core: make the engine multi-transport (Transport trait + inbound dedup)

Restructure tethercore so the engine owns a list of pluggable Transports
instead of a hardcoded SSE path. SSE moves behind the trait unchanged;
RTC (webrtc-rs data channel) and BT (BLE GATT) now have a clean socket to
implement the same interface — start(sink, status) / publish(msg).

The engine multiplexes outbound across all transports and de-dups inbound
(keyed on from+ts+text) so a clipboard echoed over two channels surfaces
once. No-op with SSE alone; required the moment RTC lands.

FFI surface is unchanged (Engine::new / start / send / stop, Message,
MessageHandler) — no binding regen needed. Round-trip example still green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/src/lib.rs

@@ -1,23 +1,26 @@
 //! tethercore — the shared sync engine.
 //!
-//! Owns the wire protocol (codec + SSE client + reconnect/backoff + send) and
-//! nothing else: no clipboard, no UI. Those stay native on each platform and
-//! talk to this engine through a 4-method seam exported via UniFFI:
+//! Owns the wire protocol (codec + transports + reconnect/backoff + send) and
+//! nothing else: no clipboard, no UI. Exposed via UniFFI behind a 4-method seam:
 //!
 //!   Engine::new(server, room, from, source)
 //!   engine.start(handler)   // inbound clipboard → handler.on_message
 //!   engine.send(text)
 //!   engine.stop()
 //!
-//! Desktop links it directly; iOS binds the .xcframework, Android the .aar.
-//! RTC mesh (the `signal` envelope) is intentionally out of scope for v1.
+//! Transports are pluggable (see `Transport`). SSE ships today; RTC and BT slot
+//! in beside it without touching the engine. The engine multiplexes outbound
+//! across all transports and de-duplicates inbound so the same clipboard
+//! arriving over two channels surfaces once.
 
+use std::collections::VecDeque;
 use std::sync::atomic::{AtomicBool, Ordering};
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
 use std::time::Duration;
 
 use futures_util::StreamExt;
 use serde::{Deserialize, Serialize};
+use tokio::runtime::Handle;
 
 uniffi::setup_scaffolding!();
 
@@ -44,13 +47,11 @@ pub struct Message {
 }
 
 /// Implemented on the foreign side (Swift/Kotlin) or natively (desktop agent).
-/// All callbacks fire on the engine's runtime thread — marshal to the UI thread
+/// Callbacks fire on the engine's runtime thread — marshal to the UI thread
 /// on the consumer side.
 #[uniffi::export(callback_interface)]
 pub trait MessageHandler: Send + Sync {
-    /// A clipboard message from another device in the room (self-echo filtered).
     fn on_message(&self, msg: Message);
-    /// Connection state changed: true once subscribed, false on drop/error.
     fn on_status(&self, connected: bool);
 }
 
@@ -62,19 +63,34 @@ struct Config {
     source: String,
 }
 
+/// Sink/Status closures a transport calls for inbound traffic and link state.
+type Sink = Arc<dyn Fn(Message) + Send + Sync>;
+type Status = Arc<dyn Fn(bool) + Send + Sync>;
+
+/// A pluggable delivery channel. The engine owns one or more; today just SSE.
+/// RTC (WebRTC data channel) and BT (BLE GATT) implement the same trait.
+trait Transport: Send + Sync {
+    fn name(&self) -> &'static str;
+    /// Spawn the inbound loop on `rt`; deliver received messages via `sink`
+    /// and link transitions via `status`. Runs until `running` clears.
+    fn start(self: Arc<Self>, rt: Handle, running: Arc<AtomicBool>, sink: Sink, status: Status);
+    /// Publish an outbound message. Fire-and-forget.
+    fn publish(&self, rt: &Handle, msg: Message);
+}
+
 #[derive(uniffi::Object)]
 pub struct Engine {
     cfg: Config,
-    http: reqwest::Client,
     rt: Arc<tokio::runtime::Runtime>,
     running: Arc<AtomicBool>,
+    transports: Vec<Arc<dyn Transport>>,
+    dedup: Arc<Mutex<Dedup>>,
 }
 
 #[uniffi::export]
 impl Engine {
     /// `server` is the bus base URL, e.g. "http://192.168.11.233:8765".
-    /// `from` is this device's stable id (used for echo suppression);
-    /// `source` is a human label like "macos" / "iphone".
+    /// `from` is this device's stable id (echo suppression); `source` a label.
     #[uniffi::constructor]
     pub fn new(server: String, room: String, from: String, source: String) -> Arc<Self> {
         let rt = tokio::runtime::Builder::new_multi_thread()
@@ -82,125 +98,198 @@ impl Engine {
             .enable_all()
             .build()
             .expect("tokio runtime");
-        Arc::new(Self {
-            cfg: Config { server, room, from, source },
+        let cfg = Config { server, room, from, source };
+        let transports: Vec<Arc<dyn Transport>> = vec![Arc::new(SseTransport {
+            cfg: cfg.clone(),
             http: reqwest::Client::new(),
+        })];
+        Arc::new(Self {
+            cfg,
             rt: Arc::new(rt),
             running: Arc::new(AtomicBool::new(false)),
+            transports,
+            dedup: Arc::new(Mutex::new(Dedup::default())),
         })
     }
 
-    /// Begin streaming. Idempotent: a second call while running is a no-op.
+    /// Begin streaming on every transport. Idempotent while already running.
     pub fn start(&self, handler: Box<dyn MessageHandler>) {
         if self.running.swap(true, Ordering::SeqCst) {
             return;
         }
         let handler: Arc<dyn MessageHandler> = Arc::from(handler);
-        let http = self.http.clone();
-        let cfg = self.cfg.clone();
-        let running = self.running.clone();
-        self.rt.spawn(async move {
-            run_loop(http, cfg, running, handler).await;
-        });
-    }
-
-    /// Publish clipboard text to the room. Fire-and-forget.
-    pub fn send(&self, text: String) {
-        let http = self.http.clone();
-        let cfg = self.cfg.clone();
-        self.rt.spawn(async move {
-            if let Err(e) = post(&http, &cfg, &text).await {
-                eprintln!("tethercore: send failed: {e}");
+        let dedup = self.dedup.clone();
+        let h_msg = handler.clone();
+        let sink: Sink = Arc::new(move |m| {
+            if dedup.lock().unwrap().seen(&m) {
+                return; // same clipboard already delivered via another transport
             }
+            h_msg.on_message(m);
         });
+        let h_status = handler.clone();
+        let status: Status = Arc::new(move |c| h_status.on_status(c));
+
+        for t in &self.transports {
+            t.clone().start(
+                self.rt.handle().clone(),
+                self.running.clone(),
+                sink.clone(),
+                status.clone(),
+            );
+        }
     }
 
-    /// Stop streaming. The reconnect loop exits at its next checkpoint.
+    /// Publish clipboard text to the room across all transports.
+    pub fn send(&self, text: String) {
+        let msg = Message {
+            kind: "clipboard".into(),
+            text,
+            from: self.cfg.from.clone(),
+            to: String::new(),
+            role: String::new(),
+            source: self.cfg.source.clone(),
+            room: self.cfg.room.clone(),
+            ts: 0, // server stamps authoritative ts
+        };
+        let rt = self.rt.handle();
+        for t in &self.transports {
+            t.publish(rt, msg.clone());
+        }
+    }
+
+    /// Stop all transports at their next checkpoint.
     pub fn stop(&self) {
         self.running.store(false, Ordering::SeqCst);
     }
 }
 
-async fn run_loop(
-    http: reqwest::Client,
-    cfg: Config,
-    running: Arc<AtomicBool>,
-    handler: Arc<dyn MessageHandler>,
-) {
-    let min = Duration::from_millis(500);
-    let max = Duration::from_secs(10);
-    let mut backoff = min;
-    while running.load(Ordering::SeqCst) {
-        match connect_stream(&http, &cfg, &running, &handler).await {
-            Ok(()) => backoff = min, // clean EOF — reconnect promptly
-            Err(e) => {
-                handler.on_status(false);
-                eprintln!("tethercore: stream error: {e}");
-            }
-        }
-        if running.load(Ordering::SeqCst) {
-            tokio::time::sleep(backoff).await;
-            backoff = (backoff * 2).min(max);
-        }
-    }
-    handler.on_status(false);
+/// Short-window de-dup keyed on the fields the server preserves. Once RTC lands,
+/// a clipboard echoed over both SSE and the data channel collapses to one.
+#[derive(Default)]
+struct Dedup {
+    recent: VecDeque<(String, i64, String)>,
 }
 
-async fn connect_stream(
-    http: &reqwest::Client,
-    cfg: &Config,
-    running: &AtomicBool,
-    handler: &Arc<dyn MessageHandler>,
-) -> Result<(), String> {
-    let url = format!(
-        "{}/api/stream?room={}",
-        cfg.server.trim_end_matches('/'),
-        cfg.room
-    );
-    let resp = http
-        .get(&url)
-        .header("X-Tether-Client", &cfg.from)
-        .send()
-        .await
-        .map_err(|e| e.to_string())?;
-    if !resp.status().is_success() {
-        return Err(format!("HTTP {}", resp.status()));
+impl Dedup {
+    fn seen(&mut self, m: &Message) -> bool {
+        let key = (m.from.clone(), m.ts, m.text.clone());
+        if self.recent.contains(&key) {
+            return true;
+        }
+        self.recent.push_back(key);
+        if self.recent.len() > 256 {
+            self.recent.pop_front();
+        }
+        false
     }
-    handler.on_status(true);
+}
 
-    // Byte-buffer the SSE stream; lines split on '\n' (ASCII, so chunk-safe).
-    let mut stream = resp.bytes_stream();
-    let mut buf: Vec<u8> = Vec::new();
-    let mut data: Vec<u8> = Vec::new();
-    while let Some(chunk) = stream.next().await {
-        if !running.load(Ordering::SeqCst) {
-            return Ok(());
-        }
-        buf.extend_from_slice(&chunk.map_err(|e| e.to_string())?);
-        while let Some(nl) = buf.iter().position(|&b| b == b'\n') {
-            let mut line: Vec<u8> = buf.drain(..=nl).collect();
-            line.pop(); // '\n'
-            if line.last() == Some(&b'\r') {
-                line.pop();
-            }
-            if line.is_empty() {
-                // Event boundary: flush accumulated data field.
-                if !data.is_empty() {
-                    if let Ok(m) = serde_json::from_slice::<Message>(&data) {
-                        if m.from != cfg.from {
-                            handler.on_message(m);
-                        }
-                    }
-                    data.clear();
-                }
-            } else if let Some(rest) = line.strip_prefix(b"data:") {
-                let rest = rest.strip_prefix(b" ").unwrap_or(rest);
-                data.extend_from_slice(rest);
-            }
-            // ':' comments (keepalives) and event:/id: lines are ignored.
-        }
+// ---------------------------------------------------------------------------
+// SSE transport — the reliable floor: server relay over text/event-stream.
+// ---------------------------------------------------------------------------
+
+struct SseTransport {
+    cfg: Config,
+    http: reqwest::Client,
+}
+
+impl Transport for SseTransport {
+    fn name(&self) -> &'static str {
+        "sse"
+    }
+
+    fn start(self: Arc<Self>, rt: Handle, running: Arc<AtomicBool>, sink: Sink, status: Status) {
+        let this = self;
+        rt.spawn(async move {
+            let min = Duration::from_millis(500);
+            let max = Duration::from_secs(10);
+            let mut backoff = min;
+            while running.load(Ordering::SeqCst) {
+                match this.stream_once(&running, &sink, &status).await {
+                    Ok(()) => backoff = min, // clean EOF — reconnect promptly
+                    Err(e) => {
+                        status(false);
+                        eprintln!("tethercore[{}]: {e}", this.name());
+                    }
+                }
+                if running.load(Ordering::SeqCst) {
+                    tokio::time::sleep(backoff).await;
+                    backoff = (backoff * 2).min(max);
+                }
+            }
+            status(false);
+        });
+    }
+
+    fn publish(&self, rt: &Handle, msg: Message) {
+        let http = self.http.clone();
+        let cfg = self.cfg.clone();
+        let name = self.name();
+        rt.spawn(async move {
+            if let Err(e) = post(&http, &cfg, &msg.text).await {
+                eprintln!("tethercore[{name}]: send failed: {e}");
+            }
+        });
+    }
+}
+
+impl SseTransport {
+    async fn stream_once(
+        &self,
+        running: &AtomicBool,
+        sink: &Sink,
+        status: &Status,
+    ) -> Result<(), String> {
+        let url = format!(
+            "{}/api/stream?room={}",
+            self.cfg.server.trim_end_matches('/'),
+            self.cfg.room
+        );
+        let resp = self
+            .http
+            .get(&url)
+            .header("X-Tether-Client", &self.cfg.from)
+            .send()
+            .await
+            .map_err(|e| e.to_string())?;
+        if !resp.status().is_success() {
+            return Err(format!("HTTP {}", resp.status()));
+        }
+        status(true);
+
+        // Byte-buffer the stream; lines split on '\n' (ASCII, so chunk-safe).
+        let mut stream = resp.bytes_stream();
+        let mut buf: Vec<u8> = Vec::new();
+        let mut data: Vec<u8> = Vec::new();
+        while let Some(chunk) = stream.next().await {
+            if !running.load(Ordering::SeqCst) {
+                return Ok(());
+            }
+            buf.extend_from_slice(&chunk.map_err(|e| e.to_string())?);
+            while let Some(nl) = buf.iter().position(|&b| b == b'\n') {
+                let mut line: Vec<u8> = buf.drain(..=nl).collect();
+                line.pop(); // '\n'
+                if line.last() == Some(&b'\r') {
+                    line.pop();
+                }
+                if line.is_empty() {
+                    if !data.is_empty() {
+                        if let Ok(m) = serde_json::from_slice::<Message>(&data) {
+                            if m.from != self.cfg.from {
+                                sink(m); // engine de-dups + forwards to handler
+                            }
+                        }
+                        data.clear();
+                    }
+                } else if let Some(rest) = line.strip_prefix(b"data:") {
+                    let rest = rest.strip_prefix(b" ").unwrap_or(rest);
+                    data.extend_from_slice(rest);
+                }
+                // ':' comments (keepalives) and event:/id: lines are ignored.
+            }
+        }
+        Ok(())
     }
-    Ok(())
 }
 
 async fn post(http: &reqwest::Client, cfg: &Config, text: &str) -> Result<(), String> {