Tty in a missing directory fails and makes no pane; a shell that never started answers runs

The host could not start the shell (its chdir failed), and the pane stayed
with no pty: a run written to it waited for a prompt for ever. Tty now
refuses a directory that is not there, `Tty: <dir>: no such directory`,
before making a pane; and a terminal whose shell the host could not start
(Pardes.shellFailed, from each host's spawn) answers any run with
`error shell gone`.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 21 insertions, 1 deletions

diff --git a/src/exec.zig b/src/exec.zig
index 0bce67d2..6e437093 100644
--- a/src/exec.zig
+++ b/src/exec.zig
@@ -341,11 +341,20 @@ pub fn spawnV9fsTty(p: *Pardes, from: usize) void {
 
 fn spawnTtyWithMount(p: *Pardes, from: usize, v9fs: bool) ?*Pane {
     if (p.panes[from] == null) return null;
+    // A directory that is not there makes no terminal: its shell could not
+    // start in it, and a dead pane would answer no run. Asked only of a host
+    // that starts shells (none, as in the core's tests, starts none).
+    const dir = p.execDir(from);
+    if (comptime hosted) if (p.host.vtable.spawn != null) if (@import("fs.zig").localPath(dir)) |local| if (!isDirectory(local)) {
+        var said: [pardes.memory.limits.host_path_cap + 48]u8 = undefined;
+        p.reportFailure(from, std.fmt.bufPrint(&said, "Tty: {s}: no such directory", .{dir}) catch "Tty: no such directory");
+        return null;
+    };
     const free = p.freeSlot() orelse {
         p.reportError(from, "Tty", error.NoPaneSlots);
         return null;
     };
-    const nt = p.newShell(free, p.execDir(from)) catch |err| {
+    const nt = p.newShell(free, dir) catch |err| {
         if (err == error.PathTooLong) {
             p.reportFailure(from, std.fmt.comptimePrint("Tty: its directory is longer than a path may be ({d} bytes)", .{pardes.memory.limits.host_path_cap}));
             return null;
@@ -360,6 +369,17 @@ fn spawnTtyWithMount(p: *Pardes, from: usize, v9fs: bool) ?*Pane {
     return nt;
 }
 
+/// Whether `path` is a directory on this host, the turn out while it asks
+/// (it may be inside a mount this editor serves).
+fn isDirectory(path: []const u8) bool {
+    var buf: [4096]u8 = undefined;
+    const z = std.fmt.bufPrintSentinel(&buf, "{s}", .{path}, 0) catch return false;
+    pardes.turn.yield();
+    defer pardes.turn.back();
+    const stat = std.Io.Dir.cwd().statFile(std.Io.Threaded.global_single_threaded.io(), z, .{}) catch return false;
+    return stat.kind == .directory;
+}
+
 /// With `BootShell replace`, a doc landing in `col` closes a lone pristine
 /// shell there, the boot's placeholder, and absorbVWeight hands its space to
 /// the doc. With `keep`, the default, the shell stays like any other pane.