Re: Per-descriptor state

[David Holland <dholland-tech%netbsd.org@localhost>]

On Sun, Apr 30, 2023 at 10:50:51PM +0700, Robert Elz wrote:
 >     Date:        Sun, 30 Apr 2023 05:25:41 +0000
 >     From:        David Holland <dholland-tech%netbsd.org@localhost>
 >     Message-ID:  <ZE371Uhm+zXvnBUl%netbsd.org@localhost>
 > 
 >   | Close-on-fork is apparently either coming or already here, not sure
 >   | which, but it's also per-descriptor.
 > 
 > We don't have it, but it will be in Posix-8.   Largely inspired by the
 > needs of threaded programs (without lots of critical sections, one cannot
 > otherwise open anything if another thread might fork, there's no
 > way to avoid race conditions, hence O_CLOFORK on open ... not sure if
 > anyone has thought of a way to add it to socket() - that doesn't look
 > to be trivial, though it might be possible to abuse one of the params
 > it has - probably domain - and add flags in upper bits ... while having
 > it able to be set/reset via fcntl is useful, to work, it needs to be
 > able to be set atomically with the operation that creates the fd, and
 > having it default "on", which would work, would break almost all existing
 > non-trivial code).

Given that the trend is to just add extra calls with flags arguments
without even pausing to consider, I'd be surprised if nobody's banged
out a socket4() yet.

 >   | But I kind of think it'd be preferable to make a way to
 >   | clone a second independent struct file for the same socket than to
 >   | start mucking with per-descriptor state.
 > 
 > When I saw mouse's message, I was thinking the exact same thing,
 > and it should be easy to extend dup3() to make that possible

If we were to add such a thing it should be called something else, not
dup, because it's a fundamentally different operation from dup and we
don't need people confusing them.

 > - however
 > I'm not sure what effects that might have on the semantics of sockets
 > (what assumptions the current code might make about there being only
 > one struct file, with all it contains, for a socket).

Indeed :-|

-- 
David A. Holland
dholland%netbsd.org@localhost