[lwip-devel] [bug #46696] accepts_pending not decreased when TCP_EVENT_ACCEPT returns error

[Ambroz Bizjak]

Follow-up Comment #5, bug #46696 (project lwip):

Hey Joel,

I wasn't considering that. But the solution is fine for applications which use
the raw API directly and do not delay accepting the connection.

Actually, I think there is a minimally invasive way to go from this design to
one which limits SYN_RCVD plus the number of ESTABLISHED but not "fully
accepted" connections. That is:
- In the accept callback, decrement lpcb->backlog as you queue up the
connection for passing up to the application.
- Once you do finally pass the connection to the higher layer (the final
"accept"), increment lpcb->backlog.
- Also take care to increment lpcb->backlog if you abandon the connection
before passing it up.

By temporarily decrementing the lpcb->backlog, you effectively make lwIP
consider your half-accepted connections in the backlog size check when SYN
packets are received.

Actually, in my application I play with lpcb->backlog to limit the total
number of PCBs used for a service, preventing unintended closing of
connections for other services. To do this, I decrement lpcb->backlog in the
accept callback, and only increment it back when the connection is closed.

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