Tty with a shell that is not there says so, and never sets the caller's shell

Tty /nonexistent or Tty fsh started the host's fallback shell without a word,
and when spawnTty made no pane, the argument became the calling pane's shell.
Tty now looks the shell up first and fails 'Tty: no shell "fsh"' when it is
neither a name on the usual paths nor a path to one; spawnTty answers the pane
it made, the one given the shell, and says why when it made none. The lookup
(host_io Shell.find) gives the turn up around its access() calls, since a
typed path may be under a mount this editor serves, and forkShell copies the
pane's shell before looking it up and checks the pane is still its own after.

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

1 files changed, 12 insertions, 5 deletions

diff --git a/src/builtins.zig b/src/builtins.zig
index fbab925c..751d4026 100644
--- a/src/builtins.zig
+++ b/src/builtins.zig
@@ -874,11 +874,18 @@ pub const Tty = struct {
     pub const takes_arg = true;
     pub const plus_arg = true;
     pub fn run(c: Ctx) void {
-        exec.spawnTty(c.p, c.id);
-        const arg = c.arg orelse return;
-        const pane = c.p.panes[c.p.active] orelse return;
-        if (!pane.isTerminal() or pane.shell != null) return;
-        pane.shell = c.p.gpa.dupe(u8, arg) catch return;
+        const arg = std.mem.trim(u8, c.arg orelse "", " \t");
+        // A shell that is not there is said, where the host would quietly
+        // start its fallback in its place.
+        if (arg.len > 0) {
+            var buf: [std.fs.max_path_bytes]u8 = undefined;
+            if (comptime pardes.hosted) if (@import("host_io.zig").Shell.find(arg, &buf) == null) {
+                var said: [320]u8 = undefined;
+                return c.p.reportFailure(c.id, std.fmt.bufPrint(&said, "Tty: no shell \"{s}\" (a name on the usual paths, or a path to one)", .{arg[0..@min(arg.len, 200)]}) catch "Tty: no such shell");
+            };
+        }
+        const pane = exec.spawnTty(c.p, c.id) orelse return;
+        if (arg.len > 0) pane.shell = c.p.gpa.dupe(u8, arg) catch null;
     }
 };