Andrew Kesterson caef63826f Document and test handing an error context between threads ...

The thread-safety section filed two different things under "does not cover,
and cannot": sharing a context between threads, and passing one to another
thread. Only the first is unsupported. Transfer already works by
construction -- the reference count is the only field the library reads
across an ownership boundary, and it is only ever touched under the pool
lock, so akerr_release_error() does not care which thread checked the slot
out. The pool is process-global, not thread-local, so a context outlives the
thread that raised it.

Calling that unsupported told readers the worker/collector shape was off the
table, which either cost them the pattern or cost them the stack trace when
they rolled their own struct instead.

Split the bullet: transfer joins the covered list and gets its own section
with the rule, the worked pattern, and the four receiving-side hazards
(PREPARE_ERROR cannot adopt, CATCH assigns over the pointer, FINISH in a
void helper still parses its return, and an unhandled error now terminates
from the collector's thread). Sharing keeps the "cannot" bullet, narrowed to
what it actually is.

err_threads_handoff.c proves it: the existing thread tests all keep every
context on the thread that raised it, so the transfer path was exercised
nowhere. Seven producers hand errors to one collector through a bounded
mutex/condvar queue -- the mutex is the thing under test, since it is what
publishes the unlocked content writes -- and the collector asserts the
context is still a live slot at refcount 1, that message and trace arrive
whole and in each producer's order, that the slot was never recycled in
flight, and that a thread which never called akerr_next_error() can release
it. A second phase reads a context whose raising thread has already exited.

Also document why copying a context by assignment is silently wrong:
stacktracebufptr is self-referential, so the copy's cursor points into the
source's buffer and the first append corrupts a slot the copier no longer
owns. TODO.md records the akerr_copy_error() shape that would fix it and the
trigger for building it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

2026-08-01 16:35:18 -04:00

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