Picoscope: use offset into driver buffer

This changes the streaming callback and process bulk to also take into
account the data offset in the buffer that the driver is writing the
data into.
Also since the callback is executed on the driver thread, it uses
atomics to exchange the indices.

While this works for now, the picoscope programmer's guide instructs to
copy the data out of the buffer inside of the callback, which suggests
that the data might be overwritten once the callback returns (and
potentially before the processBulk method has actually copied the data).
This should be investigated further.

Signed-off-by: Alexander Krimm <A.Krimm@gsi.de>

blocklib/picoscope/Picoscope.hpp

@@ -255,7 +255,8 @@ class Picoscope : public PicoscopeBlockingHelper<TPSImpl, acquisitionMode == Acq
 
 private:
     std::mutex                   g_init_mutex;
-    std::size_t                  streamingSamples = 0Z;
+    std::atomic<std::size_t>     streamingSamples = 0Z;
+    std::atomic<std::size_t>     streamingOffset  = 0Z;
     std::queue<gr::property_map> timingMessages;
 
 public:
@@ -465,12 +466,13 @@ class Picoscope : public PicoscopeBlockingHelper<TPSImpl, acquisitionMode == Acq
         ps_state.produced_worker += availableSamples;
     }
 
-    void streamingCallback(int32_t nrSamplesSigned, uint32_t, int16_t overflow)
+    void streamingCallback(int32_t nrSamplesSigned, uint32_t idx, int16_t overflow)
     requires(acquisitionMode == AcquisitionMode::Streaming)
     {
         streamingSamples = static_cast<std::size_t>(nrSamplesSigned);
+        streamingOffset  = static_cast<std::size_t>(idx);
+        // NOTE: according to the programmer's guide the data should be copied-out inside the callback function. Not sure if the driver might overwrite the buffer with new data after the callback has returned
         assert(nrSamplesSigned >= 0);
-        const auto nrSamples = static_cast<std::size_t>(nrSamplesSigned);
 
         // According to well-informed sources, the driver indicates the buffer overrun by setting all the bits of the overflow argument to true.
         if (static_cast<uint16_t>(overflow) == 0xffff) {
@@ -491,16 +493,16 @@ class Picoscope : public PicoscopeBlockingHelper<TPSImpl, acquisitionMode == Acq
     template<gr::OutputSpanLike TOutSpan>
     gr::work::Status processBulk(std::span<TOutSpan>& outputs) {
         if constexpr (acquisitionMode == AcquisitionMode::Streaming) {
-            if (const auto ec = self().driver_poll()) {
-                this->emitErrorMessage("PicoscopeError", ec.message());
-                return gr::work::Status::ERROR;
-            }
             if (streamingSamples == 0) {
                 for (auto& output : outputs) {
                     output.publish(0);
                 }
+                if (const auto ec = self().driver_poll()) {
+                    this->emitErrorMessage("PicoscopeError", ec.message());
+                    return gr::work::Status::ERROR;
+                }
             } else {
-                processDriverData(streamingSamples, 0, outputs);
+                processDriverData(streamingSamples, streamingOffset, outputs);
                 streamingSamples = 0;
             }
         } else {