From 1a73c72e3c1cae386edc4d72c2c4ecfe072bfc8a Mon Sep 17 00:00:00 2001
From: l0ng-ai <24760907+l0ng-ai@users.noreply.github.com>
Date: Sun, 23 Aug 2026 14:30:18 +0800
Subject: [PATCH] test(protocol): pin that a ShellSpec from an older peer still
 decodes
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Every wire struct here defaults its later fields so an older peer's message
degrades instead of failing the frame, and the rule is guarded — removing
`#[serde(default)]` from `PaneInfo` or `DaemonVersion` fails a test. Removing
either of `ShellSpec`'s passed the whole suite.

It is not a struct that can afford it. A `ShellSpec` crosses the wire inside
every `Spawn` that names a shell, in both directions and between builds: a
GUI talks to whatever `tty7-server` is installed on the far machine, and that
is regularly older. A required field there does not degrade — the frame fails
and the pane never starts.

`default_spawn_stays_wire_compatible_with_old_daemons` looked like it covered
this and does not: it exercises the `shell: None` legacy encoding, so it never
constructs one. This decodes the JSON an older peer sends and round-trips a
spawn that names a shell. Both defaults are checked by removal.
---
 crates/tty7-core/src/daemon/protocol.rs | 43 +++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/crates/tty7-core/src/daemon/protocol.rs b/crates/tty7-core/src/daemon/protocol.rs
index cbc4537b..5cf0a9d0 100644
--- a/crates/tty7-core/src/daemon/protocol.rs
+++ b/crates/tty7-core/src/daemon/protocol.rs
@@ -2322,6 +2322,49 @@ mod tests {
         assert_eq!(info.title, "");
     }
 
+    /// A `ShellSpec` from a peer that predates its later fields still decodes.
+    ///
+    /// This crosses the wire inside every `Spawn` that names a shell, in both
+    /// directions and between builds — a GUI talks to whatever `tty7-server`
+    /// is installed on the far machine, and that is regularly older. A field
+    /// without `#[serde(default)]` here does not degrade, it fails the frame,
+    /// and the pane never starts.
+    ///
+    /// `default_spawn_stays_wire_compatible_with_old_daemons` covers the
+    /// `shell: None` encoding and so never constructs one of these; removing
+    /// either default below passed the whole suite before this existed.
+    #[test]
+    fn a_shell_spec_from_an_older_peer_still_decodes() {
+        let spec: ShellSpec = serde_json::from_str(r#"{"program": "/bin/zsh"}"#).unwrap();
+        assert_eq!(spec.program, "/bin/zsh");
+        assert!(
+            spec.args.is_empty(),
+            "args must default, not fail the frame"
+        );
+        assert!(
+            !spec.args_are_tty7_defaults,
+            "a peer that never heard of this flag is not claiming tty7 defaults"
+        );
+
+        // And the same message inside the frame it actually travels in.
+        let msg = ClientMsg::Spawn {
+            cwd: None,
+            size: SIZE,
+            shell: Some(spec),
+            owner: None,
+            workspace: None,
+            restore: None,
+        };
+        let mut buf = Vec::new();
+        msg.encode(&mut buf).unwrap();
+        let (kind, payload) = read_frame(&mut std::io::Cursor::new(&buf)).unwrap();
+        assert_eq!(
+            ClientMsg::from_frame(kind, payload).unwrap(),
+            msg,
+            "a spawn naming a shell round-trips"
+        );
+    }
+
     /// The reason `previously_known_as` is a field and not a variant: this
     /// prompt is decoded by whatever GUI or `tty7-server` is at the other end,
     /// and one of them is regularly older than the build that sent it. A