glm-5.2
fd83fc1685
Three simplifications to the channels spec, all flowing from the review: 1. Substrate simplification (ADR-071 revised): the 9-byte chunk header is used in ALL substrates — in-line (TCP+TLS, WebTransport), native (QUIC bidi streams), and multi-connection. The ChannelsAdapter reads headers off every bidi stream it accepts; the transport's native multiplexing is a performance optimization (independent flow-control windows), not a protocol change. One wire format, one code path, one handler experience. The channel_id in the header is the correlation key across substrates. 2. Stream_type decomposition (ADR-071 revised): every stream_type is unidirectional. Bidirectionality is two stream_types (write + read), not one 'bidirectional' stream_type. Grouped in threes: 0/1/2 = data write/read/err, 3/4/5 = control write/read/err, % 3 formula. This resolves the TTY control channel's 'not actually bidirectional' flaw — control is now 3 (write, client→server) + 4 (read, server→client), each with its own reassembly buffer and EOF. Channel 0 uses [0,1] (call frames bidirectional via 0=in, 1=out). TTY uses [0,1,2,3,4]. ADR-072, 073, 074, 077 updated for the new stream_type assignments. 3. Sub-crate decomposition (ADR-081 new): channels-core (pure multiplexer — wire format, demux/mux, ChannelBidiStreamSource, ChannelManager; depends on alknet-core only, no call dependency, ALPN-blind) / channels-call (channel 0 pre-negotiation + lifecycle op registrations; depends on channels-core + alknet-call) / channels-hub (relay) / channels-worker (ChannelClient). Isolates the call-protocol coupling from the pure multiplexer so the dependency graph is honest. ADR-077 (TTY inside channels) updated: TTY now uses 5 sub-streams [0,1,2,3,4] with control properly bidirectional via 3/4; amends ADR-052's stream_type assignments for direct mode too (direct alknet/tty now uses 0-4, not 0-3). Spec docs updated: channels-wire, channels-connection, channels-adapter, channel-operations, overview, README.
194 lines
9.9 KiB
Markdown
194 lines
9.9 KiB
Markdown
# ADR-077: TTY Inside Channels — Sub-Streams, Not Wire Format
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## Status
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Accepted
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## Context
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ADR-052 defines the alknet-tty wire format: `[stream_type: u8][length: u32
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be][payload]`, a 5-byte chunk header for four sub-streams (stdin/stdout/
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stderr/control) within one bidi stream. This format is stable, implemented
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(`crates/alknet-tty/src/wire.rs`), and used for direct `alknet/tty`
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connections.
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The phase-0 research (`docs/research/alknet-channels/phase-0-findings.md`
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§DP-3, §OQ-CH-02) recommended that the TTY chunk format be "absorbed into
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channels" and that `alknet/tty` remain as a "direct-connect shortcut." But
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the research did not pin what changes in the TTY crate when a TTY session
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runs *inside* a channels connection. This is gap #5 from the architecture
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assessment — a real integration question the research hand-waved.
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The problem: the `TtyAdapter`'s current `handle()` loops `accept_bi()`,
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spawning a `drive_session` task per bidi stream that parses 5-byte TTY
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chunks off the stream. Inside a channels connection, the stream is *already
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de-chunked* by the channels layer's 9-byte format — the handler sees an
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`AsyncRead + AsyncWrite` pair, not a chunk-encoded stream. If the TTY
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adapter tries to parse 5-byte chunks off an already-de-chunked stream, it
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breaks.
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## Decision
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### Two modes for TTY, one adapter
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The `TtyAdapter` operates in two modes, determined by how it receives its
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`Connection`:
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| Mode | When | Wire format | How the adapter gets sub-streams |
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|------|------|-------------|---------------------------------|
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| **Direct (`alknet/tty` ALPN)** | Top-level QUIC/TCP connection with ALPN `alknet/tty` | TTY's 5-byte format (ADR-052, amended — see below) | `accept_bi()` → parse 5-byte chunks → split into stream_types 0-4 |
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| **Inside channels** | `channel/open` with ALPN `alknet/tty` on a channels connection | Channels' 9-byte format (ADR-071) — the channels layer de-chunks | `into_sub_streams()` (ADR-074) → five named handles for stream_types 0-4 |
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In both modes, the `TtyBackend` trait and `TtyHandle` are unchanged
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(ADR-053). The backend allocates a PTY and returns a `TtyHandle`; the
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adapter pumps data between the handle and the sub-streams. The difference is
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only in how the adapter gets the sub-streams — 5-byte chunk parsing (direct)
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vs. `into_sub_streams()` (channels).
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### The control channel is now properly bidirectional
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The phase-0 findings flagged that the TTY control channel "isn't actually
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bidirectional… the adapter ignores Exit from the client." The root cause:
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`stream_type 3` was "bidirectional" — one stream both sides wrote to, which
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is not properly multiplexed.
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ADR-071's stream_type decomposition fixes this: **every stream_type is
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unidirectional.** Control is now two halves:
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| stream_type | direction | purpose |
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|-------------|-----------|---------|
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| 3 | write (client→server) | control in: resize, signal, eof |
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| 4 | read (server→client) | control out: exit, keepalive response |
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The TTY adapter writes resize/signal/eof to `ctrl_in` (stream_type 3) and
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reads exit/keepalive from `ctrl_out` (stream_type 4). Each has its own
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reassembly buffer, its own flow control, its own EOF. The control channel
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is *actually* bidirectional — two unidirectional streams, not one shared
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stream both sides write to.
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This amends ADR-052's stream_type assignments for direct mode too: direct
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`alknet/tty` connections now use stream_types [0, 1, 2, 3, 4] (data in/out/
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err + control in/out), not [0, 1, 2, 3]. The 5-byte format's `stream_type`
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field gains value 4; the `ControlMessage` enum is unchanged (the JSON shape
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is the same; the stream_type it rides on splits from 3 into 3+4).
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### What changes in alknet-tty
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1. **The adapter's session-driving code splits into two entry points:**
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- `drive_session_direct(send, recv, backends, ...)` — the existing path:
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parse 5-byte chunks, split into stream_types 0-4, pump. Used for direct
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`alknet/tty` connections.
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- `drive_session_channels(sub_streams, backends, ...)` — the new path:
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receive `ChannelSubStreams` (five named handles: stdin=SendStream,
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stdout=RecvStream, stderr=Option<RecvStream>, ctrl_in=SendStream,
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ctrl_out=RecvStream), pump directly without chunk parsing. Used when
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the channel's `Connection` is backed by `ChannelBidiStreamSource`.
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2. **The `TtyAdapter::handle()` branches on the `Connection`'s source type.**
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The channels crate's `ChannelBidiStreamSource` is a `BidiStreamSource`
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(ADR-070); the `Connection` wraps it. The adapter detects whether the
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`Connection` is channels-backed (via a downcast or a channels-crate
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extension trait — exact ergonomics per ADR-074's implementation detail)
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and calls `drive_session_channels` instead of `drive_session_direct`.
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**This is the one place alknet-tty knows about channels.** It is a
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branch on the connection source, not a dependency on channels' wire
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format. The branch can be feature-gated (`channels` feature on
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alknet-tty) so the direct-only path has no channels dependency.
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3. **The 5-byte wire format (ADR-052) is unchanged for direct connections.**
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ADR-052's scope is now "the wire format for direct `alknet/tty`
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connections." The channels path does not use it. This amends ADR-052's
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scope — the format is not replaced, it's scoped.
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4. **The control channel works the same in both modes, now properly
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bidirectional.** In direct mode, control-in JSON rides in 5-byte chunks
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with `stream_type=3` and control-out rides with `stream_type=4`. In
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channels mode, control-in rides in 9-byte chunks with `stream_type=3`
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(write) and control-out with `stream_type=4` (read) — but the channels
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layer de-chunks them, so the adapter reads raw JSON bytes from
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`ctrl_in`/`ctrl_out` in both cases. The `ControlMessage` enum (resize,
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signal, eof, exit) is unchanged — the JSON shape is the same; only the
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stream_type assignments change (3 splits into 3+4).
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5. **The exit-chunk-is-last invariant (ADR-055) generalizes.** In direct
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mode, the exit chunk is the last 5-byte chunk on `stream_type=4` (read,
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server→client) before stream close (ADR-055, amended). In channels mode,
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the exit control message is the last data on `stream_type=4` before
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`channel/close` is sent on channel 0 (ADR-073 §channel/close). The
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ordering invariant is the same — exit before close — but the mechanism
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differs: 5-byte chunk ordering on stream_type 4 (direct) vs.
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`stream_type 4` ordering + `channel/close` after pump completion
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(channels, REQ-CH-06).
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### What does NOT change
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- **`TtyBackend` trait, `TtyHandle`, `TtyControl`** (ADR-053) — unchanged.
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Backends don't know about channels or direct mode.
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- **`DockerTtyBackend`, `LocalTtyBackend`** — unchanged. They implement
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`TtyBackend::allocate()` and return a `TtyHandle`.
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- **`ControlMessage` enum** — unchanged. The JSON shape is the same in both
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modes.
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- **The `alknet/tty` ALPN string** — unchanged. Direct connections use it;
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channels `channel/open` requests it.
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### Crate dependency
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`alknet-tty` does **not** depend on `alknet-channels` unconditionally. The
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channels-integration code is behind a `channels` feature on `alknet-tty`.
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When the feature is off, `TtyAdapter` only supports direct mode (the
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existing behavior). When the feature is on, the adapter branches into
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channels mode for channels-backed connections. This preserves ADR-003's
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no-handler-depends-on-another-handler rule for the default build; the
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feature-gated dependency is opt-in, same as `alknet-docker`'s `tty` feature
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(ADR-061).
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## Consequences
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**Positive:**
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- The TTY crate's direct mode is unchanged — existing `alknet/tty`
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deployments (browser terminals over WebSocket, direct QUIC TTY) keep
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working with the 5-byte format.
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- The channels path uses the channels layer's de-chunking — no double-
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chunking (5-byte inside 9-byte). The TTY adapter sees clean sub-streams.
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- The `TtyBackend` trait is insulated — backends don't know which mode the
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adapter is in. Docker, SSH, and local backends work in both modes without
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changes.
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- The control channel and exit-chunk invariant carry forward cleanly — the
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`ControlMessage` enum and ordering semantics are mode-independent.
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**Negative:**
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- `alknet-tty` has two session-driving entry points (`drive_session_direct`
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vs `drive_session_channels`). This is the necessary cost of supporting
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both direct and channels modes without double-chunking. The alternative
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(always use channels format, even for direct) would break existing direct
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deployments and add 4 bytes of overhead per chunk for no benefit.
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- The `channels` feature on `alknet-tty` adds a dependency edge
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(`alknet-tty` → `alknet-channels`, feature-gated). This is the same
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pattern as `alknet-docker`'s `tty` feature (ADR-061) and is opt-in.
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- ADR-052's scope is amended (from "the TTY wire format" to "the TTY wire
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format for direct connections"). This is a scope clarification, not a
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format change — the 5-byte format itself is unchanged.
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## Door type
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**One-way (scope amendment) + two-way (feature gate).** ADR-052's scope
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amendment (direct-only) is one-way — once the channels path exists,
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re-merging the formats would require unifying 5-byte and 9-byte chunk
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handling, which is a rewrite. The `channels` feature gate is two-way — it
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can be removed if channels integration is no longer needed.
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## References
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- ADR-052: alknet-tty wire format (amended — scoped to direct connections)
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- ADR-053: TtyBackend trait and TtyHandle (unchanged by this ADR)
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- ADR-055: exit-chunk-is-last (generalized by this ADR + ADR-073)
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- ADR-057: alknet-tty does not depend on alknet-call (preserved — the
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channels feature is on alknet-channels, not alknet-call)
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- ADR-071: channels wire format (the 9-byte format the channels path uses)
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- ADR-074: ChannelBidiStreamSource / `into_sub_streams` (the accessor the
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channels path uses)
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- ADR-061: DockerTtyBackend in alknet-docker (the feature-gated dependency
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pattern this ADR mirrors)
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- `docs/research/alknet-channels/phase-0-findings.md` §DP-3, §OQ-CH-02,
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§Relationship to Existing Crates / alknet-tty |