A failing _Pad primitive threw a host std::runtime_error, which no guest
catch can see. threadcall.cpp calls the checked wrappers of xthread
(_Mtx_lockX, _Cnd_waitX, _Thrd_startX), which pass a non-success code to
_Throw_C_error: _Thrd_nomem and _Thrd_timedout become
resource_unavailable_try_again, _Thrd_busy device_or_resource_busy and
_Thrd_error invalid_argument, raised through _Throw_Cpp_error, which main
now implements. A failing thread start is _Thrd_startX's
resource_unavailable_try_again.
_Release now wakes the launching thread with _Cnd_signal, as
threadcall.cpp does; libc has it since the pad was written.
guest_ampr_threads checks, in a forked child whose guest terminate
handler reports the thrown object, that _Release on the thread still
holding the pad's mutex throws std::system_error with EBUSY. It fails
with the previous host exception and with busy mapped to
invalid_argument.
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