A default constructed SurfacePipe contains a NullSurfaceSink as its
filter in mHead. This filter does nothing and is merely a placeholder.
Since most SurfacePipe objects are constructed with the default
constructor, and NullSurfaceSink has no (modified) state, we use a
singleton to represent it. Normally the SurfacePipe owns its filter, so
it needs to do a special check for NullSurfaceSink to ensure it doesn't
free it explicitly.
A Decoder object contains a default constructed SurfacePipe until it
needs to create the first frame from an image. This is a very brief
window because it does not take very long or much data to get to this
stage of decoding.
The NullSurfaceSink singleton is freed upon shutdown, however some
ISurfaceProvider objects may be lingering after this. If their Decoder
has yet to create the first frame, that means the SurfacePipe actually
contains a dangling pointer to the already freed singleton. To make
things worse, it actually tried to free the filter because it didn't
match the singleton (it got freed!).
As such, this change removes NullSurfaceSink entirely. We never use the
SurfacePipe before initializing it with a proper filter, and it would be
considered a programming error to do so. Instead let SurfacePipe::mHead
be null, and assert that it is not null when any operations are
performed on the SurfacePipe.
Milan Sreckovic
Andrew Osmond
Seth Fowler