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

[Eric Blake]

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).

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.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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