Re: AOO plugin for PD on SGI Irix

I would not expect #3 to follow from #1. #1 just means I have to try a few times due to system architecture, but a scoped_shared_lock is still a reader lock. So they should work simultaneously, you should not have to wait for a scoped_shared_lock to release the lock before the shared_lock(try_to_lock) can obtain a reader lock as long as there is no writer requesting a lock at the same time. 
Whether it's a bug or not, it does not behave as I would expect it here. 

write_guard (always non-blocking):
 
std::atomic<uint32_t> update_gen_{0}; // seqlock generation: even=idle, odd=write in progress

struct update_write_guard {
    std::atomic<uint32_t>& gen_;
    explicit update_write_guard(std::atomic<uint32_t>& g) : gen_(g) {
        g.fetch_add(1, std::memory_order_release);
    }
    ~update_write_guard() {
        gen_.fetch_add(1, std::memory_order_release);
    }
    update_write_guard(const update_write_guard&) = delete;
    update_write_guard& operator=(const update_write_guard&) = delete;
};

Before every write lock call for update_mutex_:
            update_write_guard uwg(update_gen_);
            scoped_lock lock(update_mutex_); // writer lock!

Replace reader lock try_to_lock in process() of source.cpp with this:
    auto gen_before = update_gen_.load(std::memory_order_acquire);
    if (gen_before & 1) {
        // a write is currently in progress
        LOG_DEBUG("AooSource: process blocked by active writer");
        add_xrun(nsamples);
        return kAooErrorIdle;
    }
    // we are good to go now!
    // do processing stuff here....

    // done with processing stuff.
    // Verify no write occurred while audio was being processed.
    if (update_gen_.load(std::memory_order_acquire) != gen_before) {
        LOG_DEBUG("AooSource: write detected during audio processing, xrun");
        add_xrun(nsamples);
        return kAooErrorIdle;
    }
    return kAooOk;

Similar in sink.cpp. 

This assumes that typically there's generally low probability of writer lock activity going on during process() execution.
Works well so far (at least for PD objects).
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