Generate 0.3 s 440 Hz tone.
> > >> 4) Apply "Change Pitch" effect to the track:
> > >>  % change: 5
> > >>  Use High Quality: enabled
> > >>
> > >
> > > I find that it is only sometimes repeatable.  Sometimes it works, then
> I
> > > Undo, then Repeat last effect.  After one or two passes generally I
> get the
> > > crash.
> > >
> > > So don't judge good and bad use cases hastily from one trial.
> > >
> > > Proximate cause of the crash (not freeze) for me in macOS is some
> error in
> > > memory management in the class ArrayRingBuffer<T> in the (almost
> endirely
> > > un-commented!) SBSMS library.  A buffer is accessed past its end.
> > >
> > > But bad memory accesses might have various effects with the operating
> > > system.
> > >
> > >
> > >>
> > >>
> > >> These steps do not freeze:
> > >>
> > >> 1) Launch Audacity
> > >> 2) Set Project rate to 11025
> > >> 3) Generate 0.3 s 440 Hz tone.
> > >> 4) Add a split line near the middle.
> > >> 5) Apply "Change Pitch" effect to the track:
> > >>  % change: 5
> > >>  Use High Quality: enabled
> > >>
> > >
> > >
> > > Are you sure you can undo and repeat a few times with no crash?
> > >
> > >
> > >>
> > >>
> > >> and these steps do not freeze:
> > >>
> > >> 1) Launch Audacity
> > >> 2) Set Project rate to 11025
> > >> 3) Generate 0.3 s 440 Hz tone.
> > >> 4) Add some arbitrary audio later in the track.
> > >> 5) Apply "Change Pitch" effect to the 0.3s clip at the start:
> > >>  % change: 5
> > >>  Use High Quality: enabled
> > >>
> > >
> > > Ditto ?
> > >
> > > PRL
> > >
> > >
> > >>
> > >>
> > >> The only case so far where I can reliably reproduce a crash or freeze,
> > >> is to have close to 0.3 seconds of 11025 Hz sample rate audio in a
> > >> track (not necessarily at the start of the track).
> > >>
> > >> ------------------------------------------------------------
> > >> ------------------
> > >> Check out the vibrant tech community on one of the world's most
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> > >> _______________________________________________
> > >> audacity-devel mailing list
> > >> aud...@li...
> > >> https://lists.sourceforge.net/lists/listinfo/audacity-devel
> > >>
> > >
> > >
>
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>
>
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Re: [Audacity-devel] bug 1808 sbsms

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Re: [Audacity-devel] bug 1808 sbsms