fix(udps): stop packets being dated from an earlier time base

Reported as samples sporadically carrying a previous packet's timestamp:
holes on one side of the stream and collisions on the other, in both the
Go and the MARTe2 receiver. That it appeared in both is what located it
-- the shared cause is upstream of either client. Four independent
defects, all of which end in a packet's values being placed at a time
that is not theirs.

Reassembly slot exhaustion (the "Reassembly slots full; evicting oldest"
flood). Chunk size was learnt only from fragment 0, so an out-of-order
burst destroyed a packet whose bytes had all arrived and left the slot
occupied until the 2 s GC. Slots were keyed on the counter alone, but
DATA and CONFIG number independently, so equal counters merged the two
streams. The 32-byte received-mask covered 256 of the 512 fragments the
client accepts, so a duplicate above 255 was counted as new and the
packet was delivered with a hole of stale bytes in it. And one datagram
was read per Execute(), which cannot drain a fast producer. Fixed with a
pendingTail deferral, (counter, type) keying, a 64-byte mask, a
256-datagram drain, counter-age slot reclamation, and a 1 Hz aggregated
warning in place of the per-eviction flood.

UDPStreamer dropping whole Accumulate batches. EventSem::ResetWait is
Reset-then-Wait, so a Post() landing while the sender thread was inside
ServiceClients()/SendData() was destroyed by the next Reset. The batch
was then skipped with dataReady false, readyFill was never cleared, and
the following flush overwrote it: an entire run of RT cycles never
reached the wire. The record of pending work now lives in the buffers
rather than in the semaphore edge, which also removes up to
UDPS_DATA_WAIT_MS of latency; genuine backpressure overwrites are
counted and reported. Against the unfixed code the new test sees
2999/3000 batches never consumed.

Period inflation after loss. Accumulated scalars carry no SamplingRate,
so the receiver derives dt from the sender-clock gap -- but dividing it
by the previous packet's sample count is only right while nothing is
lost. One loss doubles the reported period, which spreads a batch a full
batch past its own end and into the range the next packet claims. That
is the hole and the collision, exactly. Inferring the cycle count from
the estimate's own period is not a way out: it has a stable fixed point
wherever gap/dt is an integer, so a real rate change locks it at the old
one for good (AccumDtGTest.FollowsSustainedRateChange).

The packet counter removes the ambiguity, so all three receivers now
order on it: a DATA packet that does not advance the counter is dropped
rather than delivered, because its values are older than data already
handed over. Ordering is on the signed difference so it survives the
uint32 wrap, and the sequence resets on reconnect, where the producer's
counter restarts independently of ours. The loss count that falls out of
the same delta feeds the period estimate as cycles = prevN * (1 + lost),
which reduces exactly to gap/prevN when nothing is lost and therefore
still tracks a genuine rate change. UDPSClient::AcceptDataCounter (C++),
udpsprotocol.SequenceGate (Go), decode_data (C).

The C client's existing gap counter was wrap-unsafe and let a stale
packet rewind last_counter, which made every subsequent gap wrong; it
uses the same code now. Docs/Protocol.md gains an Ordering DATA section
stating the requirement for any receiver, including ones outside this
repository.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Common/Client/go/udpsprotocol/protocol.go

@@ -290,28 +290,37 @@ type DataSample struct {
 	HRTTimestamp uint64
 	WallTime     time.Time            // wall-clock time at UDP arrival; used as x-axis
 	Values       map[string][]float64 // key = signal name, value = []float64 with NumElements entries
+	// Lost is the number of DATA packets missing between the previous sample
+	// and this one, taken from the producer's packet counter (see
+	// SequenceGate). Consumers that derive a per-element period from the
+	// inter-packet gap need it: the gap widens with every lost packet, and
+	// dividing it by this packet's element count alone reports a period too
+	// long by exactly that factor — which walks the packet's elements past
+	// their own end and into the range the next packet claims.
+	Lost uint32
 }
 
 // parseElems reads n elements for sig from payload at offset, advancing offset.
 // Returns the slice of float64 values and the new offset.
 func parseElems(payload []byte, offset, n int, sig SignalInfo) ([]float64, int, error) {
+	sz := rawTypeSize(sig.TypeCode)
+	if sig.QuantType != QuantNone {
+		sz = quantSize(sig.QuantType)
+	}
+	// Bounds-check before allocating.  In Accumulate mode n is numSamples ×
+	// NumElements, so a malformed packet could otherwise ask for an allocation
+	// far larger than its own payload could ever justify.
+	if n < 0 || n > (len(payload)-offset)/sz {
+		return nil, offset, fmt.Errorf("data payload truncated for signal %q", sig.Name)
+	}
 	elems := make([]float64, n)
+	needed := n * sz
 	if sig.QuantType == QuantNone {
-		sz := rawTypeSize(sig.TypeCode)
-		needed := n * sz
-		if offset+needed > len(payload) {
-			return nil, offset, fmt.Errorf("data payload truncated for signal %q", sig.Name)
-		}
 		for i := 0; i < n; i++ {
 			elems[i] = readRawElement(payload, offset+i*sz, sig.TypeCode)
 		}
 		offset += needed
 	} else {
-		sz := quantSize(sig.QuantType)
-		needed := n * sz
-		if offset+needed > len(payload) {
-			return nil, offset, fmt.Errorf("data payload truncated (quant) for signal %q", sig.Name)
-		}
 		for i := 0; i < n; i++ {
 			var raw uint16
 			if sz == 1 {
@@ -331,7 +340,13 @@ func parseElems(payload []byte, offset, n int, sig SignalInfo) ([]float64, int,
 //
 // For PublishModeAccumulate the payload format is:
 //
-//	[8 HRT][4 numSamples][for each signal: accumulated scalars → numSamples elems; arrays → NumElements elems]
+//	[8 HRT][4 numSamples][for each signal: numSamples × NumElements elems]
+//
+// Every signal is accumulated, arrays included: the producer captures one full
+// snapshot of the whole signal set per RT cycle and lays the cycles out
+// contiguously per signal (UDPStreamer::SerializeAccumulated).  Reading only
+// NumElements for an array would hand every slot the first cycle's data and
+// slide all later signals into that array's tail.
 //
 // The function returns one DataSample per accumulated snapshot so the hub can
 // process each slot independently with its own timestamp.
@@ -357,28 +372,18 @@ func ParseData(payload []byte, sigs []SignalInfo, publishMode uint8, arrivalTime
 		}
 
 		// Parse per-signal data blocks (all slots for a signal are contiguous).
-		accumVals := make(map[string][]float64, len(sigs)) // scalars: numSamples values
-		fixedVals := make(map[string][]float64, len(sigs)) // arrays: NumElements values
+		accumVals := make(map[string][]float64, len(sigs)) // numSamples × NumElements
+		accumElems := make(map[string]int, len(sigs))
 
 		for _, sig := range sigs {
 			n := sig.NumElements()
-			if n == 1 {
-				// Accumulated scalar: read numSamples back-to-back elements.
-				elems, newOff, err := parseElems(payload, offset, numSamples, sig)
-				if err != nil {
-					return nil, err
-				}
-				offset = newOff
-				accumVals[sig.Name] = elems
-			} else {
-				// Fixed array (non-accumulated): one set of NumElements values.
-				elems, newOff, err := parseElems(payload, offset, n, sig)
-				if err != nil {
-					return nil, err
-				}
-				offset = newOff
-				fixedVals[sig.Name] = elems
+			elems, newOff, err := parseElems(payload, offset, numSamples*n, sig)
+			if err != nil {
+				return nil, err
 			}
+			offset = newOff
+			accumVals[sig.Name] = elems
+			accumElems[sig.Name] = n
 		}
 
 		// Build one DataSample per slot.
@@ -386,10 +391,10 @@ func ParseData(payload []byte, sigs []SignalInfo, publishMode uint8, arrivalTime
 		for k := 0; k < numSamples; k++ {
 			vals := make(map[string][]float64, len(sigs))
 			for sigName, av := range accumVals {
-				vals[sigName] = []float64{av[k]}
-			}
-			for sigName, fv := range fixedVals {
-				vals[sigName] = fv // shared read-only reference; hub does not modify
+				n := accumElems[sigName]
+				// Sub-slice of the decoded block; the hub treats values as
+				// read-only, so no copy is needed.
+				vals[sigName] = av[k*n : (k+1)*n : (k+1)*n]
 			}
 			samples[k] = DataSample{HRTTimestamp: hrt, WallTime: arrivalTime, Values: vals}
 		}