`RuntimeHandles::drop` joined its reader threads in the `Drop` body,
which runs before any field drops. The `ChildStdin` sink lives inside
`StdinWriter` in the `stdin` FIELD, so it could only be released after
the join returned -- and the join was waiting on readers blocked in
`read()` on pipes whose write ends the child still held, because the
child never received the stdin EOF that would have made it exit.
A closed cycle, entirely inside one function. Teardown hung forever.
This is the root cause of `m4_5_basedpyright_initializes_and_negotiates_
encoding` hanging indefinitely -- diagnosed with gdb stacks plus /proc fd
forensics on a wedged process, reproduced 5/5 deterministically. It also
explains why the hang looked intermittent and machine-local: a
shim-launched server orphans its real process (basedpyright's console
script spawns bundled `node` and exits, leaving it at `PPid 1`), so
nothing teardown signals can reach it, while a direct binary like clangd
or gopls is a genuine child whose pipes close on reap.
`spawn_reader`'s `cancel` flag does not help: it is consulted between
reads and around `send_timeout`, never while `read` is blocked. The
existing comment's premise -- "dropping the master closes the kernel pipe
and unblocks `read`" -- holds for a PTY master but not for pipe mode,
where `read` returns only once *every* write end closes.
The fix reuses `close_stdin`'s existing, already-idempotent mechanism at
the one site missing it. Reordering the struct's fields cannot work: a
type's `Drop::drop` body runs before all of its fields regardless of
declaration order.
Bounded claim: this delivers EOF, so it fixes children that drain stdin
to EOF -- which stdio language servers do. A child that ignores EOF, or
that stops draining while bytes are queued (the writer's `write_all` is
blocking), still wedges the join. Making the `read` itself cancellable
via the poll path already used by `spawn_group_reader` is the standing
deferral that covers those, and is deliberately not in this change.
Test: `teardown_closes_stdin_before_joining_readers`, in `--lib` so it
runs in the standard gate. It models the real shape with an orphaned
grandchild, and carries two positive controls, because this lane wrote
three reproductions that passed against the unfixed tree before one
bit. The `<&0` redirect is load-bearing: POSIX XCU 2.9.3 assigns
`/dev/null` to an asynchronous list's stdin when job control is off, so a
bare `cat &` exits immediately and proves nothing. Teardown runs on a
worker thread behind `recv_timeout` so a regression FAILS in 10s rather
than hanging -- a hanging test would reproduce the hazard being removed.
Bite verified by revert: with the fix `ok` in 2.03s; with the single
`stdin.take()` line commented out, FAILED at 10.00s on the timeout, both
controls having passed first.
Docs: framing doc added; handoff gains the drop-body-before-fields lesson
and the reproduction-needs-a-control generalization, and its section 3
caveat is corrected -- the desktop's basedpyright binary was never
broken. The `--skip basedpyright` gate entry stays for now; dropping it
is a separate proposal owed evidence.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Lv428Fth9LRtffwJSsqH7T
Levi Neuwirth
eb9b36387c