Before this commit, m_blocker was only set to null in Thread::block,
after the thread has been unblocked.
Starting with this commit, m_blocker is also set to null in
Thread::unblock.
This change will allow us to implement a missing feature of the PT_TRACE
command of the ptrace syscall - stopping the traced thread when it
exits the execve syscall.
That feature will be implemented by sending a blocking SIGSTOP to the
traced thread after it has executed the execve logic and before it
starts executing the new program in userspace.
However, since Process::exec arranges the tss to return to userspace
(the so-called "yield-teleport"), the code in Thread::block that should
be run after the thread unblocks, and sets m_blocker to null, never
actually runs.
Setting m_blocker to null in Thread::unblock allows us to avoid an
incorrect state where the thread is in a Running state but conatins a
pointer to a Blocker.
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