Camera::acquire() is not thread-safe

[Hans de Goede]

Hi All,

Camera::acquire() claims to be thread-safe,
but it relies on PipelineHandler::acquire()
to provide that thread-safety and
PipelineHandler::acquire() allows multiple
acquires of the same pipeline-handler since
a single pipeline-handler may register multiple
cameras.

So that leaves the following possible race:

1. Thread A calls Camera::acquire()
   and passed the isAccessAllowed() check

2. Thread B calls Camera::acquire()
   and passed the isAccessAllowed() check

3. Thread A successfully calls pipe_->acquire()

4. Thread B successfully calls pipe_->acquire()

5. Thread A calls setState(Private::CameraAcquired)

6. Thread B calls setState(Private::CameraAcquired)

And now we have 2 threads succesfully having
acquired the same camera which I believe is
not supposed to be possible.

Regards,

Hans