commit 41c6834c20ae0b262a4c86a0fdfbb8803a90e6e3
author Jay Logue <jaylogue@google.com> Fri Jun 26 13:59:31 2020 -0700
committer Jay Logue <jaylogue@google.com> Mon Jun 29 08:48:41 2020 -0700
tree 4e62a34ffb68e07054180b336bfef9b493192c15
parent a26364321c6ffd6103a4b8950492319835807104

Enable variable-sized packet buffers in standalone LwIP builds

-- Modified the standalone build to enable the use of variable-sized packet
buffers when running under LwIP. This makes it possible to test certain buffer
handling scenarios that don’t arise when all buffers are the same size.  Use of
variable-sized buffers is enabled by default, but can be disabled if need be
by setting LWIP_PBUF_FROM_CUSTOM_POOLS to 0 in lwipopts.h.

-- Modified the LwIP input handling function to allocate buffers for inbound
packets without reserve space.  Previously the code reserved header space which
was never used and limited the maximum size packet that could be received.

-- Modified TestRetainedPacketBuffer app to accommodate the use of variable-
sized packet buffers.

-- Fixed a long-standing bug in the definition of WEAVE_SYSTEM_CONFIG_PACKETBUFFER_CAPACITY_MAX.
Previously, this value was defined as PBUF_POOL_BUFSIZE minus the size of a pbuf
header.  However, LwIP expects PBUF_POOL_BUFSIZE to be the *payload* size of a
pbuf.  Thus one needn’t subtract the pbuf header size when computing a
“max capacity”.

-- Fixed another long-standing bug in PacketBuffer::NewWithAvailableSize().
When calling pbuf_alloc(), the length parameter is the desired *payload* size of
the buffer.  However, NewWithAvailableSize() was adding in the size of a pbuf
header before calling the function.

-- Eliminated the use of PBUF_LINK_ENCAPSULATION_HLEN in the standalone LwIP
output handling function.  This served no purpose, as evidenced by a spurious
comment about being “needed by WICED”.

-- Disabled MEMP_OVERFLOW_CHECK.  A bug exists in openweave’s version of LwIP
(since fixed upstream in commit 2fd2b68) which results in failures in the
TestPacketBuffer unit test whenever this feature is enabled.  In a nutshell, the
problem is that the runtime size of a pbuf ends up being bigger than the declared
size (e.g. PBUF_POOL_BUFSIZE), which results in a mismatch between the size
returned by AllocSize() and the value of WEAVE_SYSTEM_CONFIG_PACKETBUFFER_CAPACITY_MAX.
This triggers various asserts in TestPacketBuffer.  The temporary fix is to
disable the feature until the LwIP code is updated.

-- Cleaned up comments and code structure in lwipopts.h.