fix: a full reorder ring keeps the arriving packet and counts the overflow, instead of dropping it silently

A packet that needed a slot and found none was neither held nor delivered: no
counter recorded it and the consumer reserved a hole nothing could ever backfill.
It now commits the gap to free a slot and takes the ordering decision once more
against the sequence that commit left behind, bounded to one retry; past that it
is delivered in order with the counters it skipped as its `lost`, never flagged
late, because a packet ahead of the sequence has no hole to be looked up by. A
duplicate can now replace its slot at a full ring, as hold_packet's own comment
already promised, and the 20 ms default window is a named constant.

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

deps/udpsclient/udps_client.c

@@ -37,6 +37,8 @@
 #define UDPS_REORDER_SLOTS 64
 /** Total bytes the reorder ring may hold, so a fast stream cannot grow it without bound. */
 #define UDPS_REORDER_BYTES (4u * 1024u * 1024u)
+/** How long a gap is given to resolve before it is committed, by default. */
+#define UDPS_REORDER_WINDOW_S_DEFAULT 0.020
 
 /** Concurrent in-flight reassemblies. The server interleaves at most a couple. */
 #define UDPS_MAX_SLOTS 4
@@ -576,20 +578,16 @@ static int deliver_data(udps_client_t *c, const uint8_t *pl, size_t len,
  *
  * A duplicate counter (same datagram arriving twice on the wire) replaces the
  * existing slot so one counter never consumes two slots and both get delivered.
+ * The search for that slot therefore runs *before* the capacity checks, and the
+ * checks are applied only on the path that needs a new slot: a duplicate at a
+ * full ring consumes nothing and must still be accepted.
  * Returns non-zero and holds nothing when there is no room (ring full or byte
- * budget exhausted). */
+ * budget exhausted); `order_data` below says what the caller then does. */
 static int hold_packet(udps_client_t *c, const uint8_t *pl, size_t len,
                        uint32_t counter, double recv_time) {
     int i;
     int slot = -1;
 
-    if (c->held_count >= UDPS_REORDER_SLOTS) {
-        return -1;
-    }
-    if (c->held_bytes + len > UDPS_REORDER_BYTES) {
-        return -1;
-    }
-
     /* Duplicate counter: reuse the existing slot to avoid double delivery. */
     for (i = 0; i < UDPS_REORDER_SLOTS; i++) {
         if (c->held[i].used && c->held[i].counter == counter) {
@@ -597,17 +595,29 @@ static int hold_packet(udps_client_t *c, const uint8_t *pl, size_t len,
             break;
         }
     }
-    if (slot < 0) {
+    if (slot >= 0) {
+        /* The replacement releases the length already held, so the budget is
+         * weighed against what the slot will cost rather than what it adds. */
+        if (c->held_bytes - c->held[slot].len + len > UDPS_REORDER_BYTES) {
+            return -1;
+        }
+    } else {
+        if (c->held_count >= UDPS_REORDER_SLOTS) {
+            return -1;
+        }
+        if (c->held_bytes + len > UDPS_REORDER_BYTES) {
+            return -1;
+        }
         for (i = 0; i < UDPS_REORDER_SLOTS; i++) {
             if (!c->held[i].used) {
                 slot = i;
                 break;
             }
         }
-    }
         if (slot < 0) {
             return -1;
         }
+    }
 
     if (ensure_cap((void **)&c->held[slot].buf, &c->held[slot].cap, len) != 0) {
         return -1;
@@ -717,10 +727,27 @@ static int commit_gap(udps_client_t *c) {
  * milliseconds, and holding is what turns it back into order.  A packet behind the
  * sequence lost that race: the consumer already has a hole where it belongs, so it
  * is delivered flagged rather than discarded, which is the whole point -- a dropped
- * packet cannot fill the hole it left. */
+ * packet cannot fill the hole it left.
+ *
+ * The full ring is specified here, in one place.  A packet that needs a slot and
+ * finds none gives up on the current gap -- `commit_gap()` delivers the lowest held
+ * run and so frees at least one slot -- counts the event in
+ * `stats.reorder_overflows`, and the decision is then taken again, once, against
+ * the sequence that commit left behind.  That re-decision is why this is a bounded
+ * loop and not a straight line: the commit advances `next_counter` to the lowest
+ * held counter, which may put it past the arriving packet, and the packet is then
+ * behind the sequence rather than ahead of it.  Whatever happens, the arriving
+ * packet is held or delivered; it is never dropped, because nothing else records it
+ * and the consumer would reserve a hole for it that nothing could ever backfill.
+ * Past the cap it is delivered *in order* and not flagged late: a packet ahead of
+ * the sequence has no reserved hole, the consumer looks a late packet's hole up by
+ * counter and ignores one it cannot place, so advancing `next_counter` to it and
+ * reporting the counters it skipped as `lost` is what reserves their hole. */
 static int order_data(udps_client_t *c, const uint8_t *pl, size_t len,
                       uint32_t counter, double recv_time) {
     int32_t delta;
+    int     attempt;
+    int     rc;
 
     if (c->cfg.reorder_window_s <= 0.0) {
         /* Resequencing off: byte-for-byte the behaviour before this task. `delta`
@@ -744,28 +771,47 @@ static int order_data(udps_client_t *c, const uint8_t *pl, size_t len,
         c->next_counter = counter + 1u;
         return deliver_data(c, pl, len, counter, recv_time, 0u, 0);
     }
+    for (attempt = 0; attempt < 2; attempt++) {
         delta = (int32_t)(counter - c->next_counter);
         if (delta == 0) {
-        int rc = deliver_data(c, pl, len, counter, recv_time, 0u, 0);
+            rc = deliver_data(c, pl, len, counter, recv_time, 0u, 0);
             c->next_counter = counter + 1u;
             if (rc == 0) { rc = drain_held(c, 0u); }
             return rc;
         }
-    if (delta > 0) {
-        if (hold_packet(c, pl, len, counter, recv_time) != 0) {
-            /* No room: commit the gap now rather than growing. The counter is
-             * visible, so an undersized window is a reading and not a mystery. */
-            return commit_gap(c);
+        if (delta < 0) {
+            /* Behind the sequence. */
+            c->stats.late_delivered++;
+            return deliver_data(c, pl, len, counter, recv_time, 0u, 1);
         }
+        if (hold_packet(c, pl, len, counter, recv_time) == 0) {
             if (c->gap_deadline == 0.0) {
                 c->gap_deadline = recv_time + c->cfg.reorder_window_s;
             }
             return 0;
         }
-    /* delta < 0: behind the sequence. */
+        /* No room: give up on the gap rather than growing the ring, and say so
+         * in a counter, so an undersized window is a reading and not a mystery. */
+        c->stats.reorder_overflows++;
+        rc = commit_gap(c);
+        if (rc != 0) {
+            /* The consumer refused a delivery; nothing is freed and nothing more
+             * may be delivered, so the arriving packet is its caller's problem. */
+            return rc;
+        }
+    }
+    /* The attempt cap is spent and the ring still has no room (the byte budget
+     * cannot fit this one even with a slot free). Deliver it rather than drop it. */
+    delta = (int32_t)(counter - c->next_counter);
+    if (delta < 0) {
         c->stats.late_delivered++;
         return deliver_data(c, pl, len, counter, recv_time, 0u, 1);
     }
+    rc = deliver_data(c, pl, len, counter, recv_time, (uint32_t)delta, 0);
+    c->next_counter = counter + 1u;
+    if (rc == 0) { rc = drain_held(c, 0u); }
+    return rc;
+}
 
 /*---------------------------------------------------------------------------*/
 /*                            Fragment reassembly                            */
@@ -1187,7 +1233,7 @@ void udps_client_config_init(udps_client_config_t *cfg) {
     cfg->keepalive_interval_s = 15.0; /* server evicts silent clients at 30 s */
     cfg->recv_buffer_bytes    = 4u * 1024u * 1024u;
     cfg->max_packet_bytes     = 1u * 1024u * 1024u;
-    cfg->reorder_window_s     = 0.020;
+    cfg->reorder_window_s     = UDPS_REORDER_WINDOW_S_DEFAULT;
 }
 
 static void copy_str(char *dst, size_t cap, const char *src) {

deps/udpsclient/udps_client.h

@@ -202,6 +202,15 @@ typedef struct {
      * before the window existed they were counted in @c stale_packets and dropped.
      */
     uint64_t late_delivered;
+    /**
+     * Times a DATA packet needed a slot in the reorder ring and found none, so the
+     * current gap was committed early to make room. Every such packet is still held
+     * or delivered -- none is dropped -- but the window it was given was shorter
+     * than @c reorder_window_s asked for. A non-zero reading says the ring is
+     * undersized for the stream's reordering depth, which is a reading and not a
+     * mystery; it is the only place that shows up.
+     */
+    uint64_t reorder_overflows;
     uint64_t reconnects;
 } udps_stats_t;
 

docs/claude/build-and-run.md

@@ -63,7 +63,7 @@ handler clears either pointer without knowing which dialog it is. Presenting eit
 app's plain `GtkApplicationWindow` is supported and warning-free; it was checked.
 
 `deps/udpsclient` is a vendored C99 source pair and its diff from the upstream source
-is intentional. Four things were changed:
+is intentional. Five things were changed:
 
 1. **A reorder ring of reassembled payloads.** When a packet arrives with a higher counter
    than expected, the raw datagram is held in a fixed-size ring rather than decoded immediately.
@@ -78,11 +78,22 @@ is intentional. Four things were changed:
    timeout; the reorder deadline is a third call to the same helper, which already understood
    the pattern.
 4. **`reorder_window_s = 0` as the switch.** `udps_client_config_t::reorder_window_s`,
-   defaulted to `0.020` in `udps_client_default_config()` (`deps/udpsclient/udps_client.c:1190`),
+   defaulted to `UDPS_REORDER_WINDOW_S_DEFAULT` (0.020) in `udps_client_config_init()`,
    is the only knob. Setting it to zero disables resequencing and restores the previous
    discard-on-late behaviour exactly — so a reported regression is bisectable without reverting
    the source. **There is no command-line flag for the reorder window.** It is not surfaced on
    udpscope's command line; do not document a `--reorder-window-ms`.
+5. **The full ring never drops the arriving packet, and says so in a counter.** A packet that
+   needs a slot and finds none commits the current gap (which frees at least one, since
+   `commit_gap()` always delivers) and the ordering decision is taken once more against the
+   sequence that commit left behind — a bounded retry, because the commit can advance
+   `next_counter` past the arriving counter and turn an ahead-of-sequence packet into a behind-
+   of-sequence one. Past the retry it is delivered *in order*, with the counters it skipped as
+   its `lost`, and never flagged `late`: a packet ahead of the sequence has no reserved hole and
+   `Acquisition::backfill_from()` silently ignores a late packet it cannot place, so flagging it
+   would be the same silent loss by another route. `stats.reorder_overflows` counts the event,
+   so an undersized ring is a reading rather than a mystery; it is surfaced in the exit dump and
+   the `[reseq-probe]` line as `overflows=`.
 
 `deps/plotter` is upstream's own repository and this app's figure support was added
 there rather than worked around here: `Figure::set_min_y_gutter()` (a floor under the

src/acquisition.cpp

@@ -94,6 +94,7 @@ void Acquisition::run() {
     out.resequenced.store(s.late_resequenced, std::memory_order_relaxed);
     out.late.store(s.late_delivered, std::memory_order_relaxed);
     out.stale.store(s.stale_packets, std::memory_order_relaxed);
+    out.overflows.store(s.reorder_overflows, std::memory_order_relaxed);
     out.reconnects.store(s.reconnects, std::memory_order_relaxed);
     out.connected.store(udps_client_is_connected(client_) != 0, std::memory_order_relaxed);
   }

src/model.hpp

@@ -216,6 +216,10 @@ struct StreamStats {
   std::atomic<std::uint64_t> resequenced{0};
   /// Out-of-order packets delivered behind the sequence, past the window.
   std::atomic<std::uint64_t> late{0};
+  /// Times the client's reorder ring was full when a packet needed a slot, so the gap was
+  /// committed early. Nothing is dropped; the window was just shorter than asked for, and a
+  /// non-zero reading is what says the ring is undersized for the stream.
+  std::atomic<std::uint64_t> overflows{0};
   /// Packets the client still refused outright (a duplicate, or any late packet while
   /// resequencing is disabled). Counted since the beginning and displayed nowhere until
   /// now, which is why an inversion had to be inferred from `packets` and `frames`
@@ -547,6 +551,7 @@ class ScopeModel {
   /// Rebuilds buffers from the current channel list. Caller holds `mutex()`.
   void rebuild_locked();
 
+
  private:
   void rebuild_channels();
   /// Re-appends one synthetic signal per math channel, after the stream's own. Called under the

src/ui.cpp

@@ -10559,12 +10559,14 @@ gboolean reseq_probe(gpointer user) {
     }
   }
   std::fprintf(stderr,
-               "[reseq-probe] reseq=%llu late=%llu stale=%llu holes=%llu hole_slots=%llu "
+               "[reseq-probe] reseq=%llu late=%llu stale=%llu overflows=%llu holes=%llu "
+               "hole_slots=%llu "
                "backfilled=%llu open_holes=%zu bands=%zu bands_repaired=%zu nan_slots=%llu "
                "scanned=%llu trigger_in_hole=%d\n",
                static_cast<unsigned long long>(m.stats().resequenced.load()),
                static_cast<unsigned long long>(m.stats().late.load()),
                static_cast<unsigned long long>(m.stats().stale.load()),
+               static_cast<unsigned long long>(m.stats().overflows.load()),
                static_cast<unsigned long long>(m.stats().holes.load()),
                static_cast<unsigned long long>(m.stats().hole_slots.load()),
                static_cast<unsigned long long>(m.stats().backfilled.load()), open_holes, bands,
@@ -10809,8 +10811,8 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
     const Partition& part = model.partition();
     std::printf("[udpscope] frames=%llu packets=%llu gaps=%llu channels=%zu span=%.3fs "
                 "stride=%llu"
-                " reseq=%llu late=%llu stale=%llu holes=%llu hole_slots=%llu backfilled=%llu"
-                "\n",
+                " reseq=%llu late=%llu stale=%llu overflows=%llu holes=%llu hole_slots=%llu"
+                " backfilled=%llu\n",
                 static_cast<unsigned long long>(model.stats().frames.load()),
                 static_cast<unsigned long long>(model.stats().packets.load()),
                 static_cast<unsigned long long>(model.stats().gaps.load()),
@@ -10819,6 +10821,7 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
                 static_cast<unsigned long long>(model.stats().resequenced.load()),
                 static_cast<unsigned long long>(model.stats().late.load()),
                 static_cast<unsigned long long>(model.stats().stale.load()),
+                static_cast<unsigned long long>(model.stats().overflows.load()),
                 static_cast<unsigned long long>(model.stats().holes.load()),
                 static_cast<unsigned long long>(model.stats().hole_slots.load()),
                 static_cast<unsigned long long>(model.stats().backfilled.load()));