When packet buffer's payload buffer does not have enough contiguous
window left, the ring buffer roll over uses an incorrect size
causing the ring buffer pointer not resetting to the head.
When the payload enqueued is larger than 1/2 of the payload ring
buffer, this bug will cause the slice returned by
`PacketBuffer::enqueue` to not match the requested size, and
trigger `debug_assert` in debug profile or size mismatch panic in
`copy_from_slice` when compiled in release profile.
The size of the contiguous window should be prioritized over the size of
the contiguous window if padding were added.
For example, given the following packet buffer:
1 2 3
|---|----|------|
If 2 is used and 1 and 3 are empty, enqueing a buffer of size 4 should
be placed in contiguous window 3 and the size of 1 should not cause
any issues in enqueue.
Closes: #247
Approved by: whitequark
Dario Nieuwenhuis
Gary Guo
Scott Mabin
Kai Lüke
Dan Robertson
whitequark