feat: the mock can drop and invert packets, and carries a noisy channel

--drop-every/--drop-burst withhold datagrams the counter and stream clock
have already advanced over, and --reorder-every inverts a pair on the
wire; both are deterministic for the reason --glitch-at is, since a probe
asserts the spacing of what it finds and a random count is not a figure.
A `noise` signal gives the dense-trace and intensity-floor paths a input
that is not piecewise smooth.

verification.md records the measured readings, including the one that is
a finding rather than a baseline: a burst past kGlitchPackets (256) is
read as a producer clock glitch, each bridge advances the anchor by one
packet span against a timeline 300 ahead, and four consecutive failures
then throw the stream onto the arrival clock.

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

.gitignore

@@ -1 +1,2 @@
 build/
+__pycache__/

docs/claude/verification.md

@@ -201,8 +201,8 @@ as distinct from the fault injection below, which exercises ingest:
 - `--quiet-at S` freezes `sine` at its held value from stream second `S` onwards, so it stops
   crossing zero: the dropout trigger's input.
 
-The mock also carries one **fault injection** that no probe schedules, because what it exercises is
+The mock also carries three **fault injections** that no probe schedules, because what they exercise
-the ingest rather than the UI: `--time-glitch-at <stream seconds> --time-glitch-by <seconds>` makes
+is the ingest rather than the UI. The first: `--time-glitch-at <stream seconds> --time-glitch-by <seconds>` makes
 **one** packet's time signal jump ahead and then carries on from where the true timeline was, which
 is what a device whose clock glitches puts on the wire. Run it with the trigger armed, e.g.
 
@@ -228,6 +228,46 @@ jumping 5.040 → 16.040, and two latches **1.2 s** apart. Note the window here
 session for that port; the figures above are the default 2 s window (50 % pre), so move
 `~/.config/udpscope/<host>_<port>.toml` aside to reproduce them.
 
+The other two are **packet loss** (`--drop-every N`, `--drop-burst K`) and **wire inversion**
+(`--reorder-every N`). Both withhold a datagram the counter and the stream clock have already
+advanced over, which is what makes them loss and reordering rather than a slower producer: the
+client orders the stream by the header's counter alone (`udps_client.c:504`), counting a forward
+jump as `counter_gaps` and rejecting anything that does not advance it as `stale_packets`. There is
+**no resequencing and no interpolation** anywhere below the ring — an inverted packet's samples are
+discarded and the hole they leave is a jump in x, which is the behaviour these options are for.
+
+All of the readings below are off `UDPSCOPE_EXIT_DUMP`'s `[udpscope] frames= packets= gaps=` line,
+over an 8 s run against `--rate 10000 --batch 100 --freq 5` (100 packets a second) except where the
+burst needs the faster stream. **`gaps` is the exact reading** and repeats to the packet; `frames`
+and `packets` move by one or two with the run's own timing. Run each of these on its own — four of
+them back to back in one script starved the mock past the 2 s silence timeout and every case,
+including the clean one, then read a reconnect instead of what it was testing:
+
+- **clean**, as the reference: `frames=1029 packets=1030 gaps=0`, no warning.
+- `--drop-every 10`: `frames=925 packets=926 gaps=102`. Every tenth packet never reaches the wire,
+  so `packets` falls with `frames` and `gaps` is the count of them. No warning: a one-packet hole is
+  a one-packet jump in the stamps, far inside the glitch guard.
+- `--reorder-every 10`: `frames=925 packets=1028 gaps=102`. This is the signature to look for —
+  `packets` is the clean figure (every datagram arrived) while `frames` is the dropped one, because
+  the overtaken packet is decoded far enough to be read as stale and then thrown away. One inversion
+  costs one `gaps` (for the packet that overtook it) **and** one `stale_packets`, which udpscope does
+  not currently display; the ~104 missing frames are the only place it shows.
+- `--drop-every 1000 --drop-burst 200` at `--rate 100000 --batch 100` (1000 packets a second):
+  `frames=7000 packets=7001 gaps=1600`, eight bursts of 200, and still no warning.
+- `--drop-every 1000 --drop-burst 300`, same stream: `frames=6203 packets=6204 gaps=2400` **and the
+  guard fires** — `warning: ... jumped implausibly far ahead of the previous packet (1.299..1.29999
+  against a previous 0.99899)`, then `timing samples by arrival instead`. 300 packet spans is past
+  `kGlitchPackets` (256), so real loss is read as a clock glitch; and since each bridge advances the
+  anchor by one packet span while the true timeline is 300 ahead, the next four packets fail too and
+  `kFailureLimit` throws the stream onto the arrival clock. A burst between 256 packets and the 2 s
+  `silence_timeout_s` is the only window in which this is reachable, and it is a finding, not a
+  baseline.
+
+Keep a burst under 2 s of wire time whatever the rate: past that the client reconnects, the mock
+resets its counter and clock on the new CONNECT, and the run reads `gaps=0` with a *backwards* stamp
+warning (`does not advance across packets ... 0..0.0099 against a previous 0.3899`) — the reconnect
+path, not the loss path.
+
 **Run one self-check at a time.** Two concurrent `xvfb-run -a` instances end up on the
 same display and the second gets no GL context — it dumps `layout over 0 frames` and
 `valid=-1`, which looks like a pipeline failure and is not one. `tools/mock_udpstreamer.py`

tools/mock_udpstreamer.py

@@ -21,6 +21,8 @@ import socket
 import struct
 import time
 
+import numpy as np
+
 MAGIC = 0x53504455
 PKT_DATA, PKT_CONFIG, PKT_ACK, PKT_CONNECT, PKT_DISCONNECT = 0, 1, 2, 3, 4
 T_FLOAT64 = 9
@@ -61,6 +63,7 @@ def build_config(rate, time_mode=TIME_FULL_ARRAY, unit="V"):
         signal_desc("sine", rate=rate, time_mode=time_mode, time_idx=0, unit=unit),
         signal_desc("square", rate=rate, time_mode=time_mode, time_idx=0, unit=unit),
         signal_desc("ramp", rate=rate, time_mode=time_mode, time_idx=0, unit=unit),
+        signal_desc("noise", rate=rate, time_mode=time_mode, time_idx=0, unit=unit),
     ]
     payload = struct.pack("<I", len(sigs)) + b"".join(sigs)
     payload += bytes([PUBLISH_ACCUMULATE])
@@ -104,6 +107,9 @@ def build_data(
         1.0 if math.sin(2.0 * math.pi * freq * 0.5 * t) >= 0.0 else -1.0 for t in stamps
     ]
     ramp = [(freq * t) % 1.0 * 2.0 - 1.0 for t in stamps]
+    noisy = np.sin(100 * math.pi * np.array(stamps) + 0.5) + np.random.normal(
+        0, 0.5, batch
+    )
     # A glitch is an *inversion* of the square for `glitch_width` seconds, repeated once per
     # square period (2/freq -- the period of sin(2*pi*freq*0.5*t)). Inverting rather than
     # forcing a level makes it a narrow pulse whatever the square's polarity is, and repeating
@@ -137,7 +143,7 @@ def build_data(
         [wire_stamps[0]] * batch if time_mode == TIME_FIRST_SAMPLE else wire_stamps
     )
     payload = struct.pack("<QI", int(t0 * 1e9), batch)
-    for arr in (wire_time, sine, square, ramp):
+    for arr in (wire_time, sine, square, ramp, noisy):
         payload += struct.pack("<%dd" % batch, *arr)
     return header(PKT_DATA, counter, len(payload)) + payload
 
@@ -234,6 +240,44 @@ def main():
         "opposite side of zero, so --quiet-at is its last zero crossing: the shape a "
         "dropout trigger is read against. -1 = never",
     )
+    ap.add_argument(
+        "--drop-every",
+        type=int,
+        default=0,
+        help="withhold every Nth DATA packet from the wire -- packet loss. Deterministic "
+        "rather than a probability for the reason --glitch-at gives: a probe asserts the "
+        "spacing of the events it finds, and a random count is not a figure. The counter "
+        "and the stream clock advance over a withheld packet, which is the whole point: "
+        "the receiver orders the stream by the counter alone, so that is the only thing "
+        "that tells it a packet existed. 0 = never",
+    )
+    ap.add_argument(
+        "--drop-burst",
+        type=int,
+        default=1,
+        help="consecutive packets each --drop-every event withholds: one models a lost "
+        "datagram, a long run models a link that went away. The run length is what the "
+        "ingest's own glitch guard is sized against, since it bridges a packet whose "
+        "stamps jump more than 256 packet spans ahead. Measured at 1000 packets a second, "
+        "a burst of 200 is counted as loss and nothing else; one of 300 is read as a clock "
+        "glitch instead, and because each bridge advances the anchor by one packet span "
+        "while the true timeline is 300 ahead, the four following packets fail too and the "
+        "stream falls back to arrival timing. Keep a burst under the 2 s silence timeout "
+        "or the client reconnects and the mock restarts its clock, which is a different "
+        "test",
+    )
+    ap.add_argument(
+        "--reorder-every",
+        type=int,
+        default=0,
+        help="hold every Nth packet back and send it after the next one that goes out -- "
+        "an inversion on the wire. The receiver takes the later counter first, so the held "
+        "packet no longer advances the counter and is stale by definition: one inversion "
+        "costs one counter gap, one stale packet, and that packet's samples for good. A "
+        "--drop-every that swallows the packet after a held one deepens the inversion "
+        "rather than cancelling it, since the held packet waits for the next packet "
+        "actually sent. 0 = never",
+    )
     args = ap.parse_args()
 
     dt = args.stream_scale / args.rate  # stream seconds per sample
@@ -265,6 +309,12 @@ def main():
     t_stream = 0.0
     started = None
     next_send = time.monotonic()
+    held = None  # (counter, datagram) built but withheld, for --reorder-every
+    drop_left = 0
+    dropped = 0
+    inverted = 0
+    noted_drop = False
+    noted_reorder = False
 
     while True:
         try:
@@ -278,6 +328,8 @@ def main():
                     t_stream = 0.0
                     started = time.monotonic()
                     next_send = started
+                    held = None
+                    drop_left = 0
                     sock.sendto(build_config(declared, time_mode, args.unit), peer)
                     print(
                         "mock udpstreamer: CONNECT from %s:%d, CONFIG sent" % peer,
@@ -306,7 +358,11 @@ def main():
         if args.clock_jitter > 0.0:
             pdt = dt * (1.0 + random.uniform(-args.clock_jitter, args.clock_jitter))
         spike = 0.0
-        if args.time_glitch_at >= 0.0 and not glitched and t_stream >= args.time_glitch_at:
+        if (
+            args.time_glitch_at >= 0.0
+            and not glitched
+            and t_stream >= args.time_glitch_at
+        ):
             spike = args.time_glitch_by
             glitched = True
             print(
@@ -314,8 +370,7 @@ def main():
                 % (t_stream, spike),
                 flush=True,
             )
-        sock.sendto(
+        pkt = build_data(
-            build_data(
             counter,
             t_stream,
             pdt,
@@ -327,9 +382,46 @@ def main():
             args.glitch_at,
             args.glitch_width,
             args.quiet_at,
-            ),
-            peer,
         )
+        burst = max(1, args.drop_burst)
+        if args.drop_every > 0 and drop_left == 0 and counter % args.drop_every == 0:
+            drop_left = burst
+        if drop_left > 0:
+            drop_left -= 1
+            dropped += 1
+            if not noted_drop:
+                noted_drop = True
+                print(
+                    "mock udpstreamer: dropping packet %d and the %d after it"
+                    % (counter, burst - 1),
+                    flush=True,
+                )
+        elif (
+            args.reorder_every > 0
+            and held is None
+            and counter % args.reorder_every == 0
+        ):
+            held = (counter, pkt)
+        else:
+            sock.sendto(pkt, peer)
+            if held is not None:
+                # Out of order on the wire now: the receiver has already taken this
+                # packet's counter, so the held one no longer advances it and is stale by
+                # definition -- one counter gap for the packet that overtook it, one stale
+                # packet for the one that lost, and the loser's samples are gone.
+                sock.sendto(held[1], peer)
+                inverted += 1
+                if not noted_reorder:
+                    noted_reorder = True
+                    print(
+                        "mock udpstreamer: sent packet %d before %d"
+                        % (counter, held[0]),
+                        flush=True,
+                    )
+                held = None
+        # The counter and the stream clock advance whatever became of the datagram. That
+        # is what makes a withheld packet a *lost* one and not a slower producer: nothing
+        # in a payload says which packet it is, only the header's counter does.
         counter += 1
         t_stream += args.batch * pdt
         next_send += period
@@ -338,7 +430,11 @@ def main():
             and started is not None
             and now - started > args.duration
         ):
-            print("mock udpstreamer: done (%d packets)" % counter, flush=True)
+            print(
+                "mock udpstreamer: done (%d packets, %d dropped, %d inverted)"
+                % (counter, dropped, inverted),
+                flush=True,
+            )
             return