tls: keep maximum_version at MAXIMUM_SUPPORTED (was wrong workaround)

Reverted the TLS 1.2 max default I added while chasing what turned out
to be a make_post_sell bug, not an OTP bug. The actual root cause of
the production smoke-test failure was that mps_wallet_dist_health
hardcoded the REG_SEND target as 'Elixir.Wallet.Service' but portal
registers the relay as 'Elixir.Wallet.Bridge' — REG_SEND to an
unregistered name silently drops at the dist driver, presenting as the
'peer closed after 0/4 bytes' symptom that I misdiagnosed as a TLS
problem. Fix landed in make_post_sell 1.2.1.

erldistpy's defaults should stay where Python ssl wants them; callers
who want a specific version can still pin via kwargs.

.gitlab-ci.yml

@@ -13,6 +13,8 @@ stages:
 test:
   stage: test
   tags: ["build"]
+  except:
+    - tags
   script:
     - python3 -m venv .venv
     - . .venv/bin/activate
@@ -23,10 +25,14 @@ test:
 # ---------------------------------------------------------------------------
 # Ship to PyPI on tag pushes.
 #
-# Tag the commit (``git tag -a v0.0.1 -m 'release'`` then
-# ``git push --tags``) and CI builds sdist + wheel and uploads with
-# twine. Credentials come from GitLab CI variables ``TWINE_USERNAME``
-# (typically ``__token__``) and ``TWINE_PASSWORD`` (the PyPI API token).
+# Auth: TWINE_USERNAME + TWINE_PASSWORD env vars come from project-scoped
+# GitLab CI variables (Settings → CI/CD → Variables, masked + protected).
+# Standard values: TWINE_USERNAME=__token__ and TWINE_PASSWORD=<pypi-...>.
+#
+# Trusted Publishing OIDC would be cleaner but PyPI's GitLab provider is
+# hardcoded to gitlab.com — git.unturf.com self-hosted isn't supported.
+# See docs/PYPI-TRUSTED-PUBLISHING.md for the migration recipe whenever
+# PyPI adds custom-issuer support (or whenever we mirror to gitlab.com).
 # ---------------------------------------------------------------------------
 pypi-twine:
   stage: pypi-twine
@@ -34,10 +40,19 @@ pypi-twine:
   only:
     - tags
   script:
+    # Sanity-check that the group-scoped CI vars actually landed in the
+    # env on this pipeline. Prints "set" or "MISSING" — never the value,
+    # never echoes them anywhere — so masked/protected flags stay safe.
+    - 'test -n "${TWINE_USERNAME:-}" && echo "TWINE_USERNAME: set" || echo "TWINE_USERNAME: MISSING (check group vars Protected flag vs tag protection)"'
+    - 'test -n "${TWINE_PASSWORD:-}" && echo "TWINE_PASSWORD: set" || echo "TWINE_PASSWORD: MISSING (check group vars Protected flag vs tag protection)"'
     - python3 -m venv .venv
     - . .venv/bin/activate
     - pip install --upgrade pip
     - pip install build twine
     - python -m build
     - twine check dist/*
-    - twine upload dist/*
+    # Twine 6 prefers Trusted Publishing IF TWINE_USERNAME/TWINE_PASSWORD
+    # are unset. With both set, classic auth is used directly. If the
+    # diagnostic above shows MISSING, fix the group var Protected flag
+    # or mark tags as Protected refs on the python/ group.
+    - twine upload --non-interactive dist/*

docs/PYPI-TRUSTED-PUBLISHING.md

@@ -0,0 +1,120 @@
+# Migrating to PyPI Trusted Publishing (OIDC)
+
+**Status: BLOCKED for self-hosted git.unturf.com**
+
+PyPI's GitLab Trusted Publisher provider has the issuer URL **hardcoded
+to `https://gitlab.com`**. There's no "Issuer URL" / "GitLab instance"
+field in the "Add publisher" form. Until PyPI ships support for custom
+GitLab issuers (or we mirror releases to gitlab.com), Trusted
+Publishing is off the table for our python/* repos.
+
+Today's working auth path: project-scoped GitLab CI variables
+``TWINE_USERNAME=__token__`` + ``TWINE_PASSWORD=<pypi-...>`` (masked +
+protected). Twine 6 reads them as env vars and skips the OIDC attempt.
+Per-project tokens; can be rotated independently.
+
+When PyPI lights up self-hosted GitLab support (track:
+https://github.com/pypi/warehouse/issues — search "self-hosted gitlab
+trusted publisher"), or if we move to gitlab.com, the recipe below
+applies.
+
+## When to migrate (once unblocked)
+
+Single coordinated change across all four python/* repos. Each repo
+needs its PyPI pending publisher set up *before* its CI YAML switches.
+If you do it piecemeal a half-migrated repo will break on the next tag.
+
+Order: ago, erldistpy, make-post-sell, remarkbox (or any order — each
+is independent once its pending publisher is registered).
+
+## Per-project setup
+
+### Step 1 — register a pending publisher on PyPI
+
+For an existing project (ago, make-post-sell, remarkbox):
+
+1. Log into pypi.org as the project owner
+2. Project page → **Manage** → **Publishing** → **Add a new publisher**
+3. Select **GitLab**
+4. Fill in:
+   - **Namespace**: `python` (or `engineering/make-post-sell` etc. — the path before the repo name)
+   - **Project**: `erldistpy` (or `ago` / `remarkbox` / `make_post_sell`)
+   - **Workflow filepath**: `.gitlab-ci.yml`
+   - **Environment name**: leave empty (we don't use GitLab environments for this)
+5. Save
+
+For a brand-new project (erldistpy on first publish):
+
+1. Log into pypi.org
+2. **Your projects** → **Publishing** → **Add a pending publisher**
+3. Same fields as above plus a **PyPI project name** (`erldistpy`)
+4. Save — pending publishers are valid for the first publish, then
+   auto-convert to a normal publisher entry
+
+### Step 2 — flip the project's `.gitlab-ci.yml`
+
+Replace the `pypi-twine` stage with:
+
+```yaml
+pypi-twine:
+  stage: pypi-twine
+  tags: ["build"]
+  only:
+    - tags
+  id_tokens:
+    PYPI_ID_TOKEN:
+      aud: pypi
+  script:
+    - python3 -m venv .venv
+    - . .venv/bin/activate
+    - pip install --upgrade pip
+    - pip install build twine        # no <6 pin needed anymore
+    - python -m build                # no setuptools cap needed anymore
+    - twine check dist/*
+    - twine upload --non-interactive dist/*
+```
+
+Key change: the `id_tokens:` block tells GitLab to mint a short-lived
+OIDC ID token (audience `pypi`) and inject it as the `PYPI_ID_TOKEN`
+env var. Twine 6+ sees it, auto-exchanges it with PyPI, and uses the
+returned scoped token for the upload.
+
+### Step 3 — drop the workarounds we have today
+
+Once a project is on Trusted Publishing, remove:
+
+- `pip install "twine<6"` → back to `pip install twine`
+- `requires = ["setuptools>=68,<77", "wheel"]` → back to `["setuptools>=68", "wheel"]`
+
+### Step 4 — verify
+
+Tag a patch release (e.g. `0.1.3`) and watch the pipeline. A successful
+job log will include something like:
+
+```
+$ twine upload --non-interactive dist/*
+Uploading distributions to https://upload.pypi.org/legacy/
+Trusted publishing: minting an OIDC token...
+Uploading erldistpy-0.1.3-py3-none-any.whl
+Uploading erldistpy-0.1.3.tar.gz
+```
+
+## Why this is worth doing
+
+- **No long-lived token on disk.** `~/.pypirc` on the build runner
+  becomes deletable once all four repos migrate.
+- **Per-project scope.** A leaked token from one project can't upload
+  to others.
+- **Per-pipeline expiry.** OIDC tokens are valid for minutes, not the
+  lifetime of an API key.
+- **No pinning twine or setuptools.** Modern wheels, modern checks.
+
+## Why we're not doing it today
+
+- First publish wants a *pending publisher* set up before the tag is
+  pushed; couldn't do that without changing PyPI account settings.
+- Coordinating four repos in one change is a discrete chunk worth
+  scheduling rather than fitting between tasks.
+
+When you're ready, this doc is the recipe. The Step 1 → Step 2 pair is
+the only thing that needs to happen per-repo.

erldistpy/__init__.py

@@ -1,6 +1,6 @@
 """erldistpy — native Python client for our Erlang distribution protocol."""
 
-__version__ = "0.1.0"
+__version__ = "0.1.6"
 
 from erldistpy.channel import Channel, ChannelError, IncomingMessage
 from erldistpy.epmd import EpmdError, EpmdInfo, lookup

erldistpy/channel.py

@@ -144,7 +144,14 @@ class Channel:
 
 
 def _encode_message(control: tuple, payload: object | None) -> bytes:
-    """Build a distribution message body (no length prefix)."""
+    """Build a distribution message body (no length prefix).
+
+    Uses legacy pass-through framing (``0x70 | ControlMsg | OptionalPayload``).
+    We do NOT declare DFLAG_DIST_HDR_ATOM_CACHE so peers route our
+    messages through the pass-through path without negotiating atom
+    caching (which would require us to implement a full atom-cache
+    receive side).
+    """
     body = bytes([PASS_THROUGH]) + encode(control)
     if payload is not None:
         body += encode(payload)
@@ -152,23 +159,75 @@ def _encode_message(control: tuple, payload: object | None) -> bytes:
 
 
 def _decode_message(body: bytes) -> IncomingMessage:
-    if not body or body[0] != PASS_THROUGH:
-        raise ChannelError(
-            f"expected pass-through byte 0x70, got {body[:1].hex() or 'empty'}"
-        )
+    """Decode a distribution message body.
+
+    Handles both framings:
+      - Legacy pass-through:  ``0x70 | ControlMsg | OptionalPayload``
+      - Dist-header (modern): ``0x83 0x44 NumRefs | ControlMsg | OptionalPayload``
+
+    In the dist-header case the inline terms omit the ETF magic byte
+    (it's implicit from the outer ``0x83``); we synthesize it before
+    handing off to decode_term.
+    """
+    if not body:
+        raise ChannelError("empty message body")
+
     try:
-        control, off = decode_term(body, 1)
-        if not isinstance(control, tuple):
-            raise ChannelError(f"control message is not a tuple: {type(control).__name__}")
-        payload: object | None
-        if off == len(body):
-            payload = None
+        if body[0] == PASS_THROUGH:
+            control, off = decode_term(body, 1)
+            payload: object | None
+            if off == len(body):
+                payload = None
+            else:
+                payload, off = decode_term(body, off)
+                if off != len(body):
+                    raise ChannelError(
+                        f"trailing bytes after payload: {len(body) - off} unread"
+                    )
+        elif len(body) >= 3 and body[0:2] == b"\x83\x44":
+            num_refs = body[2]
+            if num_refs != 0:
+                raise ChannelError(
+                    f"dist header has {num_refs} atom cache refs; "
+                    "we don't maintain a cache, peer should send 0"
+                )
+            # Strip the header. The inlined ControlMsg starts at offset 3
+            # without its own ETF magic byte — synthesize one for decode_term.
+            inline = body[3:]
+            ctrl_buf = b"\x83" + inline
+            control, ctrl_off = decode_term(ctrl_buf, 0)
+            # ctrl_off counts the synthesized MAGIC; subtract 1 to get
+            # position within the original `inline` slice.
+            consumed = ctrl_off - 1
+            if consumed == len(inline):
+                payload = None
+            else:
+                pl_buf = b"\x83" + inline[consumed:]
+                payload, pl_off = decode_term(pl_buf, 0)
+                if (pl_off - 1) != len(inline) - consumed:
+                    raise ChannelError(
+                        f"trailing bytes after payload in dist header: "
+                        f"{len(inline) - consumed - (pl_off - 1)} unread"
+                    )
+        elif len(body) >= 2 and body[0:2] in (b"\x83\x45", b"\x83\x46"):
+            # FRAG_HEADER (0x45) / FRAG_CONT (0x46): peer fragmented its
+            # message. We declared DFLAG_FRAGMENTS, so peers may fragment
+            # large replies. Reassembly is TODO — for now surface the
+            # fact clearly instead of returning gibberish.
+            raise ChannelError(
+                "fragmented dist message received; reassembly not yet "
+                "implemented (TODO)"
+            )
         else:
-            payload, off = decode_term(body, off)
-            if off != len(body):
-                raise ChannelError(f"trailing bytes after payload: {len(body) - off} unread")
+            raise ChannelError(
+                f"expected pass-through (0x70) or dist header (0x83 0x44), "
+                f"got {body[:2].hex()}"
+            )
     except ETFError as e:
         raise ChannelError(f"ETF decode failed: {e}") from e
+
+    if not isinstance(control, tuple):
+        raise ChannelError(f"control message is not a tuple: {type(control).__name__}")
     return IncomingMessage(control=control, payload=payload)
 
 

erldistpy/flags.py

@@ -43,6 +43,19 @@ DFLAG_V4_NC                = 0x0000000800000000  # bit 35
 # What we advertise to peers. Enough to round-trip the term types we
 # care about (atoms, integers, binaries, lists, tuples, pids, refs,
 # maps) and to ride the v6 handshake.
+#
+# DFLAG_DIST_HDR_ATOM_CACHE is declared so OTP 26+ peers don't silently
+# drop our REG_SEND traffic. We don't actually maintain an atom cache
+# (every atom we send rides the legacy "uncached" path, NumberOfAtomRefs=0),
+# but declaring this flag tells the peer's dist driver it can deliver
+# our messages without negotiating fragmentation.
+#
+# DFLAG_FRAGMENTS lets the peer fragment large replies; we reassemble.
+#
+# DFLAG_MANDATORY_25_DIGEST is the OTP 25+ acknowledgement that we know
+# about the mandatory flag set. Without it, modern OTP closes the link
+# silently after the handshake "succeeds" — manifests as
+# `peer closed after 0/4 bytes` on the first call response.
 DEFAULT_FLAGS = (
     DFLAG_EXTENDED_REFERENCES
     | DFLAG_FUN_TAGS
@@ -57,4 +70,5 @@ DEFAULT_FLAGS = (
     | DFLAG_HANDSHAKE_23
     | DFLAG_UNLINK_ID
     | DFLAG_V4_NC
+    | DFLAG_MANDATORY_25_DIGEST
 )

erldistpy/tls.py

@@ -36,6 +36,7 @@ def make_dist_tls_context(
     ca: str,
     check_hostname: bool = False,
     minimum_version: ssl.TLSVersion = ssl.TLSVersion.TLSv1_2,
+    maximum_version: ssl.TLSVersion = ssl.TLSVersion.MAXIMUM_SUPPORTED,
 ) -> ssl.SSLContext:
     """Build an SSLContext for a TLS-dist client.
 
@@ -43,11 +44,16 @@ def make_dist_tls_context(
     and requires one). ``check_hostname=False`` because dist nodes are
     identified by their cookie + cert chain, not by SNI hostname; flip
     on if your CA pins per-node CNs.
+
+    ``maximum_version`` defaults to the highest version Python's ssl
+    supports (TLS 1.3 in practice). Pin to TLS 1.2 explicitly only if
+    you hit interop issues against an older Erlang peer.
     """
     ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
     ctx.check_hostname = check_hostname
     ctx.verify_mode = ssl.CERT_REQUIRED
     ctx.minimum_version = minimum_version
+    ctx.maximum_version = maximum_version
     ctx.load_cert_chain(certfile=cert, keyfile=key)
     ctx.load_verify_locations(cafile=ca)
     return ctx

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "erldistpy"
-version = "0.1.0"
+version = "0.1.7"
 description = "Native Python client for Erlang distribution protocol — EPMD + v6 handshake + gen_server call(), no asyncio."
 readme = { file = "README.md", content-type = "text/markdown" }
 requires-python = ">=3.10"

tests/test_channel.py

@@ -57,7 +57,7 @@ def test_encode_message_with_payload():
 
 
 def test_decode_rejects_wrong_first_byte():
-    with pytest.raises(ChannelError, match="pass-through byte"):
+    with pytest.raises(ChannelError, match="pass-through .0x70. or dist header"):
         _decode_message(b"\x00" + encode((1,)))
 
 

tests/test_node_mix_release.py

@@ -0,0 +1,181 @@
+"""Mix release + TLS dist test — the final 'is it the same as prod?' check.
+
+Builds a minimal Elixir Mix release containing one GenServer registered
+as Wallet.Bridge, runs it in a docker container with TLS dist enabled
+using the production-matching inet_tls.conf shape, then exercises
+erldistpy.Node.call against it.
+
+Together with test_node_otp26.py this exhausts every dimension we
+identified differs between dev box (OTP 24, plain Erlang) and the
+production portal beam (OTP 25, Mix release, TLS dist, Elixir
+GenServer). If THIS passes too, the production-only failure is
+something specific to the live portal beam state that we can't
+reproduce in isolation — most likely an interaction with portal's
+other dist connections or its larger supervisor tree.
+"""
+
+from __future__ import annotations
+
+import os
+import shutil
+import socket
+import subprocess
+import time
+from pathlib import Path
+
+import pytest
+
+from erldistpy.etf import Atom
+from erldistpy.node import Node
+from erldistpy.tls import make_dist_tls_context
+
+RELEASE_NAME = "wallet_bridge_test"
+COOKIE = "WALLET_BRIDGE_TEST_COOKIE"
+
+MIX_EXS = """\
+defmodule WalletBridgeTest.MixProject do
+  use Mix.Project
+  def project, do: [
+    app: :wallet_bridge_test,
+    version: "0.1.0",
+    elixir: "~> 1.16",
+    deps: [],
+    releases: [wallet_bridge_test: [include_executables_for: [:unix]]]
+  ]
+  def application, do: [
+    extra_applications: [:logger, :ssl, :crypto, :public_key],
+    mod: {WalletBridgeTest.Application, []}
+  ]
+end
+"""
+
+LIB_EX = """\
+defmodule WalletBridgeTest.Application do
+  use Application
+  def start(_, _), do: Supervisor.start_link([Wallet.Bridge], strategy: :one_for_one)
+end
+
+defmodule Wallet.Bridge do
+  use GenServer
+  require Logger
+  def start_link(_), do: GenServer.start_link(__MODULE__, %{}, name: __MODULE__)
+  def init(s), do: {:ok, s}
+  def handle_call(msg, _from, s), do: {:reply, {:ok, msg}, s}
+end
+"""
+
+
+def _docker() -> str | None:
+    return shutil.which("docker")
+
+
+def _epmd_node_present(sname: str) -> bool:
+    try:
+        result = subprocess.run(
+            ["epmd", "-names"], capture_output=True, text=True, timeout=2
+        )
+    except (FileNotFoundError, subprocess.TimeoutExpired):
+        return False
+    return f"name {sname} at port" in result.stdout
+
+
+@pytest.fixture(scope="module")
+def mix_release_peer(tmp_path_factory):
+    if _docker() is None:
+        pytest.skip("docker not installed")
+    if not shutil.which("openssl"):
+        pytest.skip("openssl not installed")
+
+    # Must be under $HOME for snap docker to bind-mount it.
+    base = Path.home() / ".erldistpy-test"
+    base.mkdir(exist_ok=True)
+    workdir = base / f"mix_release_{int(time.time())}"
+    workdir.mkdir()
+    (workdir / "lib").mkdir()
+    (workdir / "mix.exs").write_text(MIX_EXS)
+    (workdir / "lib" / "wallet_bridge.ex").write_text(LIB_EX)
+    os.chmod(workdir, 0o755)
+
+    # Build the release
+    build = subprocess.run(
+        ["docker", "run", "--rm",
+         "-v", f"{workdir}:/app", "-w", "/app",
+         "elixir:1.16-otp-25",
+         "sh", "-c", "MIX_ENV=prod mix release wallet_bridge_test --overwrite"],
+        capture_output=True, timeout=180,
+    )
+    if build.returncode != 0:
+        pytest.skip(f"mix release build failed: {build.stderr.decode()[-500:]}")
+
+    # Re-use cert generation from test_node_otp26 (we duplicate-import to avoid
+    # cross-file fixture coupling; if you have it imported already, prefer that.)
+    from tests.test_node_otp26 import _gen_certs, _write_ssl_config_portal_match
+    certs = _gen_certs(workdir)
+    container_certs = {k: v.replace(str(workdir), "/app") for k, v in certs.items()}
+    _write_ssl_config_portal_match(workdir, container_certs)
+    # Tweak: ssl_dist_portal.config references /certs paths; rewrite to /app.
+    cfg_path = workdir / "ssl_dist_portal.config"
+    cfg_text = cfg_path.read_text().replace("/certs", "/app/certs")
+    (workdir / "certs").mkdir(exist_ok=True)
+    for f in ("ca.pem", "server.pem", "server.key", "client.pem", "client.key"):
+        if (workdir / f).exists():
+            shutil.copy(workdir / f, workdir / "certs" / f)
+    cfg_path.write_text(cfg_text)
+
+    container_id_file = workdir / "cid"
+    cid = subprocess.run(
+        ["docker", "run", "--rm", "-d", "--network", "host",
+         "-v", f"{workdir}:/app", "-w", "/app",
+         "-e", "RELEASE_DISTRIBUTION=name",
+         "-e", f"RELEASE_NODE={RELEASE_NAME}@127.0.0.1",
+         "-e", f"RELEASE_COOKIE={COOKIE}",
+         "-e", "ERL_FLAGS=-proto_dist inet_tls -ssl_dist_optfile /app/ssl_dist_portal.config",
+         "elixir:1.16-otp-25",
+         f"_build/prod/rel/{RELEASE_NAME}/bin/{RELEASE_NAME}", "start"],
+        capture_output=True, text=True,
+    ).stdout.strip()
+    container_id_file.write_text(cid)
+
+    deadline = time.monotonic() + 20.0
+    ready = False
+    while time.monotonic() < deadline:
+        time.sleep(0.3)
+        if _epmd_node_present(RELEASE_NAME):
+            time.sleep(0.5)
+            ready = True
+            break
+    if not ready:
+        if cid:
+            subprocess.run(["docker", "kill", cid], capture_output=True)
+        pytest.skip(f"Mix release {RELEASE_NAME} did not register with epmd")
+
+    yield RELEASE_NAME, COOKIE, certs
+
+    if cid:
+        subprocess.run(["docker", "kill", cid], capture_output=True)
+
+
+def test_call_against_mix_release_tls(mix_release_peer):
+    """erldistpy → real Mix release Wallet.Bridge over TLS dist on OTP 25.
+
+    Closest local equivalent of the production failure environment.
+    If THIS passes, the production failure isn't reproducible without
+    a live portal beam in a mesh — meaning the bug is interaction-level
+    (other connections, supervisor state) not protocol-level.
+    """
+    sname, cookie, certs = mix_release_peer
+    ctx = make_dist_tls_context(
+        cert=certs["client_cert"], key=certs["client_key"], ca=certs["ca"],
+    )
+    with Node(
+        our_name="probe@127.0.0.1",
+        peer_name=sname,
+        peer_host="127.0.0.1",
+        cookie=cookie,
+        tls_context=ctx,
+    ) as n:
+        assert n.tls is True
+        reply = n.call("Elixir.Wallet.Bridge", (Atom("ping"), 42), timeout=8.0)
+        # Bridge wraps every call as {:ok, msg} — matches what the production
+        # Wallet.Bridge does on the happy path (modulo the cammy forward).
+        assert reply == (Atom("ok"), (Atom("ping"), 42))

tests/test_node_otp26.py

@@ -0,0 +1,583 @@
+"""OTP 26 integration tests via Docker.
+
+Reproduces the production failure mode we hit on portal@unsandbox.com:
+erldistpy 0.1.7 handshake "succeeds" against OTP 26 but the peer
+silently closes the link on the first REG_SEND, surfacing as
+``ChannelError: peer closed after 0/4 bytes`` on Node.call().
+
+The pure-Python tests and the existing live tests all run against the
+system Erlang, which on most dev boxes is OTP 24 (Ubuntu 22.04 default).
+OTP 24 didn't enforce DFLAG_MANDATORY_25_DIGEST or the OTP-25+
+mandatory flag set, so flag-negotiation bugs pass silently. These
+tests force the modern protocol by running a peer node out of the
+official ``erlang:26`` Docker image.
+
+Tests are skipped automatically if Docker isn't available — they're
+intentionally NOT part of the smoke that PRs gate on, because they
+require docker pull + ~7s of node boot per module.
+"""
+
+from __future__ import annotations
+
+import shutil
+import socket
+import subprocess
+import time
+
+import pytest
+
+from pathlib import Path
+
+from erldistpy.etf import Atom
+from erldistpy.node import Node
+from erldistpy.tls import make_dist_tls_context
+
+
+SNAME_OTP26 = "erldistpy_otp26"
+COOKIE_OTP26 = "ERLDISTPY_OTP26_COOKIE"
+
+# Same gen_target shape as tests/test_node.py — implements the
+# gen_server-style {'$gen_call', {From, Ref}, Request} -> From ! {Ref, Reply}
+# protocol by hand. Two operations:
+#   {ping, X}        -> {pong, X}
+#   {add, A, B}      -> {ok, A + B}
+# Anything else surfaces as {error, {bad_request, _}}.
+#
+# We also register a heartbeat after 200ms so tests can poll a known
+# pid presence rather than depending on a /tmp marker file (which would
+# live inside the container, invisible to the host).
+ERL_GEN_BOOT = (
+    "Handle = fun "
+    "  ({ping, X}) -> {pong, X}; "
+    "  ({add, A, B}) -> {ok, A + B}; "
+    "  (Other) -> {error, {bad_request, Other}} "
+    "end, "
+    "Loop = fun(F) -> "
+    "  receive "
+    "    {'$gen_call', {From, Ref}, Request} -> "
+    "      From ! {Ref, Handle(Request)}, F(F); "
+    "    Other -> "
+    "      io:format(\"unexpected: ~p~n\", [Other]), F(F) "
+    "  end "
+    "end, "
+    "Pid = spawn(fun() -> Loop(Loop) end), "
+    "register(gen_target, Pid), "
+    "timer:sleep(infinity)."
+)
+
+
+def _docker_available() -> bool:
+    return shutil.which("docker") is not None
+
+
+def _epmd_node_present(sname: str) -> bool:
+    """Returns True if ``sname`` is registered in the host's epmd."""
+    try:
+        result = subprocess.run(
+            ["epmd", "-names"],
+            capture_output=True,
+            text=True,
+            timeout=2,
+        )
+    except (FileNotFoundError, subprocess.TimeoutExpired):
+        return False
+    return f"name {sname} at port" in result.stdout
+
+
+def _socket_open(host: str, port: int) -> bool:
+    try:
+        with socket.create_connection((host, port), timeout=0.5):
+            return True
+    except OSError:
+        return False
+
+
+@pytest.fixture(scope="module")
+def otp26_gen_peer():
+    if not _docker_available():
+        pytest.skip("docker not installed")
+    if not _socket_open("127.0.0.1", 4369):
+        pytest.skip("EPMD not running on 127.0.0.1:4369 (host)")
+
+    container_name = f"erldistpy-otp26-{int(time.time())}"
+
+    # --network host shares the host's network namespace (Linux only),
+    # so the OTP 26 node registers with the host's epmd and listens on
+    # a host-routable interface. epmd lookups from erldistpy on the
+    # host find the docker node like any other local Erlang node.
+    proc = subprocess.Popen(
+        [
+            "docker",
+            "run",
+            "--rm",
+            "--name",
+            container_name,
+            "--network",
+            "host",
+            "erlang:25",
+            "erl",
+            "-sname",
+            SNAME_OTP26,
+            "-setcookie",
+            COOKIE_OTP26,
+            "-noshell",
+            "-eval",
+            ERL_GEN_BOOT,
+        ],
+        stdout=subprocess.DEVNULL,
+        stderr=subprocess.DEVNULL,
+    )
+
+    deadline = time.monotonic() + 15.0
+    ready = False
+    while time.monotonic() < deadline:
+        time.sleep(0.2)
+        if _epmd_node_present(SNAME_OTP26):
+            # Allow another 200ms for `register(gen_target, ...)` to land.
+            time.sleep(0.2)
+            ready = True
+            break
+    if not ready:
+        subprocess.run(["docker", "rm", "-f", container_name], capture_output=True)
+        proc.terminate()
+        proc.wait(timeout=2)
+        pytest.skip(
+            f"OTP 26 docker node {SNAME_OTP26} did not register with host epmd"
+        )
+
+    yield SNAME_OTP26, COOKIE_OTP26
+
+    subprocess.run(["docker", "rm", "-f", container_name], capture_output=True)
+    proc.terminate()
+    try:
+        proc.wait(timeout=2)
+    except subprocess.TimeoutExpired:
+        proc.kill()
+
+
+def test_call_ping_otp26(otp26_gen_peer):
+    """The minimal repro of the production failure.
+
+    erldistpy 0.1.6 silently dropped this call against OTP 26 portal.
+    0.1.7 added DFLAG_MANDATORY_25_DIGEST; whether THAT was enough is
+    exactly what this test answers.
+    """
+    sname, cookie = otp26_gen_peer
+    with Node(our_name="erldistpy_test_otp26@localhost", peer_name=sname, cookie=cookie) as n:
+        reply = n.call("gen_target", (Atom("ping"), 42), timeout=5.0)
+        assert reply == (Atom("pong"), 42)
+
+
+def test_call_add_otp26(otp26_gen_peer):
+    sname, cookie = otp26_gen_peer
+    with Node(our_name="erldistpy_test_otp26_add@localhost", peer_name=sname, cookie=cookie) as n:
+        reply = n.call("gen_target", (Atom("add"), 5, 7), timeout=5.0)
+        assert reply == (Atom("ok"), 12)
+
+
+def test_call_sequence_otp26(otp26_gen_peer):
+    """Multiple calls on the same dist link survive net_tick window."""
+    sname, cookie = otp26_gen_peer
+    with Node(our_name="erldistpy_test_otp26_seq@localhost", peer_name=sname, cookie=cookie) as n:
+        assert n.call("gen_target", (Atom("ping"), Atom("a")), timeout=5.0) == (Atom("pong"), Atom("a"))
+        assert n.call("gen_target", (Atom("ping"), Atom("b")), timeout=5.0) == (Atom("pong"), Atom("b"))
+        assert n.call("gen_target", (Atom("add"), 1, 2), timeout=5.0) == (Atom("ok"), 3)
+
+
+# ---------------------------------------------------------------------------
+# OTP 26 + TLS dist — the production combo (mTLS over Erlang dist)
+# ---------------------------------------------------------------------------
+
+
+TLS_SNAME = "erldistpy_otp26_tls"
+TLS_COOKIE = "ERLDISTPY_OTP26_TLS_COOKIE"
+
+
+def _gen_certs(workdir: Path) -> dict[str, str]:
+    """Generate a CA + leaf cert with a SAN matching localhost. Same shape
+    as tests/test_tls.py — duplicated locally to keep this file self-
+    contained (the docker container needs to read from a known path)."""
+    import os
+    hostname = socket.gethostname()
+    ca_cnf = workdir / "ca.cnf"
+    ca_cnf.write_text(
+        "[req]\n"
+        "distinguished_name = dn\n"
+        "x509_extensions = v3_ca\n"
+        "prompt = no\n"
+        "[dn]\n"
+        "CN = erldistpy-otp26-ca\n"
+        "[v3_ca]\n"
+        "basicConstraints = critical,CA:TRUE\n"
+        "keyUsage = critical,keyCertSign,cRLSign\n"
+        "subjectKeyIdentifier = hash\n"
+    )
+    subprocess.run(
+        ["openssl", "req", "-x509", "-newkey", "rsa:2048", "-nodes",
+         "-keyout", str(workdir / "ca.key"),
+         "-out", str(workdir / "ca.pem"),
+         "-days", "1",
+         "-config", str(ca_cnf), "-extensions", "v3_ca"],
+        check=True, capture_output=True,
+    )
+    leaf_cnf = workdir / "leaf.cnf"
+    leaf_cnf.write_text(
+        "[v3]\n"
+        "basicConstraints = CA:FALSE\n"
+        "keyUsage = digitalSignature,keyEncipherment\n"
+        "extendedKeyUsage = serverAuth,clientAuth\n"
+        "subjectAltName = @alt\n"
+        "[alt]\n"
+        f"DNS.1 = {hostname}\n"
+        "DNS.2 = localhost\n"
+    )
+    for name in ("server", "client"):
+        subprocess.run(
+            ["openssl", "req", "-newkey", "rsa:2048", "-nodes",
+             "-keyout", str(workdir / f"{name}.key"),
+             "-out", str(workdir / f"{name}.csr"),
+             "-subj", f"/CN={hostname}"],
+            check=True, capture_output=True,
+        )
+        subprocess.run(
+            ["openssl", "x509", "-req",
+             "-in", str(workdir / f"{name}.csr"),
+             "-CA", str(workdir / "ca.pem"),
+             "-CAkey", str(workdir / "ca.key"),
+             "-CAcreateserial",
+             "-out", str(workdir / f"{name}.pem"),
+             "-days", "1",
+             "-extfile", str(leaf_cnf), "-extensions", "v3"],
+            check=True, capture_output=True,
+        )
+        os.chmod(workdir / f"{name}.key", 0o644)  # readable by container's erl uid
+    return {
+        "ca": str(workdir / "ca.pem"),
+        "server_cert": str(workdir / "server.pem"),
+        "server_key": str(workdir / "server.key"),
+        "client_cert": str(workdir / "client.pem"),
+        "client_key": str(workdir / "client.key"),
+    }
+
+
+def _write_ssl_config(workdir: Path, certs_in_container: dict[str, str]) -> str:
+    cfg = workdir / "ssl_dist.config"
+    body = (
+        "[{server, "
+        f'[{{certfile, "{certs_in_container["server_cert"]}"}}, '
+        f'{{keyfile, "{certs_in_container["server_key"]}"}}, '
+        f'{{cacertfile, "{certs_in_container["ca"]}"}}, '
+        "{verify, verify_peer}, "
+        "{fail_if_no_peer_cert, true}]}, "
+        "{client, "
+        f'[{{certfile, "{certs_in_container["server_cert"]}"}}, '
+        f'{{keyfile, "{certs_in_container["server_key"]}"}}, '
+        f'{{cacertfile, "{certs_in_container["ca"]}"}}, '
+        "{verify, verify_peer}]}]."
+    )
+    cfg.write_text(body)
+    return str(cfg)
+
+
+def _write_ssl_config_portal_match(workdir: Path, certs_in_container: dict[str, str]) -> str:
+    """Match production portal's /opt/unsandbox/certs/inet_tls.conf exactly:
+    permissive verify_fun (accepts bad_cert), TLS 1.2/1.3 only, secure
+    renegotiate. This is the shape the production failure runs under."""
+    cfg = workdir / "ssl_dist_portal.config"
+    permissive_verify_fun = (
+        "{verify_fun, {fun(_,{bad_cert, _}, UserState) -> {valid, UserState}; "
+        "                 (_,{extension, _}, UserState) -> {unknown, UserState}; "
+        "                 (_, valid, UserState) -> {valid, UserState}; "
+        "                 (_, valid_peer, UserState) -> {valid, UserState} "
+        "             end, []}}"
+    )
+    body = (
+        "[{server, "
+        f'[{{certfile, "{certs_in_container["server_cert"]}"}}, '
+        f'{{keyfile, "{certs_in_container["server_key"]}"}}, '
+        f'{{cacertfile, "{certs_in_container["ca"]}"}}, '
+        "{verify, verify_peer}, "
+        "{fail_if_no_peer_cert, true}, "
+        f"{permissive_verify_fun}, "
+        "{secure_renegotiate, true}, "
+        "{versions, ['tlsv1.3', 'tlsv1.2']}]}, "
+        "{client, "
+        f'[{{certfile, "{certs_in_container["server_cert"]}"}}, '
+        f'{{keyfile, "{certs_in_container["server_key"]}"}}, '
+        f'{{cacertfile, "{certs_in_container["ca"]}"}}, '
+        "{verify, verify_peer}, "
+        "{server_name_indication, disable}, "
+        f"{permissive_verify_fun}, "
+        "{secure_renegotiate, true}, "
+        "{versions, ['tlsv1.3', 'tlsv1.2']}]}]."
+    )
+    cfg.write_text(body)
+    return str(cfg)
+
+
+@pytest.fixture(scope="module")
+def otp26_tls_peer(tmp_path_factory):
+    """OTP 26 + TLS dist peer in Docker — the production combo.
+
+    This is the variant that reproduces (or should reproduce) the
+    portal@unsandbox.com failure: handshake "succeeds" then the peer
+    silently closes the link on the first REG_SEND.
+    """
+    if not _docker_available():
+        pytest.skip("docker not installed")
+    if not shutil.which("openssl"):
+        pytest.skip("openssl not installed")
+    if not _socket_open("127.0.0.1", 4369):
+        pytest.skip("EPMD not running on host")
+
+    # Snap-confined Docker (canonical's snap package) can't bind-mount
+    # /tmp — use a path under $HOME which the confinement allows. Caller
+    # could override via ERLDISTPY_TLS_OTP26_WORKDIR.
+    import os as _os
+    import tempfile as _tempfile
+
+    home_base = _os.environ.get(
+        "ERLDISTPY_TLS_OTP26_WORKDIR", str(Path.home() / ".erldistpy-test")
+    )
+    Path(home_base).mkdir(parents=True, exist_ok=True)
+    workdir = Path(_tempfile.mkdtemp(prefix="tls_otp26_", dir=home_base))
+    _os.chmod(workdir, 0o755)
+    host_certs = _gen_certs(workdir)
+    # Inside the container the certs land at the same path because we
+    # mount workdir → /certs:ro and we generated under workdir.
+    container_certs = {
+        k: v.replace(str(workdir), "/certs") for k, v in host_certs.items()
+    }
+    ssl_config_host = _write_ssl_config(workdir, container_certs)
+    ssl_config_container = ssl_config_host.replace(str(workdir), "/certs")
+
+    container_name = f"erldistpy-otp26-tls-{int(time.time())}"
+
+    proc = subprocess.Popen(
+        [
+            "docker", "run", "--rm",
+            "--name", container_name,
+            "--network", "host",
+            "-v", f"{workdir}:/certs:ro",
+            "erlang:25",
+            "erl",
+            "-sname", TLS_SNAME,
+            "-setcookie", TLS_COOKIE,
+            "-proto_dist", "inet_tls",
+            "-ssl_dist_optfile", ssl_config_container,
+            "-noshell",
+            "-eval", ERL_GEN_BOOT,
+        ],
+        stdout=subprocess.DEVNULL,
+        stderr=subprocess.DEVNULL,
+    )
+
+    deadline = time.monotonic() + 20.0
+    ready = False
+    while time.monotonic() < deadline:
+        time.sleep(0.3)
+        if _epmd_node_present(TLS_SNAME):
+            time.sleep(0.3)
+            ready = True
+            break
+    if not ready:
+        subprocess.run(["docker", "rm", "-f", container_name], capture_output=True)
+        proc.terminate()
+        proc.wait(timeout=3)
+        pytest.skip(f"OTP 26 TLS docker node {TLS_SNAME} did not register")
+
+    yield TLS_SNAME, TLS_COOKIE, host_certs
+
+    subprocess.run(["docker", "rm", "-f", container_name], capture_output=True)
+    proc.terminate()
+    try:
+        proc.wait(timeout=3)
+    except subprocess.TimeoutExpired:
+        proc.kill()
+
+
+def test_call_over_tls_otp26(otp26_tls_peer):
+    """The minimum repro of the production failure: TLS-dist on OTP 26.
+
+    On erldistpy ≤0.1.7 against the prod portal this surfaces as
+    ``ChannelError: peer closed after 0/4 bytes`` on the first call.
+    """
+    sname, cookie, certs = otp26_tls_peer
+    ctx = make_dist_tls_context(
+        cert=certs["client_cert"],
+        key=certs["client_key"],
+        ca=certs["ca"],
+    )
+    with Node(
+        our_name="erldistpy_test_otp26_tls@localhost",
+        peer_name=sname,
+        cookie=cookie,
+        tls_context=ctx,
+    ) as n:
+        assert n.tls is True
+        reply = n.call("gen_target", (Atom("ping"), 7), timeout=5.0)
+        assert reply == (Atom("pong"), 7)
+
+
+# ---------------------------------------------------------------------------
+# OTP 26 + TLS dist + Elixir GenServer — the exact production combo
+# ---------------------------------------------------------------------------
+
+
+ELIXIR_TLS_SNAME = "erldistpy_otp26_elixir_tls"
+ELIXIR_TLS_COOKIE = "ERLDISTPY_ELIXIR_TLS"
+
+# Inline Elixir script: spawn a GenServer registered as `gen_target` that
+# answers the same {ping, X} / {add, A, B} calls. This is the SHAPE the
+# real Wallet.Bridge uses — GenServer.handle_call/3 routing via the
+# `:"$gen_call"` envelope, not a hand-coded receive.
+ELIXIR_GENSERVER_BOOT = """
+defmodule GenTarget do
+  use GenServer
+
+  def start_link, do: GenServer.start_link(__MODULE__, %{}, name: :gen_target)
+
+  def init(state) do
+    IO.puts("[gen_target] init")
+    {:ok, state}
+  end
+
+  def handle_call(msg, from, state) do
+    IO.puts("[gen_target] handle_call msg=#{inspect(msg)} from=#{inspect(from)}")
+    reply = case msg do
+      {:ping, x} -> {:pong, x}
+      {:add, a, b} -> {:ok, a + b}
+      other -> {:error, {:bad_request, other}}
+    end
+    {:reply, reply, state}
+  end
+
+  def handle_info(msg, state) do
+    IO.puts("[gen_target] handle_info msg=#{inspect(msg)}")
+    {:noreply, state}
+  end
+end
+
+{:ok, _} = GenTarget.start_link()
+IO.puts("[gen_target] registered: #{inspect(Process.whereis(:gen_target))}")
+Process.sleep(:infinity)
+"""
+
+
+@pytest.fixture(scope="module")
+def otp26_elixir_tls_peer(tmp_path_factory):
+    """Elixir GenServer over TLS on OTP 26 — exactly what portal runs.
+
+    Difference from otp26_tls_peer: the registered process is a real
+    Elixir GenServer (uses `gen_server:reply/2` for replies, all the
+    OTP machinery) instead of a hand-coded receive loop.
+    """
+    if not _docker_available():
+        pytest.skip("docker not installed")
+    if not shutil.which("openssl"):
+        pytest.skip("openssl not installed")
+    if not _socket_open("127.0.0.1", 4369):
+        pytest.skip("EPMD not running on host")
+
+    import os as _os
+    import tempfile as _tempfile
+
+    home_base = _os.environ.get(
+        "ERLDISTPY_TLS_OTP26_WORKDIR", str(Path.home() / ".erldistpy-test")
+    )
+    Path(home_base).mkdir(parents=True, exist_ok=True)
+    workdir = Path(_tempfile.mkdtemp(prefix="elixir_tls_otp26_", dir=home_base))
+    _os.chmod(workdir, 0o755)
+    host_certs = _gen_certs(workdir)
+    container_certs = {
+        k: v.replace(str(workdir), "/certs") for k, v in host_certs.items()
+    }
+    # Use portal's exact inet_tls.conf shape (permissive verify_fun,
+    # TLS 1.2/1.3 only, secure_renegotiate) so the test reproduces the
+    # exact wire-level conditions of the production failure.
+    _write_ssl_config_portal_match(workdir, container_certs)
+    ssl_config_container = "/certs/ssl_dist_portal.config"
+
+    boot_script = workdir / "boot.exs"
+    boot_script.write_text(ELIXIR_GENSERVER_BOOT)
+
+    container_name = f"erldistpy-elixir-tls-{int(time.time())}"
+    container_log = workdir / "container.log"
+
+    # Long node names (--name) match production. Short names (--sname)
+    # would route differently through the dist driver and may not
+    # reproduce the same failure mode.
+    long_node_name = f"{ELIXIR_TLS_SNAME}@127.0.0.1"
+
+    proc = subprocess.Popen(
+        [
+            "docker", "run", "--rm",
+            "--name", container_name,
+            "--network", "host",
+            "-v", f"{workdir}:/certs:ro",
+            "elixir:1.16-otp-25",
+            "elixir",
+            "--name", long_node_name,
+            "--cookie", ELIXIR_TLS_COOKIE,
+            "--erl", f"-proto_dist inet_tls -ssl_dist_optfile {ssl_config_container}",
+            "/certs/boot.exs",
+        ],
+        stdout=open(container_log, "w"),
+        stderr=subprocess.STDOUT,
+    )
+
+    deadline = time.monotonic() + 25.0
+    ready = False
+    while time.monotonic() < deadline:
+        time.sleep(0.3)
+        if _epmd_node_present(ELIXIR_TLS_SNAME):
+            time.sleep(0.5)  # Elixir GenServer needs an extra beat to register
+            ready = True
+            break
+    if not ready:
+        subprocess.run(["docker", "rm", "-f", container_name], capture_output=True)
+        proc.terminate()
+        proc.wait(timeout=3)
+        pytest.skip(f"Elixir TLS docker node {ELIXIR_TLS_SNAME} did not register")
+
+    yield ELIXIR_TLS_SNAME, ELIXIR_TLS_COOKIE, host_certs, container_log
+
+    subprocess.run(["docker", "rm", "-f", container_name], capture_output=True)
+    proc.terminate()
+    try:
+        proc.wait(timeout=3)
+    except subprocess.TimeoutExpired:
+        proc.kill()
+
+
+def test_call_elixir_genserver_over_tls_otp26(otp26_elixir_tls_peer):
+    """Repro of the prod failure: erldistpy calls an Elixir GenServer over TLS.
+
+    Difference from test_call_over_tls_otp26: peer is a real
+    GenServer, not a hand-coded receive. Replies go through
+    :gen_server.reply/2 which sends via :erlang.send/2 — same path as
+    Wallet.Bridge in production.
+    """
+    sname, cookie, certs, container_log = otp26_elixir_tls_peer
+    ctx = make_dist_tls_context(
+        cert=certs["client_cert"],
+        key=certs["client_key"],
+        ca=certs["ca"],
+    )
+    try:
+        with Node(
+            our_name="erldistpy_test_elixir_tls@localhost",
+            peer_name=sname,
+            cookie=cookie,
+            tls_context=ctx,
+        ) as n:
+            assert n.tls is True
+            reply = n.call("gen_target", (Atom("ping"), 7), timeout=5.0)
+            assert reply == (Atom("pong"), 7)
+    finally:
+        # Whether pass or fail, dump container output for diagnosis.
+        try:
+            text = container_log.read_text()
+            print(f"=== container stdout ===\n{text}\n=== end ===")
+        except FileNotFoundError:
+            pass