[lwip-devel] [bug #59651] tcp_write() without TCP_WRITE_FLAG_COPY

[Oleg Kobrin]

URL:
  <https://savannah.nongnu.org/bugs/?59651>

                 Summary: tcp_write() without TCP_WRITE_FLAG_COPY
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: kiabrin
            Submitted on: Чт 10 дек 2020 14:54:16
                Category: IPv4
                Severity: 3 - Normal
              Item Group: Change Request
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None
            lwIP version: git head

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Details:

Debugging the code I found the problem.
Consider you have large output traffic, including one large buffer, which is
used statically.
You can use double buffering to avoid modification of this buffer while it is
not sent.

In call to tcp_write() you are using no TCP_WRITE_FLAG_COPY. But the
pcb->snd_buf is decreased, and after some time you experiense data loss as you
cannot write more data.

Obviousely, tcp_write() without TCP_WRITE_FLAG_COPY should not decrease
snd_buf, as no buffer space are used. But the problem is we cannot just add 

if (apiflags & TCP_WRITE_FLAG_COPY)

before 

    pcb->snd_buf -= len;

as tcp_output() depends on this value. If where is a way to determine if nexp
pbuf in unsent queue is PBUF_ROM type and not to increase snd_buf value, maybe
it sould work?




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