Automarks are similar to bookmarks placed by the terminal, allowing the
user to selectively remove a single command and its output from the
terminal scrollback.
This commit implements a single way to add marks: automatically placing
them when the shell becomes interactive.
To make sure the shell behaves correctly after its expected prompt
position changes, the terminal layer forces a resize event to be passed
to the shell on such (possibly) partial clears; this also has the nice
side effect of fixing the disappearing prompt on the preexisting "clear
including history" action: Fixes#4192.
MasterPTY::read called DoubleBuffer::read which takes a mutex (which
may block) while holding m_slave's spinlock. If it did block, and was
later rescheduled on a different physical CPU, we would deadlock on
re-locking m_slave inside the unblock callback. (Since our recursive
spinlock implementation is processor based and not process based)
MasterPTY's double buffer unblock callback would take m_slave's
spinlock and then call evaluate_block_conditions() which would take
BlockerSet's spinlock, while on the other hand, BlockerSet's
add_blocker would take BlockerSet's spinlock, and then call
should_add_blocker, which would call unblock_if_conditions_are_met,
which would then call should_unblock, which will finally call
MasterPTY::can_read() which will take m_slave's spinlock.
Resolve this by moving the call to evaluate_block_conditions() out of
the scope of m_slave's spinlock, as there's no need to hold the lock
while calling it anyways.
