char: remove watch callback on chardev detach from frontend
If a frontend device releases the chardev (via unplug), the chr handlers are set to NULL via qdev's exit callbacks invoking qemu_chr_add_handlers(). If the chardev had a pending operation, a callback will be invoked, which will try to access data in the just-released frontend, causing a segfault. Ensure the callbacks are disabled when frontends release chardevs. This was seen when a virtio-serial port was unplugged when heavy guest->host IO was in progress (causing a callback to be registered). In the window in which the throttling was active, unplugging ports caused a qemu segfault. https://bugzilla.redhat.com/show_bug.cgi?id=985205 CC: <qemu-stable@nongnu.org> Reported-by: NSibiao Luo <sluo@redhat.com> Reviewed-by: NGerd Hoffmann <kraxel@redhat.com> Signed-off-by: NAmit Shah <amit.shah@redhat.com>
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