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>

Cargo.lock

@@ -246,6 +246,12 @@ version = "1.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "75b325c5dbd37f80359721ad39aca5a29fb04c89279657cffdda8736d0c0b9d2"
 
+[[package]]
+name = "equivalent"
+version = "1.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "877a4ace8713b0bcf2a4e7eec82529c029f1d0619886d18145fea96c3ffe5c0f"
+
 [[package]]
 name = "errno"
 version = "0.3.14"
@@ -382,6 +388,12 @@ dependencies = [
  "wasi",
 ]
 
+[[package]]
+name = "hashbrown"
+version = "0.17.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ed5909b6e89a2db4456e54cd5f673791d7eca6732202bbf2a9cc504fe2f9b84a"
+
 [[package]]
 name = "heck"
 version = "0.5.0"
@@ -403,6 +415,16 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "indexmap"
+version = "2.14.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d466e9454f08e4a911e14806c24e16fba1b4c121d1ea474396f396069cf949d9"
+dependencies = [
+ "equivalent",
+ "hashbrown",
+]
+
 [[package]]
 name = "ioctl-rs"
 version = "0.1.6"
@@ -736,6 +758,15 @@ dependencies = [
  "zmij",
 ]
 
+[[package]]
+name = "serde_spanned"
+version = "0.6.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bf41e0cfaf7226dca15e8197172c295a782857fcb97fad1808a166870dee75a3"
+dependencies = [
+ "serde",
+]
+
 [[package]]
 name = "serial"
 version = "0.4.0"
@@ -889,6 +920,7 @@ dependencies = [
  "dirs",
  "fs2",
  "futures",
+ "libc",
  "serde",
  "serde_json",
  "spacesh-core",
@@ -897,6 +929,7 @@ dependencies = [
  "thiserror",
  "tokio",
  "tokio-util",
+ "toml",
 ]
 
 [[package]]
@@ -986,6 +1019,47 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "toml"
+version = "0.8.23"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dc1beb996b9d83529a9e75c17a1686767d148d70663143c7854d8b4a09ced362"
+dependencies = [
+ "serde",
+ "serde_spanned",
+ "toml_datetime",
+ "toml_edit",
+]
+
+[[package]]
+name = "toml_datetime"
+version = "0.6.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "22cddaf88f4fbc13c51aebbf5f8eceb5c7c5a9da2ac40a13519eb5b0a0e8f11c"
+dependencies = [
+ "serde",
+]
+
+[[package]]
+name = "toml_edit"
+version = "0.22.27"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "41fe8c660ae4257887cf66394862d21dbca4a6ddd26f04a3560410406a2f819a"
+dependencies = [
+ "indexmap",
+ "serde",
+ "serde_spanned",
+ "toml_datetime",
+ "toml_write",
+ "winnow",
+]
+
+[[package]]
+name = "toml_write"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5d99f8c9a7727884afe522e9bd5edbfc91a3312b36a77b5fb8926e4c31a41801"
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.24"
@@ -1200,6 +1274,15 @@ version = "0.52.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "589f6da84c646204747d1270a2a5661ea66ed1cced2631d546fdfb155959f9ec"
 
+[[package]]
+name = "winnow"
+version = "0.7.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "df79d97927682d2fd8adb29682d1140b343be4ac0f08fd68b7765d9c059d3945"
+dependencies = [
+ "memchr",
+]
+
 [[package]]
 name = "winreg"
 version = "0.10.1"

Cargo.toml

@@ -26,5 +26,7 @@ portable-pty = "0.8"
 alacritty_terminal = "0.25"
 fs2 = "0.4"
 dirs = "5"
+toml = "0.8"
+libc = "0.2"
 clap = { version = "4", features = ["derive"] }
 clap_complete = "4"

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);