[lwip-users] RFC: pbuf chains and their reference counts

[Leon Woestenberg]

Hello,

there are some subtleties in the pbuf_free() code regarding pbuf chains.

case 1: equal reference counts for each pbuf.
=> lower all counts, and if zero, free memory.

case 2: a non-decreasing reference count (i.e. staying equal or increasing)
=> lower all counts, de-allocate pbufs with reference count 0 until a pbuf
with non-zero is reached. (same as case 1 really).

case 3: a non-increasing reference count (i.e. staying equal or decreasing)
=> cannot be freed unambigiously
=> should NEVER occur, but CAN occur if:
- User has used pbuf_ref() instead of pbuf_ref_chain().
- User has chained differently referenced pbufs.

case 4: a complex variation of reference counts.
=> should NEVER occur
=> cannot be freed unambigiously

I will be running a code audit against the use of pbuf_ref() and
pbuf_ref_chain()
and will introduce asserts for the above two cases 3 and 4.

If this breaks anything, it was broken in the first place.

Regards,

Leon.