issues: libc_children's tty_interrupt can stall at the nested shell's line

issues/libc-childrens-tty-interrupt-can-stall-at-the-nested-shells-line.md

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+---
+status: open
+kind: defect
+opened: 2026-11-01
+---
+
+# libc_children's tty_interrupt can stall at the nested shell's line
+
+`cargo test --test toyos-build -- libc_children` on the 2-CPU QEMU x86-64 boot
+sometimes ends with `tty_interrupt: stalled at the nested line held`.
+1. The probe types `shell` into its shell, and the nested prompt prints.
+2. It then types `cat` into the nested shell. That line never becomes a held
+   foreground line.
+3. The probe's own 120 s hang ceiling ends the run with exit 2.
+
+**Evidence.** These are local QEMU runs, with logs in
+[`pr-1111/r2/tty`](https://github.com/ToyOSOrg/evidence/tree/8b3306f1ba244133a2c28e8d7c09fcc48c473c8e/pr-1111/r2/tty):
+
+| revision | runs | stalled |
+|---|---|---|
+| main `24a538c88` | 4 | 1 |
+| `f5ae75f6b` (#1111's step 1 on that main) | 6 | 0 |
+| `dd9bf974d` (#1111's head) | 3 | 2 |
+
+One of the head's two failures was this stall. The other was
+`signal_quit: stalled driving the child that reloads`.
+
+It reproduces on main, so it does not come with #1111. Its cause is unknown.
+The kept serial log covers only boot, so nothing yet says which side lost the
+line: the nested shell's stdin read, the surface's report of its foreground
+line, or a wake between them.
+
+**Exit:** find what the nested shell waits on when it stalls, fix it, and
+show `libc_children` green across repeated runs. `signal_quit`'s stall may be
+the same defect.