kern: send parent a SIGCHLD when the debugger has detached

[kevans]

Summary

The practical scenario that leads to this is porch(1) spawning some
utility and sending it a SIGSTOP as a debugging aide. The user then
attaches a debugger and walks through how some specific input is
processed, then detaches to allow the script to continue. When ptrace
is detached, the process resumes execution but the parent is never
notified and may be stuck in wait(2) for it to continue or terminate.

Other platforms seem to re-suspend the process after the debugger is
detached, but neither behavior seems unreasonable. Just notifying the
parent that the child has resumed is a relatively low-risk departure
from our current behavior and had apparently been considered in the
past, based on pre-existing comments.

Move p_flag and p_xsig handling into childproc_continued(), as just
sending the SIGCHLD here isn't really useful without P_CONTINUED set
and the other caller already sets these up as well.

[kib]

sys/kern/kern_sig.c

3779

Assert the p is locked (no need to assert p_pptr there).

[markj]

Should we note this behaviour in ptrace.2?

[kevans]

Assert that we have the proc lock and note the new behavior in ptrace(2)

[kib]

lib/libsys/ptrace.2

476

Start new paragraph.

481

A note could be added to wait.2, it would be even more natural. I do not suggest to remove this text.

[kevans]

Address review commentary:

• Add a paragraph break in the PT_DETACH description
• Also note the wait(2) interaction in that paragraph, to be explicit
• Amend the WCONTINUED description in wait(2) to note that ptrace(2) may cause the process to be noted as continued as well, not just a SIGCONT.

[kib]

lib/libsys/wait.2

457