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, 18 insertions, 7 deletions

diff --git a/src/pardes.zig b/src/pardes.zig
index 58354b95..429f33b9 100644
--- a/src/pardes.zig
+++ b/src/pardes.zig
@@ -1193,7 +1193,7 @@ test "first document fallback splits like Tty from the originating pane" {
             const free = p.freeSlot().?;
             const doc = try p.openTutorView(free);
             exec.placeDoc(p, from, free, doc);
-            exec.spawnTty(tty, from);
+            _ = exec.spawnTty(tty, from);
             p.sync();
             tty.sync();
             try std.testing.expectEqual(@as(usize, 2), p.ncol);
@@ -1516,11 +1516,22 @@ test "Tty+fish, one word a tag can hold, opens a terminal on that shell" {
     const p = try Pardes.init(std.testing.allocator, .{ .tty_only = true });
     defer p.deinit();
     while (p.nextEffect()) |_| {}
-    try std.testing.expect(p.executeBuiltinLine(0, "Tty+fish"));
+    // sh, the one shell every host has.
+    try std.testing.expect(p.executeBuiltinLine(0, "Tty+sh"));
     try std.testing.expect(p.active != 0);
-    try std.testing.expectEqualStrings("fish", p.panes[p.active].?.shell.?);
-    p.acknowledgeShell(p.active, "/usr/bin/fish", true);
-    try std.testing.expect(std.mem.startsWith(u8, tagline.curTail(p.panes[p.active].?), " Tty+fish "));
+    try std.testing.expectEqualStrings("sh", p.panes[p.active].?.shell.?);
+    p.acknowledgeShell(p.active, "/bin/sh", false);
+    try std.testing.expect(std.mem.startsWith(u8, tagline.curTail(p.panes[p.active].?), " Tty+sh "));
+    p.active = 0;
+    // A shell that is not there is said, not quietly replaced, and the
+    // pane that asked gets no shell of its own.
+    const before = p.active;
+    const caller = p.panes[0].?;
+    try std.testing.expect(p.executeBuiltinLine(0, "Tty /nonexistent"));
+    try std.testing.expect(p.active == before and caller.shell == null);
+    try std.testing.expect(std.mem.indexOf(u8, caller.msg[0..caller.msg_len], "no shell \"/nonexistent\"") != null);
+    try std.testing.expect(p.executeBuiltinLine(0, "Tty fsh-not-a-shell"));
+    try std.testing.expect(caller.shell == null);
     // Only a word that says so splits at `+`: `Dump+x.zon` is no Dump.
     try std.testing.expect(!p.executeBuiltinLine(0, "Dump+x.zon"));
     try std.testing.expect(!p.executeBuiltinLine(0, "Msg+hello"));
@@ -7059,7 +7070,7 @@ test "raw tty Escape only leaves at a shell prompt" {
     if (platform == .web) return;
     const p = try Pardes.init(std.testing.allocator, .{ .cols = 80, .rows = 24, .file = "mise.toml" });
     defer p.deinit();
-    exec.spawnTty(p, 0);
+    _ = exec.spawnTty(p, 0);
     const shell = p.active;
     const pane = p.panes[shell].?;
     var buf: [256]u8 = undefined;
@@ -7090,7 +7101,7 @@ test "raw tty Ctrl-B switches to editor mode without sending child input" {
     if (platform == .web) return;
     const p = try Pardes.init(std.testing.allocator, .{ .file = "mise.toml" });
     defer p.deinit();
-    exec.spawnTty(p, 0);
+    _ = exec.spawnTty(p, 0);
     const shell = p.active;
     p.sync();
     var buf: [256]u8 = undefined;