[Bug 236035] ptrace: SIGTRAP and EXIT race

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236035

            Bug ID: 236035
           Summary: ptrace: SIGTRAP and EXIT race
           Product: Base System
           Version: 12.0-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: robert.ayrapetyan at gmail.com

When tracing a process using ptrace and there are multiple threads in the
tracing process hitting the same breakpoint, sometimes main thread exits
(WIFEXITED(status) is TRUE) before last queued TRAP_BKPT signal(s) have been
delivered to the tracing process. So a final breakpoint hits counter is less
than it should be.

Steps to reproduce:

cd /tmp
git clone https://github.com/rayrapetyan/ptrace_bug_poc.git
cd ptrace_bug_poc
mkdir build
cd build
cmake ..
make

Run ~20 times:

/tmp/ptrace_bug_poc/build/src/ptrace_test/ptrace_test
/tmp/ptrace_bug_poc/build/src/mt_example/mt_example 0x201385

-------
Note: make sure 0x201385 is a call to <printf at plt> in
"/tmp/ptrace_bug_poc/build/src/mt_example/mt_example":
gdb /tmp/ptrace_bug_poc/build/src/mt_example/mt_example
disassemble foo
-------

Wait fo appearance of:
"BOOM! Invalid BP hits counter (hits: 1, tid: XXXX)"
at the end of the output (most of the times it will be "SUCCESS")

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