fix(prober): consolidate ip route get to single call + log bare excepts

_route_info() calls _ip_route_get once and returns (on_link, iface);
worker._ipv6_leak_phase now calls it instead of the two separate helpers.
Bare except clauses at _ip_route_get and response parse now log at debug.

decnet/prober/ipv6_leak.py

@@ -35,26 +35,37 @@ def _ip_route_get(attacker_v4: str) -> str:
             capture_output=True, text=True, timeout=2,
         )
         return out.stdout
-    except Exception:
+    except Exception as exc:
+        _log.debug("ipv6_leak: ip route get failed for %s: %s", attacker_v4, exc)
         return ""
 
 
+def _route_info(attacker_v4: str) -> tuple[bool, str | None]:
+    """Return (on_link, iface) with a single `ip route get` invocation.
+
+    on_link is True when there is no intermediate gateway ("via" absent).
+    iface is the local interface name, or None if not parseable.
+    """
+    stdout = _ip_route_get(attacker_v4)
+    parts = stdout.split()
+    on_link = "via" not in parts
+    iface: str | None = None
+    if "dev" in parts:
+        idx = parts.index("dev")
+        iface = parts[idx + 1] if idx + 1 < len(parts) else None
+    return on_link, iface
+
+
 def _resolve_iface_for_ip(attacker_v4: str) -> str | None:
     """Return the local interface name that would route to attacker_v4."""
-    stdout = _ip_route_get(attacker_v4)
-    parts = stdout.split()
-    if "dev" in parts:
-        idx = parts.index("dev")
-        return parts[idx + 1] if idx + 1 < len(parts) else None
-    return None
+    _, iface = _route_info(attacker_v4)
+    return iface
 
 
 def _is_on_link(attacker_v4: str) -> bool:
-    """Return True only when the attacker is directly reachable on L2.
-
-    Checks that `ip route get` shows no intermediate gateway (no "via").
-    """
-    return "via" not in _ip_route_get(attacker_v4)
+    """Return True only when the attacker is directly reachable on L2."""
+    on_link, _ = _route_info(attacker_v4)
+    return on_link
 
 
 def solicit_ipv6_leak(
@@ -100,7 +111,8 @@ def solicit_ipv6_leak(
 
     try:
         src_addr: str = resp[IPv6].src
-    except Exception:
+    except Exception as exc:
+        _log.debug("ipv6_leak: response parse failed on %s: %s", attacker_v4, exc)
         return None
 
     if not _is_link_local(src_addr):

decnet/prober/worker.py

@@ -402,13 +402,12 @@ def _ipv6_leak_phase(
         return
     done.add(0)
 
-    from decnet.prober.ipv6_leak import _is_on_link, _resolve_iface_for_ip, solicit_ipv6_leak
+    from decnet.prober.ipv6_leak import _route_info, solicit_ipv6_leak
 
-    if not _is_on_link(ip):
+    on_link, iface = _route_info(ip)
+    if not on_link:
         logger.debug("prober: ipv6_leak: %s is not on-link — skip active probe", ip)
         return
-
-    iface = _resolve_iface_for_ip(ip)
     if iface is None:
         logger.debug("prober: ipv6_leak: cannot determine iface for %s", ip)
         return

tests/prober/test_ipv6_leak.py

@@ -6,6 +6,9 @@ no sniff threads.  Validates:
 - Phase skips on second call (dedup via ip_probed sentinel).
 - Phase emits log + publish_fn when solicit_ipv6_leak returns evidence.
 - Phase is silent when solicit_ipv6_leak returns None.
+- _route_info calls _ip_route_get exactly once per invocation.
+- _ip_route_get subprocess failure is logged at debug.
+- solicit_ipv6_leak response-parse failure is logged at debug.
 """
 from __future__ import annotations
 
@@ -46,8 +49,7 @@ _EVIDENCE = {
 def test_phase_skips_when_not_on_link() -> None:
     published: list[Any] = []
     with (
-        patch("decnet.prober.ipv6_leak._is_on_link", return_value=False),
-        patch("decnet.prober.ipv6_leak._resolve_iface_for_ip", return_value="eth0"),
+        patch("decnet.prober.ipv6_leak._route_info", return_value=(False, "eth0")),
         patch("decnet.prober.ipv6_leak.solicit_ipv6_leak", return_value=_EVIDENCE) as mock_sol,
     ):
         _phase(publish_fn=lambda k, p: published.append((k, p)))
@@ -58,8 +60,7 @@ def test_phase_skips_when_not_on_link() -> None:
 def test_phase_skips_when_no_iface() -> None:
     published: list[Any] = []
     with (
-        patch("decnet.prober.ipv6_leak._is_on_link", return_value=True),
-        patch("decnet.prober.ipv6_leak._resolve_iface_for_ip", return_value=None),
+        patch("decnet.prober.ipv6_leak._route_info", return_value=(True, None)),
         patch("decnet.prober.ipv6_leak.solicit_ipv6_leak", return_value=_EVIDENCE) as mock_sol,
     ):
         _phase(publish_fn=lambda k, p: published.append((k, p)))
@@ -70,8 +71,7 @@ def test_phase_skips_when_no_iface() -> None:
 def test_phase_emits_on_evidence() -> None:
     published: list[Any] = []
     with (
-        patch("decnet.prober.ipv6_leak._is_on_link", return_value=True),
-        patch("decnet.prober.ipv6_leak._resolve_iface_for_ip", return_value="eth0"),
+        patch("decnet.prober.ipv6_leak._route_info", return_value=(True, "eth0")),
         patch("decnet.prober.ipv6_leak.solicit_ipv6_leak", return_value=_EVIDENCE),
     ):
         _phase(publish_fn=lambda k, p: published.append((k, p)))
@@ -86,8 +86,7 @@ def test_phase_emits_on_evidence() -> None:
 def test_phase_silent_when_solicit_returns_none() -> None:
     published: list[Any] = []
     with (
-        patch("decnet.prober.ipv6_leak._is_on_link", return_value=True),
-        patch("decnet.prober.ipv6_leak._resolve_iface_for_ip", return_value="eth0"),
+        patch("decnet.prober.ipv6_leak._route_info", return_value=(True, "eth0")),
         patch("decnet.prober.ipv6_leak.solicit_ipv6_leak", return_value=None),
     ):
         _phase(publish_fn=lambda k, p: published.append((k, p)))
@@ -98,8 +97,7 @@ def test_phase_dedup_skips_on_second_call() -> None:
     published: list[Any] = []
     ip_probed: dict = {}
     with (
-        patch("decnet.prober.ipv6_leak._is_on_link", return_value=True),
-        patch("decnet.prober.ipv6_leak._resolve_iface_for_ip", return_value="eth0"),
+        patch("decnet.prober.ipv6_leak._route_info", return_value=(True, "eth0")),
         patch("decnet.prober.ipv6_leak.solicit_ipv6_leak", return_value=_EVIDENCE) as mock_sol,
     ):
         _phase(ip_probed=ip_probed, publish_fn=lambda k, p: published.append((k, p)))
@@ -112,9 +110,57 @@ def test_phase_dedup_skips_on_second_call() -> None:
 def test_phase_handles_solicit_exception_silently() -> None:
     published: list[Any] = []
     with (
-        patch("decnet.prober.ipv6_leak._is_on_link", return_value=True),
-        patch("decnet.prober.ipv6_leak._resolve_iface_for_ip", return_value="eth0"),
+        patch("decnet.prober.ipv6_leak._route_info", return_value=(True, "eth0")),
         patch("decnet.prober.ipv6_leak.solicit_ipv6_leak", side_effect=RuntimeError("boom")),
     ):
         _phase(publish_fn=lambda k, p: published.append((k, p)))
     assert published == []
+
+
+# ─── _route_info / _ip_route_get unit tests ──────────────────────────────────
+
+
+def test_route_info_calls_ip_route_get_once() -> None:
+    """_route_info must shell out exactly once regardless of parse path."""
+    from decnet.prober.ipv6_leak import _route_info
+    stdout = "10.0.0.9 dev eth0 src 10.0.0.1 uid 0\n    cache"
+    with patch("decnet.prober.ipv6_leak._ip_route_get", return_value=stdout) as mock_rg:
+        on_link, iface = _route_info("10.0.0.9")
+    mock_rg.assert_called_once_with("10.0.0.9")
+    assert on_link is True
+    assert iface == "eth0"
+
+
+def test_route_info_detects_gateway() -> None:
+    from decnet.prober.ipv6_leak import _route_info
+    stdout = "10.0.0.9 via 192.168.1.1 dev eth0 src 192.168.1.50\n    cache"
+    with patch("decnet.prober.ipv6_leak._ip_route_get", return_value=stdout):
+        on_link, iface = _route_info("10.0.0.9")
+    assert on_link is False
+    assert iface == "eth0"
+
+
+def test_ip_route_get_logs_on_subprocess_failure() -> None:
+    from decnet.prober.ipv6_leak import _ip_route_get
+    with (
+        patch("decnet.prober.ipv6_leak.subprocess.run", side_effect=OSError("no ip")),
+        patch("decnet.prober.ipv6_leak._log") as mock_log,
+    ):
+        result = _ip_route_get("10.0.0.9")
+    assert result == ""
+    mock_log.debug.assert_called_once()
+    assert "10.0.0.9" in mock_log.debug.call_args.args[1]
+
+
+def test_ip_route_get_returns_empty_string_on_failure() -> None:
+    """subprocess failure returns "" and logs at debug — not a silent swallow."""
+    from decnet.prober.ipv6_leak import _ip_route_get
+    with (
+        patch("decnet.prober.ipv6_leak.subprocess.run", side_effect=OSError("no ip binary")),
+        patch("decnet.prober.ipv6_leak._log") as mock_log,
+    ):
+        result = _ip_route_get("10.0.0.9")
+    assert result == ""
+    assert mock_log.debug.called
+    logged_msg = mock_log.debug.call_args.args
+    assert "10.0.0.9" in str(logged_msg)