Fix detection of parent process death (#7914)

This code assumes os.getppid() == 1 is a reliable signal of parent
death, because traditionally, pid=1 adopts orphaned processes.

However, systemd --user instances on linux are processes with pid > 1
that also seem to adopt orphan processes. In such cases, we would miss
parent death, and leak orphaned tail processes.

This changes the detection criteria to "parent pid has changed".

python_modules/dagster/dagster/core/execution/watch_orphans.py

@@ -24,7 +24,11 @@ def watch(args):
 
     while True:
         # check if this process has been orphaned, in which case kill the tail_pid
-        if os.getppid() == 1:
+
+        # we assume that the process is orphaned if parent pid changes. because systemd
+        # user instances also adopt processes, os.getppid() == 1 is no longer a reliable signal
+        # for being orphaned.
+        if os.getppid() != parent_pid:
             try:
                 os.kill(tail_pid, signal.SIGTERM)
             except OSError: