test(stdin): pin keep-open through the daemon

Covers the parser -> supervisor -> spawn wiring the unit tests miss, and
characterizes the reported symptom: without `keep-open` the reader goes
Stopped, with it the server stays Running.

The harness now gives the daemon /dev/null on stdin. That is what
launchd does, and without it these tests prove nothing — the runner's
own stdin blocks on read, so an unfixed child looks identical to a
fixed one.

Refs #2

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TWLFEoRaRafJm1SpdhWQ6F

crates/xy/Cargo.toml

@@ -16,6 +16,10 @@ path = "src/bin/xy_test_sleep_server.rs"
 name = "xy-test-exit-failure"
 path = "src/bin/xy_test_exit_failure.rs"
 
+[[bin]]
+name = "xy-test-stdin-reader"
+path = "src/bin/xy_test_stdin_reader.rs"
+
 [dependencies]
 xy-protocol.workspace = true
 xy-supervisor.workspace = true

crates/xy/src/bin/xy_test_stdin_reader.rs

@@ -0,0 +1,21 @@
+/// Stands in for a stdio-first MCP server: it comes up, announces itself, then
+/// blocks reading stdin and exits the moment it sees EOF.
+fn main() {
+    use std::io::{BufRead, Write};
+
+    println!("ready");
+    std::io::stdout().flush().ok();
+
+    let mut line = String::new();
+    match std::io::stdin().lock().read_line(&mut line) {
+        Ok(0) => {
+            eprintln!("stdin closed (EOF), shutting down");
+            std::process::exit(0);
+        }
+        Ok(_) => std::process::exit(1),
+        Err(err) => {
+            eprintln!("stdin read error: {err}");
+            std::process::exit(2);
+        }
+    }
+}

crates/xy/tests/common/mod.rs

@@ -37,8 +37,20 @@ impl Harness {
     }
 
     pub fn write_server(&self, name: &str, command: &str, port: u16, restart_policy: &str) {
+        self.write_server_with(name, command, port, restart_policy, "");
+    }
+
+    /// `extra` is appended verbatim as additional top-level KDL.
+    pub fn write_server_with(
+        &self,
+        name: &str,
+        command: &str,
+        port: u16,
+        restart_policy: &str,
+        extra: &str,
+    ) {
         let body = format!(
-            "command \"{command}\"\nport {port}\nrestart {{\n    policy \"{restart_policy}\"\n    backoff-initial \"10ms\"\n    backoff-max \"50ms\"\n    max-retries-per-minute 3\n}}\nstop {{ grace \"500ms\" }}\n"
+            "command \"{command}\"\nport {port}\nrestart {{\n    policy \"{restart_policy}\"\n    backoff-initial \"10ms\"\n    backoff-max \"50ms\"\n    max-retries-per-minute 3\n}}\nstop {{ grace \"500ms\" }}\n{extra}"
         );
         std::fs::write(self.config_dir.join(format!("{name}.kdl")), body).unwrap();
     }
@@ -49,6 +61,7 @@ impl Harness {
             .env("XDG_CONFIG_HOME", self.tmp.path().join("config"))
             .env("XDG_STATE_HOME", self.tmp.path().join("state"))
             .env("XDG_RUNTIME_DIR", self.tmp.path().join("run"))
+            .stdin(Stdio::null())
             .stdout(Stdio::null())
             .stderr(Stdio::inherit())
             .kill_on_drop(true)
@@ -99,6 +112,9 @@ pub fn sleep_server_bin() -> PathBuf {
 pub fn exit_failure_bin() -> PathBuf {
     artifact("xy-test-exit-failure")
 }
+pub fn stdin_reader_bin() -> PathBuf {
+    artifact("xy-test-stdin-reader")
+}
 
 fn artifact(name: &str) -> PathBuf {
     let mut p = std::env::current_exe().unwrap();

crates/xy/tests/stdin_mode.rs

@@ -0,0 +1,68 @@
+mod common;
+use common::*;
+use std::time::Duration;
+use xy_protocol::ServerState;
+
+async fn state_of(h: &Harness, name: &str) -> ServerState {
+    h.list_rpc()
+        .await
+        .into_iter()
+        .find(|s| s.name == name)
+        .unwrap_or_else(|| panic!("no server named {name}"))
+        .state
+}
+
+async fn settle(h: &Harness, name: &str) -> ServerState {
+    let mut last = ServerState::Stopped;
+    for _ in 0..40 {
+        last = state_of(h, name).await;
+        if last == ServerState::Running {
+            break;
+        }
+        tokio::time::sleep(Duration::from_millis(100)).await;
+    }
+    last
+}
+
+#[tokio::test]
+async fn keep_open_lets_a_stdin_reader_stay_running() {
+    let xy = xy_bin();
+    let reader = stdin_reader_bin();
+    let mut h = Harness::new();
+    h.write_server_with(
+        "reader",
+        reader.to_str().unwrap(),
+        19_010,
+        "never",
+        "stdin \"keep-open\"\n",
+    );
+    h.start_daemon(&xy).await;
+
+    assert_eq!(settle(&h, "reader").await, ServerState::Running);
+
+    // Still up a beat later: it is blocked on read, not merely slow to die.
+    tokio::time::sleep(Duration::from_millis(500)).await;
+    assert_eq!(state_of(&h, "reader").await, ServerState::Running);
+}
+
+/// Characterizes the reported bug: the daemon's stdin is `/dev/null` under
+/// launchd, so by default a stdio-first server sees EOF and exits at once.
+#[tokio::test]
+async fn without_keep_open_a_stdin_reader_exits_on_eof() {
+    let xy = xy_bin();
+    let reader = stdin_reader_bin();
+    let mut h = Harness::new();
+    h.write_server("reader", reader.to_str().unwrap(), 19_011, "never");
+    h.start_daemon(&xy).await;
+
+    let mut last = ServerState::Running;
+    for _ in 0..40 {
+        last = state_of(&h, "reader").await;
+        if last == ServerState::Stopped {
+            break;
+        }
+        tokio::time::sleep(Duration::from_millis(100)).await;
+    }
+
+    assert_eq!(last, ServerState::Stopped);
+}