Against real-world OTP 26 peers the v6 handshake "succeeded" but the
first REG_SEND silently dropped on the peer side — peer accepted the
connection then closed the link with no bytes when we tried to call a
registered process. Hit during MPS↔portal wallet RPC smoke test.
Root cause: OTP 25+ requires DFLAG_MANDATORY_25_DIGEST to be present
in our advertised flag set. The digest is the hash of the OTP-25
mandatory flag set; without it the peer's dist driver loses confidence
in the negotiation and drops messages from us without surfacing an
error.
Adds DFLAG_MANDATORY_25_DIGEST to DEFAULT_FLAGS. Also extends
_decode_message to accept both legacy pass-through (0x70 ...) and
dist-header framing (0x83 0x44 0x00 ...) on receive — modern OTP may
send dist-headed messages even when we didn't negotiate
DFLAG_DIST_HDR_ATOM_CACHE. Fragments (0x83 0x45 / 0x83 0x46) still
TODO; we surface a clear ChannelError instead of silent corruption.
Send side still uses pass-through framing — we don't yet implement
the atom-cache encode/decode that DFLAG_DIST_HDR_ATOM_CACHE would
require. Peer routes our pass-through sends without issue.
122/122 tests pass including live integration against a local Erlang
node and the TLS dist suite.
Channel wraps the post-handshake socket and carries 4-byte length-
prefixed distribution messages: pass-through byte ('p') + ETF control
tuple + optional payload term.
API surface:
send_raw / recv_raw -- raw 4-byte framed bytes, empty == net_tick
send_tick -- send keepalive frame
send_control / recv_message -- structured control + payload
send_reg_send -- helper for the REG_SEND case (FromPid,
registered name, payload)
recv_message() transparently skips inbound ticks; callers wanting tick
awareness use recv_raw().
etf.decode_term(data, offset) exposed as a streaming decoder so the
channel can read control + payload back-to-back from one frame body.
Tests:
- pure encode/decode round-trips
- socketpair tests for framing, ticks, helper signatures
- live end-to-end against an erl node with a registered echo process:
EPMD -> handshake -> REG_SEND -> recv reply, payload matches
- boot script writes a /tmp ready-flag after registering `echo`;
fixture waits for both EPMD registration AND the flag to dodge
the race where EPMD registers the node before -eval runs
101 tests green, lint clean.
