code review 12170045: syscall: add Dup3 on Linux

syscall: add Dup3 on Linux

With dup3, we can avoid an extra system call on some machines
while holding syscall.ForkLock. Currently we have to
syscall.Dup + syscall.CloseOnExec.

On machines with Linux and a new enough kernel, this can just
be dup3.

[bradfitz, 2013-07-31 20:12:46]

Hello golang-dev@googlegroups.com,

I'd like you to review this change to
https://go.googlecode.com/hg/

[r, 2013-08-01 00:59:46]

Do we know the kernel is new enough? Would dup4 be 33% better?

[bradfitz, 2013-08-01 01:01:28]

This doesn't use it yet.

A future CL will conditionally use it, checking ENOSYS etc.
 On Jul 31, 2013 5:59 PM, <r@golang.org> wrote:

> Do we know the kernel is new enough? Would dup4 be 33% better?
>
>
https://codereview.appspot.**com/12170045/<https://codereview.appspot.com/121...
>

[r, 2013-08-01 01:01:57]

LGTM

[bradfitz, 2013-08-01 06:38:57]

*** Submitted as https://code.google.com/p/go/source/detail?r=c92197d20d7d ***

syscall: add Dup3 on Linux

With dup3, we can avoid an extra system call on some machines
while holding syscall.ForkLock. Currently we have to
syscall.Dup + syscall.CloseOnExec.

On machines with Linux and a new enough kernel, this can just
be dup3.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/12170045

[iant, 2013-08-02 23:15:25]

On Wed, Jul 31, 2013 at 1:12 PM,  <bradfitz@golang.org> wrote:
>
> syscall: add Dup3 on Linux
>
> With dup3, we can avoid an extra system call on some machines
> while holding syscall.ForkLock. Currently we have to
> syscall.Dup + syscall.CloseOnExec.
>
> On machines with Linux and a new enough kernel, this can just
> be dup3.

No objection to the CL, but I wonder how we can use this.  What we
need in the net package is really dup+O_CLOEXEC.  Unfortunately dup3
is dup2+O_CLOEXEC: it requires you to specify the new descriptor
number.  I don't see how the net package can reliably determine a new
descriptor number to use.

That is to say, what we should use in the net package is not dup3, but
fcntl(fd.sysfd, F_DUPFD_CLOEXEC), which has been available since
kernel version 2.6.24.

Ian

[bradfitz, 2013-08-03 00:14:14]

On Fri, Aug 2, 2013 at 4:15 PM, Ian Lance Taylor <iant@golang.org> wrote:

> On Wed, Jul 31, 2013 at 1:12 PM,  <bradfitz@golang.org> wrote:
> >
> > syscall: add Dup3 on Linux
> >
> > With dup3, we can avoid an extra system call on some machines
> > while holding syscall.ForkLock. Currently we have to
> > syscall.Dup + syscall.CloseOnExec.
> >
> > On machines with Linux and a new enough kernel, this can just
> > be dup3.
>
> No objection to the CL, but I wonder how we can use this.  What we
> need in the net package is really dup+O_CLOEXEC.  Unfortunately dup3
> is dup2+O_CLOEXEC: it requires you to specify the new descriptor
> number.  I don't see how the net package can reliably determine a new
> descriptor number to use.
>

Yeah, I realized that later too. And then I laughed, thinking of Rob's dup4
remark.


> That is to say, what we should use in the net package is not dup3, but
> fcntl(fd.sysfd, F_DUPFD_CLOEXEC), which has been available since
> kernel version 2.6.24.
>

Nice. I'll try that.