v1.6.0: ATR-hunt deadlock + idle-loop trap fixes

Add hunt-side live ETU recovery so a card already at a non-default F/D
(e.g. Samsung F=512/D=32 -> 16 CLK/bit) can be decoded without a 372-rate
ATR, and bound the all-0xFF hunt so an idle/unsyncable line can never loop
forever.

- _recover_etu_in_hunt(): measure the live ETU from DATA edges while still
  in FIND START, then re-derive clock_skip; reuses _confirm_etu validation.
- _atr_hunt_failure_action(): pure recover/retry/resume_data decision, with
  ATR_HUNT_ETU_ATTEMPTS=3 / ATR_HUNT_IDLE_RESETS=3 / ATR_HUNT_FAIL_LIMIT=12.
- Persistent counters reset only on a successful ATR parse (not RST re-arm),
  so RST-cycling phones cannot starve the escape.
- 8 new unit tests (55 total).

Validated on the full trace suite (test_8 56/0/OK, test_7 65/0/OK, xiaomi
640, xiaomi_cold 473, 4gmodem 327, A55 407/395, S21p 399/375, target S21p
369; 0 CONCAT, RESULT: OK) with no regressions.

AGENTS.md

@@ -255,6 +255,55 @@ mis-frame at the source) is the next decoder step; until then the gate
 makes the corruption visible in live and offline captures instead of
 silently producing wrong APDUs.
 
+### v1.6.0: ATR-hunt deadlock + idle-loop trap fixes
+
+A live Samsung cold-boot capture that never emits a 372-rate ATR exposed two
+defects in the ATR hunt (`state == 'FIND START'`):
+
+1. **Deadlock.** `RST low at start` arms the hunt at the spec-default
+   `clock_skip=372` with `bit_samples=None`.  The ETU-recovery routine
+   `_measure_etu()` is only reachable from the `DATA` state, which the hunt
+   can only enter after reading a valid `TS=0x3B/0x3F` (or a plausible
+   CLA/INS header) **at 372**.  A card already running at a non-default F/D
+   (Samsung `F=512/D=32` -> 16 CLK/bit) produces none, so the hunt reads
+   idle `0xFF` forever and never reaches the ETU measurement -- a circular
+   dependency.  This is why Samsung (non-default F/D) fails where
+   default-F/D devices succeed: not clock rate, but `F/D`.
+
+2. **Trap.** The all-`0xFF` branch unconditionally reset `_atr_hunt_count=0`
+   and returned to `FIND START`, so the bounded fall-through to `DATA` was
+   unreachable.  With RST re-arm also resetting the counter, the hunt could
+   spin forever.
+
+Fixes:
+
+- **Hunt-side ETU recovery.** `_recover_etu_in_hunt()` runs `_measure_etu()`
+  + `_derive_clock_skip_from_etu()` from the hunt after the default-rate
+  hunt has already failed a few rounds, so a fast device is read at its
+  true rate.  It reuses the DATA-path `_confirm_etu()` validation, so a
+  noise-locked ETU self-clears after 2 parity failures.  No-op for normal
+  ATRs (parsed on hunt 1).
+- **Bounded escape.** `_atr_hunt_failure_action(byte)` (pure, unit-tested)
+  returns `recover` / `retry` / `resume_data`.  ETU recovery every 2 failed
+  rounds up to 3 times; then a hard bound (`_atr_fail_total >= 12`) or the
+  all-`0xFF` idle cap (3) falls through to the existing "resuming DATA
+  without synthetic ATR" path (clearing `bit_samples` so the DATA path
+  re-measures).
+- **Persistent counters.** `_atr_fail_total`, `_hunt_etu_attempts`,
+  `_atr_idle_resets` are reset only on a successful ATR parse, **not** on
+  RST re-arm -- otherwise a phone that power-cycles RST while never sending
+  a valid ATR would starve the escape.  Constants: `ATR_HUNT_ETU_ATTEMPTS=3`,
+  `ATR_HUNT_IDLE_RESETS=3`, `ATR_HUNT_FAIL_LIMIT=12`.
+
+Verification: no regression on the full trace suite (test_8 56/0/OK,
+test_7 65/0/OK, xiaomi 640, xiaomi_cold 473, 4gmodem 327, A55 407/395,
+S21p 399/375, target S21p_coldboot 369, all `RESULT: OK`, 0 CONCAT);
+54/54 unit tests (7 new hunt-escape regression tests).  VERSION -> 1.6.0.
+
+Known limitation: no on-disk fixture reproduces the exact live failure (all
+real captures contain a 372 ATR), so end-to-end proof needs a re-captured
+`.sr` from the failing phone; the unit tests cover the loop-escape logic.
+
 ### v1.5.0: invalid-INS one-byte re-frame (residual CONCATs → 0)
 
 The last CONCAT family (4gmodem/A55_sim2/S21p_sim2) is now **reframed at

pd.py

@@ -28,7 +28,7 @@ from .gsmtap_stream import (GsmtapStreamSender,
     GSMTAP_SIM_RST_EVENT, GSMTAP_SIM_VCC_EVENT,
     GSMTAP_FLAG_BAD_FCS)
 
-VERSION = '1.5.0'
+VERSION = '1.6.0'
 
 
 
@@ -276,6 +276,15 @@ MAX_TPDU_LEN = 271  # ISO 7816-3 T=0: header(5) + proc + 255 + SW(2) < 271
 IDLE_RESYNC_ETU = 4  # idle bit-periods marking an exchange boundary
 MAX_RESYNC_ATTEMPTS = 1024  # bail out if no idle gap found in that many tries
 
+# ATR-hunt failure bounds.  When the default-rate (372 CLK/bit) hunt keeps
+# failing, the card is either idle or already running at a non-default F/D
+# (e.g. Samsung F=512/D=32 -> 16 CLK/bit).  Without a bound the hunt reads
+# idle 0xFF forever at the wrong rate and can never reach the DATA state,
+# where the live ETU is measured -- a deadlock.  These cap the retries.
+ATR_HUNT_ETU_ATTEMPTS = 3   # live-ETU recovery attempts before resuming DATA
+ATR_HUNT_IDLE_RESETS = 3    # all-0xFF hunt resets before resuming DATA
+ATR_HUNT_FAIL_LIMIT = 12    # total failed hunt rounds before resuming DATA
+
 
 class Decoder(srd.Decoder):
     api_version = 3
@@ -482,6 +491,15 @@ class Decoder(srd.Decoder):
         # ever present) eventually falls back to T=0 parsing instead of
         # looping forever.
         self._atr_hunt_count = 0
+        # Persistent hunt-failure accounting (reset only when an ATR is
+        # parsed, NOT on RST re-arm): a device that power-cycles RST while
+        # never sending a valid ATR would otherwise starve the escape below.
+        # _atr_fail_total counts every failed hunt round across activations;
+        # _hunt_etu_attempts caps live-ETU recovery; _atr_idle_resets caps the
+        # all-0xFF reset loop.
+        self._atr_fail_total = 0
+        self._hunt_etu_attempts = 0
+        self._atr_idle_resets = 0
         # PPS is only attempted once after an ATR/warm reset.  Prevents
         # idle-line 0xFF bytes from being parsed as an endless stream of
         # PPS requests when the card does not actually perform PPS.
@@ -946,6 +964,57 @@ class Decoder(srd.Decoder):
         self._edge_read = True
         self.log("etu recovered (samples):", self.bit_samples)
 
+    def _recover_etu_in_hunt(self):
+        '''Recover the live bit period from the DATA line while still in the
+        ATR hunt.
+
+        The hunt reads at the spec-default 372 CLK/bit.  A device already
+        running at a non-default F/D (Samsung F=512/D=32 -> 16 CLK/bit) or a
+        mid-session capture with no ATR sends no 372-rate bytes, so every
+        candidate reads as idle 0xFF and the hunt can never reach the DATA
+        state -- where _measure_etu() normally lives.  Measure the ETU here
+        (after the default-rate hunt has already failed a few rounds) and
+        re-derive clock_skip so subsequent hunt reads use the true rate.
+        Reuses the DATA-path measurement and its _confirm_etu() validation.'''
+        if self._samples_per_clock is None:
+            self._measure_clock_period()
+        self._measure_etu()
+        self._derive_clock_skip_from_etu()
+        # Start a fresh default-rate counting window at the recovered rate.
+        self._atr_hunt_count = 0
+        self.log("hunt ETU recovery attempt", self._hunt_etu_attempts,
+                 "bit_samples", self.bit_samples,
+                 "clock_skip", self.clock_skip)
+
+    def _atr_hunt_failure_action(self, byte):
+        '''Decide what to do after a failed ATR hunt round.
+
+        Returns one of:
+          'recover'     -- stop and recover the live ETU, then re-hunt
+          'retry'       -- go back to FIND START (bounded)
+          'resume_data' -- give up on the ATR and resume T=0 framing
+
+        The counters are persistent across RST re-arms (reset only when an
+        ATR is parsed) so a phone that cycles RST while never emitting a
+        valid ATR cannot starve the escape.  Pure function of state + byte
+        so it is unit-testable without libsigrokdecode.'''
+        self._atr_fail_total += 1
+        # Live-ETU recovery every 2 failures, up to ATR_HUNT_ETU_ATTEMPTS.
+        if (self.bit_samples is None
+                and self._hunt_etu_attempts < ATR_HUNT_ETU_ATTEMPTS
+                and self._atr_fail_total >=
+                    2 * (self._hunt_etu_attempts + 1)):
+            self._hunt_etu_attempts += 1
+            return 'recover'
+        # Hard bound: never hunt forever on an idle/unsyncable line.
+        if self._atr_fail_total >= ATR_HUNT_FAIL_LIMIT:
+            return 'resume_data'
+        if byte == 0xFF and self._atr_idle_resets >= ATR_HUNT_IDLE_RESETS:
+            return 'resume_data'
+        if self._atr_hunt_count < 8:
+            return 'retry'
+        return 'resume_data'
+
     def _confirm_etu(self, parity_ok):
         '''Validate ETU measurement after _measure_etu.  Call after each byte
         decode in _read_byte_edges.  After 3 successful decodes the
@@ -1564,24 +1633,40 @@ class Decoder(srd.Decoder):
             ts_attempts += 1
         if (byte not in (0x3b, 0x3f)):
             self._atr_hunt_count += 1
-            # No valid ATR found this round -- go back to hunting for the
-            # next falling edge.  Never fabricate a synthetic ATR: a stray
-            # 0x3B/0x3F byte inside command traffic (e.g. the FID '3F00' of a
-            # SELECT) would otherwise be mis-committed as a fake 2-byte ATR
-            # and destroy the surrounding command.  The hunt is bounded by
-            # _atr_hunt_count only to avoid logging forever on a dead line.
-            if (self._atr_hunt_count < 8):
-                self.log("No valid TS byte, back to FIND START (hunt {count})".format(count=self._atr_hunt_count))
+            # No valid ATR found this round.  Never fabricate a synthetic
+            # ATR: a stray 0x3B/0x3F byte inside command traffic (e.g. the
+            # FID '3F00' of a SELECT) would otherwise be mis-committed as a
+            # fake 2-byte ATR and destroy the surrounding command.
+            action = self._atr_hunt_failure_action(byte)
+            if action == 'recover':
+                # Default-rate hunt has failed a few rounds: the card may be
+                # idle or already at a non-default F/D.  Measure the live ETU
+                # and re-hunt at that rate instead of looping on idle 0xFF.
+                self.log("ATR hunt failing; recovering live ETU")
+                self._recover_etu_in_hunt()
                 self.state = 'FIND START'
                 return
+            if action == 'retry':
                 if byte == 0xFF:
-                self.log("line appears idle (all 0xFF), resetting ATR hunt")
+                    self._atr_idle_resets += 1
+                    self.log("line appears idle (all 0xFF), resetting ATR "
+                             "hunt (#{n})".format(n=self._atr_idle_resets))
                     self._atr_hunt_count = 0
+                else:
+                    self.log("No valid TS byte, back to FIND START "
+                             "(hunt {count})".format(count=self._atr_hunt_count))
                 self.state = 'FIND START'
                 return
             # Give up hunting for an ATR here without emitting anything;
             # resume T=0 framing from the next byte instead of inventing one.
-            self.log("No valid ATR after {count} hunts; resuming DATA without synthetic ATR".format(count=self._atr_hunt_count))
+            # Clear bit_samples so the DATA path measures the ETU fresh.
+            self.log("No valid ATR after {count} hunts (fails={fails}); "
+                     "resuming DATA without synthetic ATR".format(
+                         count=self._atr_hunt_count,
+                         fails=self._atr_fail_total))
+            self.bit_samples = None
+            self._etu_confirm_count = 0
+            self._etu_fail_count = 0
             self.hasT0 = True
             self.hasT1 = False
             self.state = 'DATA'
@@ -1595,6 +1680,9 @@ class Decoder(srd.Decoder):
             return
         else:
             self._atr_hunt_count = 0
+            self._atr_fail_total = 0
+            self._hunt_etu_attempts = 0
+            self._atr_idle_resets = 0
         self.ATR.append(byte)
         # TS=0x3F means inverse convention: logic 1 = LOW, logic 0 = HIGH.
         # All subsequent byte reads must invert bits.

tests/test_gsmtap.py

@@ -248,6 +248,93 @@ class TestDeglitchEdges(unittest.TestCase):
         self.assertEqual(result, [(0, 0), (100, 0)])
 
 
+class TestAtrHuntEscape(unittest.TestCase):
+    """Regression: the ATR hunt must never loop forever on an idle/unsyncable
+    line, and must recover the live ETU when the card is already at a
+    non-default F/D (Samsung 16 CLK/bit) with no 372-rate ATR coming."""
+
+    def setUp(self):
+        _, pd = _load_iso7816()
+        self.pd = pd
+        self.inst = object.__new__(pd.Decoder)
+        self.inst.bit_samples = None
+        self.inst._atr_hunt_count = 0
+        self.inst._atr_fail_total = 0
+        self.inst._hunt_etu_attempts = 0
+        self.inst._atr_idle_resets = 0
+
+    def _hunt_round(self, byte):
+        """Mirror handle_atr's bookkeeping around _atr_hunt_failure_action."""
+        self.inst._atr_hunt_count += 1
+        action = self.inst._atr_hunt_failure_action(byte)
+        if action == 'recover':
+            self.inst._atr_hunt_count = 0
+        elif action == 'retry' and byte == 0xFF:
+            self.inst._atr_idle_resets += 1
+            self.inst._atr_hunt_count = 0
+        return action
+
+    def test_retry_then_recover_after_two_failures(self):
+        self.assertEqual(self._hunt_round(0x00), 'retry')
+        self.assertEqual(self._hunt_round(0x00), 'recover')
+        self.assertEqual(self.inst._hunt_etu_attempts, 1)
+
+    def test_recovery_attempts_capped_then_hard_limit(self):
+        actions = [self._hunt_round(0x00) for _ in range(200)]
+        actions = actions[:actions.index('resume_data') + 1]
+        self.assertEqual(actions.count('recover'),
+                         self.pd.ATR_HUNT_ETU_ATTEMPTS)
+        self.assertEqual(actions[-1], 'resume_data')
+
+    def test_all_ff_idle_loop_is_bounded(self):
+        seen = []
+        for _ in range(200):
+            seen.append(self._hunt_round(0xFF))
+            if seen[-1] == 'resume_data':
+                break
+        self.assertEqual(seen[-1], 'resume_data')
+        self.assertLess(len(seen), 20)
+
+    def test_idle_ff_retries_before_cap(self):
+        self.inst.bit_samples = 100  # ETU known -> recovery disabled
+        self.inst._hunt_etu_attempts = self.pd.ATR_HUNT_ETU_ATTEMPTS
+        self.inst._atr_idle_resets = self.pd.ATR_HUNT_IDLE_RESETS - 1
+        self.assertEqual(self.inst._atr_hunt_failure_action(0xFF), 'retry')
+
+    def test_idle_resets_capped_then_resume(self):
+        self.inst.bit_samples = 100
+        self.inst._hunt_etu_attempts = self.pd.ATR_HUNT_ETU_ATTEMPTS
+        self.inst._atr_idle_resets = self.pd.ATR_HUNT_IDLE_RESETS
+        self.assertEqual(self.inst._atr_hunt_failure_action(0xFF),
+                         'resume_data')
+
+    def test_non_ff_resumes_when_count_high(self):
+        self.inst.bit_samples = 100
+        self.inst._hunt_etu_attempts = self.pd.ATR_HUNT_ETU_ATTEMPTS
+        self.inst._atr_hunt_count = 8
+        self.assertEqual(self.inst._atr_hunt_failure_action(0x00),
+                         'resume_data')
+
+    def test_no_recovery_when_etu_known(self):
+        self.inst.bit_samples = 100
+        for _ in range(6):
+            self.assertEqual(self._hunt_round(0x00), 'retry')
+        self.assertEqual(self.inst._hunt_etu_attempts, 0)
+
+    def test_recover_etu_in_hunt_runs_measurement(self):
+        calls = []
+        self.inst._samples_per_clock = 4.0  # skip CLK-period measurement
+        self.inst._measure_etu = lambda: calls.append('measure')
+        self.inst._derive_clock_skip_from_etu = lambda: calls.append('derive')
+        self.inst._measure_clock_period = lambda: calls.append('period')
+        self.inst.log = lambda *a, **k: None
+        self.inst.clock_skip = 372
+        self.inst._atr_hunt_count = 5
+        self.inst._recover_etu_in_hunt()
+        self.assertEqual(calls, ['measure', 'derive'])
+        self.assertEqual(self.inst._atr_hunt_count, 0)
+
+
 class TestEmbeddedExchanges(unittest.TestCase):
     """CONCAT detection: structurally valid packets that embed complete T=0
     exchanges (a byte-drop mis-frame swallowed several real exchanges into