Re: kern/54435: reading TCP urgent data with MSG_OOB doesn't clear poll(2) status

[Valery Ushakov <uwe%stderr.spb.ru@localhost>]

The following reply was made to PR kern/54435; it has been noted by GNATS.

From: Valery Ushakov <uwe%stderr.spb.ru@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: Michael van Elst <mlelstv%serpens.de@localhost>
Subject: Re: kern/54435: reading TCP urgent data with MSG_OOB doesn't clear
 poll(2) status
Date: Sat, 3 Aug 2019 23:51:59 +0300

 On Sat, Aug 03, 2019 at 20:25:01 +0000, Michael van Elst wrote:
 
 >  OOB data is not delivered in the stream. Instead you get information
 >  that 'urgent data' lies ahead. recv(MSG_OOB) can be used to peek at
 >  that data, but it doesn't consume the stream or that data.
 
 Can you cite a chapter and verse for that?
 
 
 >  The correct answer to 'urgent data' is to consume the stream to
 >  the point where the OOB data lies ahead. Then, since a normal
 >  recv() won't deliver the urgent data to the application, use
 >  recv(MSG_OOB) to read it. At that point, the poll condition
 >  already has been cleared by reading the stream.
 
 Except that this is not what happens, as far as I can tell.  Consider
 the output from the test quoted in the PR:
 
 1) we read non-urgent "a"
 
   poll: revents = 0x1: IN
   recv() = 1
   a
 
 
 2) At this point we have urgent "b" and there's no normal data
 preceding it.
 
   poll: revents = 0x82: PRI RDBAND
   recv(MSG_OOB) = 1
   b
 
 the test "used recv(MSG_OOB) to read it".
 
 
 3) "At that point, the poll condition already has been cleared by
    reading the stream" - unfortunately, that's not what happens:
 
   poll: revents = 0x82: PRI RDBAND
   oobrecv: recv(MSG_OOB): Invalid argument
 
 The test had consumed urgent "b" and tries to read further - using
 MSG_OOB since that's how the test interprets POLLPRI.  It fails b/c
 there's no urgent data.  A modified test that uses plain recv() at
 this point would read (non-urgent) "c".
 
 E.g. on linux the same test would work with
 
   reading urgent data with MSG_OOB
   poll: revents = 0x1: IN
   recv() = 1
   a
   poll: revents = 0x2: PRI
   recv(MSG_OOB) = 1
   b
   poll: revents = 0x1: IN
   recv() = 1
   c
   poll: revents = 0x1: IN
   recv() = 0
 
 or, dependent on timing:
 
   reading urgent data with MSG_OOB
   poll: revents = 0x3: IN PRI
   recv(MSG_OOB) = 1
   b
   poll: revents = 0x1: IN
   recv() = 1
   a
   poll: revents = 0x1: IN
   recv() = 1
   c
   poll: revents = 0x1: IN
   recv() = 0
 
 -uwe