6850720: Allow POSIX_SPAWN to be used for ProcessImpl on Linux

[David Lloyd]

On Fri, Oct 19, 2018 at 1:56 AM Thomas Stüfe <thomas.stuefe at gmail.com> wrote:
>
> Hi David,
>
> (for convenience, here the old thread:
> http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-September/055333.html)
>
> I would rather not expose a third way to spawn to the end user. As a
> test switch for developers, this seems fine.
>
> AFAIK (and someone please correct me if I am wrong) posix_spawn is not
> the magical third way beside fork and vfork, it is just a wrapper
> around fork/vfork and exec. Especially posix_spawn(), if using fork(),
> will still have the same pitfalls raw fork() has - quasi-random ENOMEM
> if we hit the overcommit heuristic, and inferior performance compared
> to vfork. So you have nothing gained, you have just relegated the
> fork/vfork decision down to a different layer, one which you have
> little control over. And since that fork/vfork decision is kind of
> important, you need that contol.

We've gained the same thing that was gained in [1] and for the same
reason: vfork is deprecated and it is an error to continue to rely
directly on it.  The only non-deprecated means of using vfork is via
posix_spawn.

And, posix_spawn is not the game of roulette that you make it out to
be.  To quote the manual page:

"The child process is created using vfork(2) instead of fork(2) when
either of the following is true:

*  the spawn-flags element of the attributes object pointed to by
attrp contains the GNU-specific flag POSIX_SPAWN_USEVFORK; or

*  file_actions is NULL and the spawn-flags element of the attributes
object  pointed  to  by  attrp  does  not  contain
POSIX_SPAWN_SETSIGMASK,  POSIX_SPAWN_SETSIGDEF,
POSIX_SPAWN_SETSCHEDPARAM, POSIX_SPAWN_SETSCHEDULER,
POSIX_SPAWN_SETPGROUP, or POSIX_SPAWN_RESETIDS.

In other words, vfork(2) is used if the caller requests it, or if
there is no cleanup expected in the child before it exec(3)s the
requested file."

It is true that we aren't explicitly requesting it, however, in our
case file_actions is NULL and attrp is also NULL, so posix_spawn will
use vfork* on Linux.

* Actually neither fork nor vfork use anything but the clone()
syscall.  To quote the sources, "The Linux implementation of
posix_spawn{p} uses the clone syscall directly with CLONE_VM and
CLONE_VFORK flags and an allocated stack."

> Note that glibc has a flag to force always vfork behavior:
> POSIX_SPAWN_USEVFORK.

Sure, but I don't really see this as necessary if glibc is already
following the vfork-like path.  Another thing to know is that at least
in the 2.26 sources I have checked out, the POSIX_SPAWN_USEVFORK flag
is completely ignored.  See also [2].

[1] https://bugs.openjdk.java.net/browse/JDK-8161360
[2] https://github.com/bminor/glibc/commit/ccfb2964726512f6669fea99a43afa714e2e6a80

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- DML