[Libguestfs] [nbdkit PATCH 8/8] rate: Atomically set CLOEXEC on fds

[Richard W.M. Jones]

On Wed, Jul 31, 2019 at 05:01:52PM -0500, Eric Blake wrote:
> On 7/31/19 4:31 PM, Eric Blake wrote:
> > The rate filter is potentially opening fds in one thread while another
> > thread is processing a fork() in the plugin.  Although the file is not
> > open for long, we MUST atomically use CLOEXEC to avoid fd leaks.  This
> > one is a bit harder to observe using only the sh plugin, because the
> > window is small; you'll have better success at catching the leak by
> > using gdb or recompiling code to insert strategic sleeps.
> 
> In fact, I have to tweak this commit message: you CAN'T observe this one
> with the sh plugin unless you recompile it to use #define THREAD_MODEL
> NBDKIT_THREAD_MODEL_SERIALIZE_REQUESTS, as well as introducing the
> timing hacks mentioned above (that's because with our current
> SERIALIZE_ALL_REQUESTS, there is never more than one thread in
> filter/plugin code at a time).

The current nbdkit-sh-plugin is only SERIALIZE_ALL_REQUESTS in order
to make writing the shell scripts a bit more sane.  I believe it could
be fully PARALLEL.

(As an aside: Ideally in future we'll allow the thread model to be
specified by the plugin dynamically.  It's one of the things I thought
I had listed in the TODO file - it wasn't there so I've added it now.)

> But it does raise an interesting point - if we hit platforms that are
> unable to support atomic CLOEXEC, one possibility is a patch that forces
> SERIALIZE_ALL_REQUESTS as the maximum parallelism allowed on that
> platform (while remaining at our goal of PARALLEL on more competent
> systems) - once we do that, the lacking systems will be serialized to
> the point that there is no race window where one thread can fork() while
> another is obtaining an fd.

Yup.  But probably better to encourage those platforms to support
atomic CLOEXEC everywhere.

Rich.

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