[lwip-devel] [bug #11400] ARP multi-packet-queue modifies TCP unsent/unacked segment packet pbuf chain into packet queue

[Jonathan Larmour]

Follow-up Comment #35, bug #11400 (project lwip):

Hmmm, oh dear it looks like you're right. I hadn't thought there would be a
problem because I assumed the ref count would sort it out. But the API
contract with the user is not like that. This issue - whether chains survive
a call - is exactly the same as with TCP, where it may get put on a queue.
But in that case there is a "copy" flag (originating from netconn_write) to
indicate what should be done. By the time the code path reaches ARP, that
information is lost. And in the case of UDP and netconn_send, it never
existed.

So unless we change the UDP raw API to also have a 'copy' flag, and then pass
that around (or perhaps better, add in a new different pbuf flag e.g.
PBUF_FLAG_COPY). At which point it clearly becomes far too much effort, so
Simon's pbuf_copy() implementation _is_ probably the best thing to do right
now after all. We should consider changing the API for the future to allow
this optimisation.

About the last question and pbuf_ref. Yes you must not call pbuf_free(b). And
there's a lot of API stuff which should be pointed out more clearly IMHO!

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