pty/ctl exec fails the write when the shell cannot start; a directory removed under a shell leaves its name

`exec` on pty/ctl restarted a shell in a directory that was gone, the
write succeeding and the pane dead; and the pane was then renamed
`<dir> (deleted)` from /proc, so a restart failed even once the directory
was back. `exec` is now refused up front, ENOENT, where the directory is
gone; a shell the host cannot start fails the waiting write (late
failure, as a Save's); and a `(deleted)` cwd is no name to take.

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

1 files changed, 33 insertions, 1 deletions

diff --git a/src/host_io.zig b/src/host_io.zig
index 1db0dac9..10bcf0d7 100644
--- a/src/host_io.zig
+++ b/src/host_io.zig
@@ -1286,7 +1286,14 @@ pub fn shellCwd(pid: libc.pid_t, buf: []u8) ?[]const u8 {
             const path = std.fmt.bufPrintSentinel(&pbuf, "/proc/{d}/cwd", .{pid}, 0) catch return null;
             const n = libc.readlink(path, buf.ptr, buf.len);
             if (n <= 0 or n >= buf.len) return null;
-            return buf[0..@intCast(n)];
+            const link = buf[0..@intCast(n)];
+            // A directory removed under the shell reads `<path> (deleted)`:
+            // no directory to be named by, so the name it had stays, and a
+            // restart works there once it is back.
+            if (std.mem.endsWith(u8, link, " (deleted)")) {
+                _ = std.Io.Dir.cwd().statFile(std.Io.Threaded.global_single_threaded.io(), link, .{}) catch return null;
+            }
+            return link;
         },
         .macos, .ios, .tvos, .watchos, .visionos => {
             var info: proc_vnodepathinfo = undefined;
@@ -1330,6 +1337,31 @@ test "shell cwd rejects truncation and preserves an owned child path longer than
     try std.testing.expectEqualStrings(directory, shellCwd(child.pid, &result) orelse return error.MissingCwd);
 }
 
+test "a shell whose directory was removed is not named `... (deleted)`" {
+    if (comptime builtin.os.tag != .linux) return error.SkipZigTest;
+    if (!haveFile("/bin/sh")) return error.SkipZigTest;
+    const io = std.testing.io;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    try tmp.dir.createDirPath(io, "gone");
+    var path_buf: [4096]u8 = undefined;
+    const directory = path_buf[0..try tmp.dir.realPathFile(io, "gone", &path_buf)];
+    const rcs: Shell.PromptFiles = .{};
+    const child = try forkShell(null, 0, &rcs, "/bin/sh", directory, 24, 80, null);
+    defer {
+        _ = libc.kill(child.pid, libc.SIG.KILL);
+        _ = libc.waitpid(child.pid, null, 0);
+        _ = libc.close(child.file.handle);
+    }
+    var sh: TestShell = .{ .master = child.file.handle, .pid = child.pid };
+    try std.testing.expect(writeFd(child.file.handle, "printf '\\160ardes-cwd-ready\\n'\n"));
+    try std.testing.expect(sh.waitText("pardes-cwd-ready", 5_000));
+    var result: [4096]u8 = undefined;
+    try std.testing.expectEqualStrings(directory, shellCwd(child.pid, &result) orelse return error.MissingCwd);
+    try tmp.dir.deleteDir(io, "gone");
+    try std.testing.expect(shellCwd(child.pid, &result) == null);
+}
+
 extern "c" fn tcgetpgrp(fd: c_int) libc.pid_t;
 
 const occ_max_depth: u8 = 8;