memoryd: a reported turn decided after loopd has its answer, from brokerd and the log

Co-Authored-By: ornith-1.5-35b-a3b <agent@wntrmute.dev>

crates/memoryd/src/honcho/state.rs

@@ -0,0 +1,45 @@
+//! Asking `brokerd` for a session's state on its `state.sock` (M5c spec, section 7): one question
+//! per connection, one final answer, as on every socket between the roles.
+
+use std::os::unix::net::UnixStream;
+use std::path::Path;
+use std::time::Duration;
+
+use bxcore::{
+    Envelope, Message, PROTOCOL_VERSION, SessionId, SessionState, SessionStateReply, read_frame,
+    write_frame,
+};
+
+/// How long `brokerd` may leave the connection without a byte, sending or answering.
+pub const WAIT: Duration = Duration::from_secs(5);
+
+/// The runbook entry for a `brokerd` that does not answer.
+pub const RUNBOOK: &str = "see docs/runbook.md#broker-state-unavailable";
+
+/// `brokerd`'s answer for `session`, asked on `socket`. `wait` is the read and the write timeout;
+/// the connect has none, and fails at once when nothing listens. Any error, an error frame
+/// included, is `Err` with one line that names the socket or `brokerd`.
+pub fn ask(
+    socket: &Path,
+    session: &SessionId,
+    wait: Duration,
+) -> Result<SessionStateReply, String> {
+    let line = |e: &dyn std::fmt::Display| format!("{}: {e}", socket.display());
+    let mut stream = UnixStream::connect(socket).map_err(|e| line(&e))?;
+    stream.set_read_timeout(Some(wait)).map_err(|e| line(&e))?;
+    stream.set_write_timeout(Some(wait)).map_err(|e| line(&e))?;
+    let question = Envelope {
+        v: PROTOCOL_VERSION,
+        id: 1,
+        r#final: true,
+        msg: Message::SessionState(SessionState {
+            session: session.clone(),
+        }),
+    };
+    write_frame(&mut stream, &question).map_err(|e| line(&e))?;
+    match read_frame(&mut stream).map_err(|e| line(&e))?.msg {
+        Message::SessionStateReply(reply) => Ok(reply),
+        Message::Error(e) => Err(format!("brokerd: {}", e.detail)),
+        _ => Err("brokerd answered with the wrong message".to_string()),
+    }
+}