I'm not sure all the situations it affects, to be honest. The fix
aimed to address the strange symptom here:
http://forums.rockbox.org/index.php/topic,50793.0.html
It turns out that ringbuf_add_cross was used when handles were
butted up against one another with the first parameter equal to
the last, which it interprets as being an empty case when it should
be interpreted as full in the context it was used. To fix this,
introduce full/empty variants of ringbuf_add_cross and ringbuf_sub
and use them at the appropriate time.
The other way to address the problem is ensure there's always at
least a space byte between the end of one handle and the start of
another but this make the code a bit trickier to reason about than
using additional function variants.
bufopen() may yield after creating a handle and so do some more
locking so that the buffering thread doesn't mess things up by
moving anything or not seeing the yet-to-be linked-in allocation.
Add alignof() macro to use proper method to get alignment of
struct memory_handle. That should be useful in general anyway.
It's merely defined as __alignof__ but looks nicer.
Change-Id: If21739eaa33a4f6c084a28ee5b3c8fceecfd87ce
eefc7c73e2