When the context passed to Nomad Allocation Exec is cancelled, the
process is not terminated. Instead, just the WebSocket connection is
closed. In order to terminate long-running processes, a special
character is injected into the standard input stream. This character is
parsed by the tty line discipline (tty has to be true). The line
discipline sends a SIGQUIT signal to the process, terminating it and
producing a core dump (in a file called 'core'). The SIGQUIT signal can
be caught but isn't by default, which is why the runner is destroyed if
the program does not terminate during a grace period after the signal
was sent.
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