feat(spaceshd): configurable default shell

Add a config.rs module that loads optional ~/.spacesh/config.toml and a
default_shell() resolver: SPACESH_SHELL env -> config.toml default_shell ->
login shell from the passwd DB -> $SHELL -> /bin/sh. The passwd lookup
matters under launchd where $SHELL is absent. All plain-panel spawn sites in
server.rs now use the resolver. Adds toml + libc deps.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/spaceshd/Cargo.toml

@@ -22,3 +22,5 @@ thiserror.workspace = true
 futures.workspace = true
 fs2.workspace = true
 dirs.workspace = true
+toml.workspace = true
+libc.workspace = true

crates/spaceshd/src/config.rs

@@ -0,0 +1,101 @@
+//! Daemon configuration loaded from `~/.spacesh/config.toml`.
+//!
+//! The file is optional; every field has a sane fallback so a missing or
+//! partial config never breaks startup.
+
+use std::path::Path;
+use serde::Deserialize;
+
+#[derive(Debug, Clone, Default, Deserialize)]
+pub struct Config {
+    /// Shell launched for plain (no-command) panels. When unset, the daemon
+    /// auto-detects the user's login shell.
+    #[serde(default)]
+    pub default_shell: Option<String>,
+}
+
+impl Config {
+    /// Load `~/.spacesh/config.toml`. Any error (missing file, bad TOML) yields defaults.
+    pub fn load() -> Self {
+        let Ok(dir) = crate::lifecycle::spacesh_dir() else { return Self::default() };
+        Self::from_path(&dir.join("config.toml"))
+    }
+
+    pub fn from_path(path: &Path) -> Self {
+        match std::fs::read_to_string(path) {
+            Ok(s) => toml::from_str(&s).unwrap_or_default(),
+            Err(_) => Self::default(),
+        }
+    }
+}
+
+/// Resolve the shell to spawn for a plain panel.
+///
+/// Order: `SPACESH_SHELL` env → `config.toml` `default_shell` → login shell
+/// from the passwd DB → `$SHELL` → `/bin/sh`. The passwd lookup matters under
+/// launchd, where `$SHELL` is typically absent (so a bash fallback would win).
+pub fn default_shell() -> String {
+    if let Ok(s) = std::env::var("SPACESH_SHELL") {
+        if !s.is_empty() { return s; }
+    }
+    if let Some(s) = Config::load().default_shell {
+        if !s.is_empty() { return s; }
+    }
+    if let Some(s) = login_shell() {
+        return s;
+    }
+    if let Ok(s) = std::env::var("SHELL") {
+        if !s.is_empty() { return s; }
+    }
+    "/bin/sh".into()
+}
+
+/// The current user's login shell from the passwd database (`getpwuid`).
+#[cfg(unix)]
+fn login_shell() -> Option<String> {
+    use std::ffi::CStr;
+    // SAFETY: getpwuid returns a pointer into a static buffer valid until the
+    // next libc passwd call; we copy the string out immediately on this thread.
+    unsafe {
+        let pw = libc::getpwuid(libc::getuid());
+        if pw.is_null() { return None; }
+        let shell = (*pw).pw_shell;
+        if shell.is_null() { return None; }
+        let s = CStr::from_ptr(shell).to_str().ok()?;
+        if s.is_empty() { None } else { Some(s.to_string()) }
+    }
+}
+
+#[cfg(not(unix))]
+fn login_shell() -> Option<String> { None }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn missing_file_is_default() {
+        let c = Config::from_path(Path::new("/no/such/spacesh/config.toml"));
+        assert!(c.default_shell.is_none());
+    }
+
+    #[test]
+    fn parses_default_shell() {
+        let dir = std::env::temp_dir().join("spacesh-cfg-test");
+        std::fs::create_dir_all(&dir).unwrap();
+        let path = dir.join("config.toml");
+        std::fs::write(&path, "default_shell = \"/bin/zsh\"\n").unwrap();
+        let c = Config::from_path(&path);
+        assert_eq!(c.default_shell.as_deref(), Some("/bin/zsh"));
+        let _ = std::fs::remove_file(&path);
+    }
+
+    #[test]
+    fn env_override_wins() {
+        let _serial = crate::test_support::serial();
+        std::env::set_var("SPACESH_SHELL", "/tmp/fake-shell");
+        let s = default_shell();
+        std::env::remove_var("SPACESH_SHELL");
+        assert_eq!(s, "/tmp/fake-shell");
+    }
+}

crates/spaceshd/src/main.rs

@@ -1,3 +1,4 @@
+mod config;
 mod event_log;
 mod event_store;
 mod hooks;

crates/spaceshd/src/server.rs

@@ -298,7 +298,7 @@ async fn handle_request(
                 let _ = out.send(err(id, "NOT_FOUND", "workspace")).await; return;
             };
             let sid = reg.new_surface_id();
-            let shell = command.clone().unwrap_or_else(|| std::env::var("SHELL").unwrap_or_else(|_| "/bin/sh".into()));
+            let shell = command.clone().unwrap_or_else(crate::config::default_shell);
             let spec = SurfaceSpec {
                 command: shell, args: args.clone(), cwd: ws.path.clone(),
                 agent_label: command, cols, rows, autostart: false,
@@ -333,7 +333,7 @@ async fn handle_request(
             };
             let ws = reg.workspace(&ws_id).cloned().unwrap();
             let new_sid = reg.new_surface_id();
-            let shell = command.clone().unwrap_or_else(|| std::env::var("SHELL").unwrap_or_else(|_| "/bin/sh".into()));
+            let shell = command.clone().unwrap_or_else(crate::config::default_shell);
             let spec = SurfaceSpec { command: shell, args, cwd: ws.path.clone(), agent_label: command, cols: 80, rows: 24, autostart: false };
             let (env, hooks_active) = spawn_env(&new_sid, &spec);
             match crate::surface::spawn_from_spec(new_sid.clone(), ws_id.clone(), &spec, env, hooks_active, state_tx.clone(), exit_tx.clone()) {
@@ -406,7 +406,7 @@ async fn handle_request(
                 let slot = slots.get(i);
                 let new_sid = reg.new_surface_id();
                 let command = slot.and_then(|s| s.command.clone());
-                let shell = command.clone().unwrap_or_else(|| std::env::var("SHELL").unwrap_or_else(|_| "/bin/sh".into()));
+                let shell = command.clone().unwrap_or_else(crate::config::default_shell);
                 let args = slot.map(|s| s.args.clone()).unwrap_or_default();
                 let spec = SurfaceSpec { command: shell, args, cwd: ws.path.clone(), agent_label: command, cols: 80, rows: 24, autostart: false };
                 let (env, hooks_active) = spawn_env(&new_sid, &spec);