Let emulator tell debugger to stop

src/emulator.rs

@@ -151,6 +151,7 @@ pub struct Emulator {
     linked: Arc<AtomicBool>,
     renderers: HashMap<SimId, TextureSink>,
     messages: HashMap<SimId, mpsc::Sender<Toast>>,
+    debuggers: HashMap<SimId, DebugInfo>,
     eye_contents: Vec<u8>,
     audio_samples: Vec<f32>,
 }
@@ -174,6 +175,7 @@ impl Emulator {
             linked,
             renderers: HashMap::new(),
             messages: HashMap::new(),
+            debuggers: HashMap::new(),
             eye_contents: vec![0u8; 384 * 224 * 2],
             audio_samples: vec![0.0; EXPECTED_FRAME_SIZE],
         })
@@ -288,6 +290,48 @@ impl Emulator {
         Ok(())
     }
 
+    fn start_debugging(&mut self, sim_id: SimId, sender: DebugSender) {
+        let debug = DebugInfo {
+            sender,
+            stop_reason: Some(DebugStopReason::Trapped),
+        };
+        self.debuggers.insert(sim_id, debug);
+        self.state
+            .store(EmulatorState::Debugging, Ordering::Release);
+    }
+
+    fn stop_debugging(&mut self, sim_id: SimId) {
+        self.debuggers.remove(&sim_id);
+        if self.debuggers.is_empty() {
+            self.state.store(EmulatorState::Running, Ordering::Release);
+        }
+    }
+
+    fn debug_interrupt(&mut self, sim_id: SimId) {
+        let Some(debugger) = self.debuggers.get_mut(&sim_id) else {
+            self.stop_debugging(sim_id);
+            return;
+        };
+        if !matches!(debugger.stop_reason, Some(DebugStopReason::Trapped)) {
+            debugger.stop_reason = Some(DebugStopReason::Trapped);
+            if debugger
+                .sender
+                .send(DebugEvent::Stopped(DebugStopReason::Trapped))
+                .is_err()
+            {
+                self.stop_debugging(sim_id);
+            }
+        }
+    }
+
+    fn debug_continue(&mut self, sim_id: SimId) {
+        let Some(debugger) = self.debuggers.get_mut(&sim_id) else {
+            self.stop_debugging(sim_id);
+            return;
+        };
+        debugger.stop_reason = None;
+    }
+
     pub fn run(&mut self) {
         loop {
             let idle = self.tick();
@@ -320,8 +364,14 @@ impl Emulator {
         let p1_state = self.sim_state[SimId::Player1.to_index()].load(Ordering::Acquire);
         let p2_state = self.sim_state[SimId::Player2.to_index()].load(Ordering::Acquire);
         let state = self.state.load(Ordering::Acquire);
-        let p1_running = state == EmulatorState::Running && p1_state == SimState::Ready;
+        // Don't emulate if the state is "paused", or if any sim is paused in the debugger
-        let p2_running = state == EmulatorState::Running && p2_state == SimState::Ready;
+        let running = match state {
+            EmulatorState::Paused => false,
+            EmulatorState::Running => true,
+            EmulatorState::Debugging => self.debuggers.values().all(|d| d.stop_reason.is_none()),
+        };
+        let p1_running = running && p1_state == SimState::Ready;
+        let p2_running = running && p2_state == SimState::Ready;
         let mut idle = !p1_running && !p2_running;
         if p1_running && p2_running {
             Sim::emulate_many(&mut self.sims);
@@ -405,6 +455,18 @@ impl Emulator {
             EmulatorCommand::Resume => {
                 self.resume_sims();
             }
+            EmulatorCommand::StartDebugging(sim_id, debugger) => {
+                self.start_debugging(sim_id, debugger);
+            }
+            EmulatorCommand::StopDebugging(sim_id) => {
+                self.stop_debugging(sim_id);
+            }
+            EmulatorCommand::DebugInterrupt(sim_id) => {
+                self.debug_interrupt(sim_id);
+            }
+            EmulatorCommand::DebugContinue(sim_id) => {
+                self.debug_continue(sim_id);
+            }
             EmulatorCommand::ReadRegister(sim_id, register, done) => {
                 let Some(sim) = self.sims.get_mut(sim_id.to_index()) else {
                     return;
@@ -476,6 +538,10 @@ pub enum EmulatorCommand {
     StopSecondSim,
     Pause,
     Resume,
+    StartDebugging(SimId, DebugSender),
+    StopDebugging(SimId),
+    DebugInterrupt(SimId),
+    DebugContinue(SimId),
     ReadRegister(SimId, VBRegister, oneshot::Sender<u32>),
     ReadMemory(SimId, Range<u32>, Vec<u8>, oneshot::Sender<Vec<u8>>),
     SetAudioEnabled(bool, bool),
@@ -499,6 +565,24 @@ pub enum SimState {
 pub enum EmulatorState {
     Paused,
     Running,
+    Debugging,
+}
+
+type DebugSender = tokio::sync::mpsc::UnboundedSender<DebugEvent>;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum DebugStopReason {
+    // The debugger told us to pause
+    Trapped,
+}
+
+struct DebugInfo {
+    sender: DebugSender,
+    stop_reason: Option<DebugStopReason>,
+}
+
+pub enum DebugEvent {
+    Stopped(DebugStopReason),
 }
 
 #[derive(Clone)]

src/gdbserver.rs

@@ -1,22 +1,19 @@
 use anyhow::{bail, Result};
 use registers::REGISTERS;
-use request::{Request, RequestKind};
+use request::{Request, RequestKind, RequestSource};
 use response::Response;
 use std::{
     sync::{Arc, Mutex},
     thread,
 };
 use tokio::{
-    io::{AsyncWriteExt as _, BufReader, BufWriter},
+    io::{AsyncWriteExt as _, BufReader},
-    net::{
+    net::{TcpListener, TcpStream},
-        tcp::{OwnedReadHalf, OwnedWriteHalf},
-        TcpListener, TcpStream,
-    },
     select,
-    sync::oneshot,
+    sync::{mpsc, oneshot},
 };
 
-use crate::emulator::{EmulatorClient, EmulatorCommand, SimId};
+use crate::emulator::{DebugEvent, DebugStopReason, EmulatorClient, EmulatorCommand, SimId};
 
 mod registers;
 mod request;
@@ -81,8 +78,8 @@ async fn run_server(
     let Some(stream) = try_connect(port, status).await else {
         return;
     };
-    let connection = GdbConnection::new(sim_id, client, stream);
+    let mut connection = GdbConnection::new(sim_id, client);
-    match connection.run().await {
+    match connection.run(stream).await {
         Ok(()) => {
             *status.lock().unwrap() = GdbServerStatus::Stopped;
         }
@@ -130,191 +127,191 @@ impl GdbServerStatus {
 struct GdbConnection {
     sim_id: SimId,
     client: EmulatorClient,
-    stream_in: BufReader<OwnedReadHalf>,
-    stream_out: BufWriter<OwnedWriteHalf>,
     ack_messages: bool,
-    request_buf: Vec<u8>,
+    stop_reason: Option<DebugStopReason>,
     response_buf: Option<Vec<u8>>,
     memory_buf: Option<Vec<u8>>,
 }
 
 impl GdbConnection {
-    fn new(sim_id: SimId, client: EmulatorClient, stream: TcpStream) -> Self {
+    fn new(sim_id: SimId, client: EmulatorClient) -> Self {
-        let (rx, tx) = stream.into_split();
         Self {
             sim_id,
             client,
-            stream_in: BufReader::new(rx),
-            stream_out: BufWriter::new(tx),
             ack_messages: true,
-            request_buf: vec![],
+            stop_reason: None,
             response_buf: None,
             memory_buf: None,
         }
     }
-    async fn run(mut self) -> Result<()> {
+    async fn run(&mut self, stream: TcpStream) -> Result<()> {
-        println!("Connected for {}", self.sim_id);
+        let (debug_sink, mut debug_source) = mpsc::unbounded_channel();
-        self.client.send_command(EmulatorCommand::Pause);
+        let (rx, mut tx) = stream.into_split();
+        let mut request_source = RequestSource::new(BufReader::new(rx));
+        self.client
+            .send_command(EmulatorCommand::StartDebugging(self.sim_id, debug_sink));
         loop {
-            let mut req = Request::read(&mut self.stream_in, &mut self.request_buf).await?;
+            let response = select! {
-            println!("received {:02x?}", req);
+                maybe_event = debug_source.recv() => {
-
+                    let Some(event) = maybe_event else {
-            if req.kind == RequestKind::Signal {
+                        // debugger has stopped running
-                self.client.send_command(EmulatorCommand::Pause);
+                        break;
-                let res = self
-                    .response()
-                    .write_str("T05;thread:p1.t1;threads:p1.t1;reason:trap;");
-                self.send(res).await?;
-                continue;
-            }
-
-            if req.match_str("QStartNoAckMode") {
-                let res = self.response().write_str("OK");
-                self.send(res).await?;
-                self.ack_messages = false;
-            } else if req.match_str("qSupported:") {
-                let res = self
-                    .response()
-                    .write_str("multiprocess+;swbreak+;vContSupported+;PacketSize=10000");
-                self.send(res).await?;
-            } else if req.match_str("QThreadSuffixSupported")
-                || req.match_str("QListThreadsInStopReply")
-                || req.match_str("QEnableErrorStrings")
-            {
-                let res = self.response().write_str("OK");
-                self.send(res).await?;
-            } else if req.match_str("qHostInfo") {
-                let res = self.response().write_str(&format!(
-                    "triple:{};endian:little;ptrsize:4;",
-                    hex::encode("v810-unknown-vb")
-                ));
-                self.send(res).await?;
-            } else if req.match_str("qProcessInfo") {
-                let res = self.response().write_str(&format!(
-                    "pid:1;triple:{};endian:little;ptrsize:4;",
-                    hex::encode("v810-unknown-vb")
-                ));
-                self.send(res).await?;
-            } else if req.match_str("qRegisterInfo") {
-                let mut get_reg_info = || {
-                    let register = req.match_hex::<usize>()?;
-                    REGISTERS.get(register)
-                };
-                let Some(reg_info) = get_reg_info() else {
-                    self.send_empty().await?;
-                    continue;
-                };
-                let res = self.response().write_str(&reg_info.to_description());
-                self.send(res).await?;
-            } else if req.match_str("vCont?") {
-                let res = self.response().write_str("vCont;c;");
-                self.send(res).await?;
-            } else if req.match_str("qC") {
-                // The v810 has no threads, so report that the "current thread" is 1.
-                let res = self.response().write_str("QCp1.t1");
-                self.send(res).await?;
-            } else if req.match_str("qfThreadInfo") {
-                let res = self.response().write_str("mp1.t1");
-                self.send(res).await?;
-            } else if req.match_str("qsThreadInfo") {
-                let res = self.response().write_str("l");
-                self.send(res).await?;
-            } else if req.match_str("k") {
-                bail!("debug process was killed");
-            } else if req.match_str("?") {
-                let res = self.response().write_str("T00;thread:p1.t1;threads:p1.t1;");
-                self.send(res).await?;
-            } else if req.match_str("c") || req.match_str("vCont;c:") {
-                self.client.send_command(EmulatorCommand::Resume);
-                // Don't send a response until we hit a breakpoint or get interrupted
-            } else if req.match_str("p") {
-                let mut read_register = || {
-                    let register_index = req.match_hex::<usize>()?;
-                    let register = REGISTERS.get(register_index)?.to_vb_register();
-                    let (tx, rx) = ::oneshot::channel();
-                    self.client.send_command(EmulatorCommand::ReadRegister(
-                        self.sim_id,
-                        register,
-                        tx,
-                    ));
-                    rx.recv().ok()
-                };
-                let Some(value) = read_register() else {
-                    self.send_empty().await?;
-                    continue;
-                };
-                let res = self.response().write_hex(value);
-                self.send(res).await?;
-            } else if req.match_str("m") {
-                let mut read_memory = || {
-                    let start = req.match_hex::<u32>()?;
-                    if !req.match_str(",") {
-                        return None;
                     };
-                    let size = req.match_hex::<u32>()?;
+                    self.handle_event(event)
-                    let mut buf = self.memory_buf.take().unwrap_or_default();
-                    buf.clear();
-                    let (tx, rx) = ::oneshot::channel();
-                    self.client.send_command(EmulatorCommand::ReadMemory(
-                        self.sim_id,
-                        start..(start + size),
-                        buf,
-                        tx,
-                    ));
-                    rx.recv().ok()
-                };
-                let Some(memory) = read_memory() else {
-                    self.send_empty().await?;
-                    continue;
-                };
-                let mut res = self.response();
-                for byte in &memory {
-                    res = res.write_hex(*byte);
                 }
-                self.memory_buf = Some(memory);
+                maybe_request = request_source.recv() => {
-                self.send(res).await?;
+                    let req = maybe_request?;
-            } else if req.match_str("x") {
+                    self.handle_request(req)?
-                let mut read_memory = || {
+                }
-                    let start = req.match_hex::<u32>()?;
+            };
-                    if !req.match_str(",") {
+            if let Some(res) = response {
-                        return None;
+                let buffer = res.finish();
-                    };
+                match std::str::from_utf8(&buffer) {
-                    let size = req.match_hex::<u32>()?;
+                    Ok(text) => println!("response: {text}"),
-                    let mut buf = self.memory_buf.take().unwrap_or_default();
+                    Err(_) => println!("response: {buffer:02x?}"),
-                    buf.clear();
+                }
-                    let (tx, rx) = ::oneshot::channel();
+                tx.write_all(&buffer).await?;
-                    self.client.send_command(EmulatorCommand::ReadMemory(
+                self.response_buf = Some(buffer);
-                        self.sim_id,
+                tx.flush().await?;
-                        start..(start + size),
+            }
-                        buf,
+        }
-                        tx,
+        Ok(())
-                    ));
+    }
-                    rx.recv().ok()
+
-                };
+    fn handle_event(&mut self, event: DebugEvent) -> Option<Response> {
-                let Some(memory) = read_memory() else {
+        let res = match event {
-                    self.send_empty().await?;
+            DebugEvent::Stopped(reason) => {
-                    continue;
+                if self.stop_reason.is_some_and(|r| r == reason) {
+                    return None;
+                }
+                self.stop_reason = Some(reason);
+                self.response()
+                    .write_str(debug_stop_reason_string(self.stop_reason))
+            }
+        };
+        Some(res)
+    }
+
+    fn handle_request(&mut self, mut req: Request<'_>) -> Result<Option<Response>> {
+        println!("received {:02x?}", req);
+
+        if req.kind == RequestKind::Signal {
+            self.client
+                .send_command(EmulatorCommand::DebugInterrupt(self.sim_id));
+            return Ok(None); // we'll send a message when the emulator reports it has stopped
+        }
+
+        let res = if req.match_str("QStartNoAckMode") {
+            let res = self.response().write_str("OK");
+            self.ack_messages = false;
+            res
+        } else if req.match_str("qSupported:") {
+            self.response()
+                .write_str("multiprocess+;swbreak+;vContSupported+;PacketSize=10000")
+        } else if req
+            .match_some_str([
+                "QThreadSuffixSupported",
+                "QListThreadsInStopReply",
+                "QEnableErrorStrings",
+            ])
+            .is_some()
+        {
+            self.response().write_str("OK")
+        } else if req.match_str("qHostInfo") {
+            self.response().write_str(&format!(
+                "triple:{};endian:little;ptrsize:4;",
+                hex::encode("v810-unknown-vb")
+            ))
+        } else if req.match_str("qProcessInfo") {
+            self.response().write_str(&format!(
+                "pid:1;triple:{};endian:little;ptrsize:4;",
+                hex::encode("v810-unknown-vb")
+            ))
+        } else if req.match_str("qRegisterInfo") {
+            let mut get_reg_info = || {
+                let register = req.match_hex::<usize>()?;
+                REGISTERS.get(register)
+            };
+            if let Some(reg_info) = get_reg_info() {
+                self.response().write_str(&reg_info.to_description())
+            } else {
+                self.response()
+            }
+        } else if req.match_str("vCont?") {
+            self.response().write_str("vCont;c;")
+        } else if req.match_str("qC") {
+            // The v810 has no threads, so report that the "current thread" is 1.
+            self.response().write_str("QCp1.t1")
+        } else if req.match_str("qfThreadInfo") {
+            self.response().write_str("mp1.t1")
+        } else if req.match_str("qsThreadInfo") {
+            self.response().write_str("l")
+        } else if req.match_str("k") {
+            bail!("debug process was killed");
+        } else if req.match_str("?") {
+            self.response()
+                .write_str(debug_stop_reason_string(self.stop_reason))
+        } else if req.match_some_str(["c", "vCont;c:"]).is_some() {
+            self.client
+                .send_command(EmulatorCommand::DebugContinue(self.sim_id));
+            self.stop_reason = None;
+            // Don't send a response until we hit a breakpoint or get interrupted
+            return Ok(None);
+        } else if req.match_str("p") {
+            let mut read_register = || {
+                let register_index = req.match_hex::<usize>()?;
+                let register = REGISTERS.get(register_index)?.to_vb_register();
+                let (tx, rx) = ::oneshot::channel();
+                self.client
+                    .send_command(EmulatorCommand::ReadRegister(self.sim_id, register, tx));
+                rx.recv().ok()
+            };
+            if let Some(value) = read_register() {
+                self.response().write_hex(value)
+            } else {
+                self.response()
+            }
+        } else if let Some(op) = req.match_some_str(["m", "x"]) {
+            let mut read_memory = || {
+                let start = req.match_hex::<u32>()?;
+                if !req.match_str(",") {
+                    return None;
                 };
+                let size = req.match_hex::<u32>()?;
+                let mut buf = self.memory_buf.take().unwrap_or_default();
+                buf.clear();
+                let (tx, rx) = ::oneshot::channel();
+                self.client.send_command(EmulatorCommand::ReadMemory(
+                    self.sim_id,
+                    start..(start + size),
+                    buf,
+                    tx,
+                ));
+                rx.recv().ok()
+            };
+            if let Some(memory) = read_memory() {
                 let mut res = self.response();
-                if memory.is_empty() {
+                if op == "m" {
-                    res = res.write_str("OK");
+                    // send the hex-encoded byte stream
+                    for byte in &memory {
+                        res = res.write_hex(*byte);
+                    }
                 } else {
+                    // send the raw byte stream
                     for byte in &memory {
                         res = res.write_byte(*byte);
                     }
                 }
                 self.memory_buf = Some(memory);
-                self.send(res).await?;
+                res
             } else {
-                // unrecognized command
+                self.response()
-                self.send_empty().await?;
             }
-        }
+        } else {
-    }
+            // unrecognized command
-
+            self.response()
-    async fn send_empty(&mut self) -> std::io::Result<()> {
+        };
-        let res = self.response();
+        Ok(Some(res))
-        self.send(res).await
     }
 
     fn response(&mut self) -> Response {
@@ -323,15 +320,18 @@ impl GdbConnection {
             self.ack_messages,
         )
     }
+}
 
-    async fn send(&mut self, res: Response) -> std::io::Result<()> {
+impl Drop for GdbConnection {
-        let buffer = res.finish();
+    fn drop(&mut self) {
-        match std::str::from_utf8(&buffer) {
+        self.client
-            Ok(text) => println!("response: {text}"),
+            .send_command(EmulatorCommand::StopDebugging(self.sim_id));
-            Err(_) => println!("response: {buffer:02x?}"),
+    }
-        }
+}
-        self.stream_out.write_all(&buffer).await?;
+
-        self.response_buf = Some(buffer);
+fn debug_stop_reason_string(reason: Option<DebugStopReason>) -> &'static str {
-        self.stream_out.flush().await
+    match reason {
+        Some(DebugStopReason::Trapped) => "T05;thread:p1.t1;threads:p1.t1;reason:trap;",
+        None => "T00;thread:p1.t1;threads:p1.t1;",
     }
 }

src/gdbserver/request.rs

@@ -35,66 +35,7 @@ impl std::fmt::Debug for Request<'_> {
     }
 }
 
-impl<'a> Request<'a> {
+impl Request<'_> {
-    pub async fn read<R: AsyncRead + Unpin>(
-        reader: &mut R,
-        buffer: &'a mut Vec<u8>,
-    ) -> Result<Self> {
-        buffer.clear();
-
-        let mut char = reader.read_u8().await?;
-        while char == b'+' {
-            // just ignore positive acks
-            char = reader.read_u8().await?;
-        }
-        if char == b'-' {
-            bail!("no support for negative acks");
-        }
-        if char == 0x03 {
-            // This is how the client "cancels an in-flight request"
-            buffer.push(char);
-            return Ok(Self {
-                kind: RequestKind::Signal,
-                buffer,
-            });
-        }
-        if char != b'$' {
-            // Messages are supposed to start with a dollar sign
-            bail!("malformed message");
-        }
-
-        // now read the body
-        let mut checksum = 0u8;
-        char = reader.read_u8().await?;
-        while char != b'#' {
-            if char == b'}' {
-                // escape character
-                checksum = checksum.wrapping_add(char);
-                char = reader.read_u8().await?;
-                checksum = checksum.wrapping_add(char);
-                buffer.push(char ^ 0x20);
-            } else {
-                checksum = checksum.wrapping_add(char);
-                buffer.push(char);
-            }
-            char = reader.read_u8().await?;
-        }
-
-        let mut checksum_bytes = [b'0'; 2];
-        reader.read_exact(&mut checksum_bytes).await?;
-        let (real_checksum, 2) = u8::from_radix_16(&checksum_bytes) else {
-            bail!("invalid checksum");
-        };
-        if checksum != real_checksum {
-            bail!("mismatched checksum");
-        }
-
-        Ok(Self {
-            kind: RequestKind::Command,
-            buffer,
-        })
-    }
-
     pub fn match_str(&mut self, prefix: &str) -> bool {
         if let Some(new_buffer) = self.buffer.strip_prefix(prefix.as_bytes()) {
             self.buffer = new_buffer;
@@ -103,6 +44,13 @@ impl<'a> Request<'a> {
         false
     }
 
+    pub fn match_some_str<'a, I: IntoIterator<Item = &'a str>>(
+        &mut self,
+        prefixes: I,
+    ) -> Option<&'a str> {
+        prefixes.into_iter().find(|&prefix| self.match_str(prefix))
+    }
+
     pub fn match_hex<I: FromRadix16>(&mut self) -> Option<I> {
         match I::from_radix_16(self.buffer) {
             (_, 0) => None,
@@ -113,3 +61,119 @@ impl<'a> Request<'a> {
         }
     }
 }
+
+pub struct RequestSource<R> {
+    reader: R,
+    buffer: Vec<u8>,
+    state: RequestReadState,
+}
+
+impl<R: AsyncRead + Unpin> RequestSource<R> {
+    pub fn new(reader: R) -> Self {
+        Self {
+            reader,
+            buffer: vec![],
+            state: RequestReadState::Header,
+        }
+    }
+
+    pub async fn recv(&mut self) -> Result<Request<'_>> {
+        let mut char = self.reader.read_u8().await?;
+        if matches!(self.state, RequestReadState::Start) {
+            self.buffer.clear();
+            self.state = RequestReadState::Header;
+        }
+        if matches!(self.state, RequestReadState::Header) {
+            // Just ignore positive acks
+            while char == b'+' {
+                char = self.reader.read_u8().await?;
+            }
+            if char == b'-' {
+                bail!("no support for negative acks");
+            }
+            if char == 0x03 {
+                // This is how the client "cancels an in-flight request"
+                self.buffer.push(char);
+                self.state = RequestReadState::Start;
+                return Ok(Request {
+                    kind: RequestKind::Signal,
+                    buffer: &self.buffer,
+                });
+            }
+            if char != b'$' {
+                // Messages are supposed to start with a dollar sign
+                bail!("malformed message");
+            }
+            self.state = RequestReadState::Body {
+                checksum: 0,
+                escaping: false,
+            };
+            char = self.reader.read_u8().await?;
+        }
+        while let RequestReadState::Body { checksum, escaping } = &mut self.state {
+            if char == b'#' && !*escaping {
+                self.state = RequestReadState::Checksum {
+                    expected: *checksum,
+                    actual: 0,
+                    digits: 0,
+                };
+                char = self.reader.read_u8().await?;
+                break;
+            }
+            *checksum = checksum.wrapping_add(char);
+
+            if *escaping {
+                // escaped character
+                self.buffer.push(char ^ 0x20);
+                *escaping = false;
+            } else if char == b'}' {
+                // next character will be escaped
+                *escaping = true;
+            } else {
+                self.buffer.push(char);
+            }
+            char = self.reader.read_u8().await?;
+        }
+        while let RequestReadState::Checksum {
+            expected,
+            actual,
+            digits,
+        } = &mut self.state
+        {
+            let digit = match char {
+                b'0'..=b'9' => char - b'0',
+                b'a'..=b'f' => char - b'a' + 10,
+                b'A'..=b'F' => char - b'A' + 10,
+                _ => bail!("invalid checksum"),
+            };
+            *actual = (*actual << 4) + digit;
+            *digits += 1;
+            if *digits == 2 {
+                if *expected != *actual {
+                    bail!("mismatched checksum");
+                }
+                self.state = RequestReadState::Start;
+                return Ok(Request {
+                    kind: RequestKind::Command,
+                    buffer: &self.buffer,
+                });
+            }
+            char = self.reader.read_u8().await?;
+        }
+        unreachable!();
+    }
+}
+
+enum RequestReadState {
+    Start,
+    Header,
+    Body {
+        checksum: u8,
+        escaping: bool,
+    },
+    Checksum {
+        expected: u8,
+        actual: u8,
+        digits: u8,
+    },
+}

src/window/game.rs

@@ -83,20 +83,18 @@ impl GameWindow {
         });
         ui.menu_button("Emulation", |ui| {
             let state = self.client.emulator_state();
-            let can_interact = self.client.sim_state(self.sim_id) == SimState::Ready;
+            let is_ready = self.client.sim_state(self.sim_id) == SimState::Ready;
+            let can_pause = is_ready && state == EmulatorState::Running;
             if state == EmulatorState::Running {
-                if ui.add_enabled(can_interact, Button::new("Pause")).clicked() {
+                if ui.add_enabled(is_ready, Button::new("Pause")).clicked() {
                     self.client.send_command(EmulatorCommand::Pause);
                     ui.close_menu();
                 }
-            } else if ui
+            } else if ui.add_enabled(can_pause, Button::new("Resume")).clicked() {
-                .add_enabled(can_interact, Button::new("Resume"))
-                .clicked()
-            {
                 self.client.send_command(EmulatorCommand::Resume);
                 ui.close_menu();
             }
-            if ui.add_enabled(can_interact, Button::new("Reset")).clicked() {
+            if ui.add_enabled(is_ready, Button::new("Reset")).clicked() {
                 self.client
                     .send_command(EmulatorCommand::Reset(self.sim_id));
                 ui.close_menu();