Tty and Shell refuse a directory: not a shell

A directory passes access(X_OK), so Tty /etc made a pane whose shell
exited 127 and Shell /etc was taken. The shell lookup now refuses a
directory, and both say "not a shell: /etc is a directory" (Tty and
Shell share one refusal, host_io.Shell.refusal). shellset's golden takes
the shared wording (re-recorded by name).

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

1 files changed, 21 insertions, 1 deletions

diff --git a/src/host_io.zig b/src/host_io.zig
index f83c7d47..10a85d8f 100644
--- a/src/host_io.zig
+++ b/src/host_io.zig
@@ -834,7 +834,7 @@ pub const Shell = struct {
             @memcpy(buf[0..bin.len], bin);
             buf[bin.len] = 0;
             const p: [*:0]const u8 = @ptrCast(buf);
-            return if (libc.access(p, X_OK) == 0) p else null;
+            return if (libc.access(p, X_OK) == 0 and !isDirectory(p)) p else null;
         }
         for (bin_dirs) |dir| {
             if (dir.len + bin.len + 1 > buf.len) continue;
@@ -847,6 +847,26 @@ pub const Shell = struct {
         return null;
     }
 
+    /// A directory passes access(X_OK) (it can be searched): no shell.
+    pub fn isDirectory(path: [*:0]const u8) bool {
+        const d = libc.opendir(path) orelse return false;
+        _ = libc.closedir(d);
+        return true;
+    }
+
+    /// Why `bin` is no shell, for the words that name one (Tty, Shell);
+    /// null when it is one.
+    pub fn refusal(bin: []const u8, said: []u8) ?[]const u8 {
+        var buf: [std.fs.max_path_bytes]u8 = undefined;
+        if (find(bin, &buf) != null) return null;
+        if (std.mem.indexOfScalar(u8, bin, '/') != null and bin.len < buf.len) {
+            @memcpy(buf[0..bin.len], bin);
+            buf[bin.len] = 0;
+            if (isDirectory(@ptrCast(&buf))) return std.fmt.bufPrint(said, "not a shell: {s} is a directory", .{bin}) catch "not a shell: a directory";
+        }
+        return std.fmt.bufPrint(said, "no shell \"{s}\" (a name on the usual paths, or a path to one)", .{bin}) catch "no such shell";
+    }
+
     fn fallback(buf: *[std.fs.max_path_bytes]u8) [*:0]const u8 {
         for (fallbacks) |f| {
             @memcpy(buf[0..f.len], f);