fix: rewritten __poll() using psutil

Author: fstagni

src/DIRAC/Core/Utilities/Subprocess.py

@@ -162,7 +162,6 @@ def __init__(self, timeout=False, bufferLimit=52428800):
 
         self.child = None
         self.childPID = 0
-        self.childKilled = False
         self.callback = None
         self.bufferList = []
         self.cmdSeq = []
@@ -252,7 +251,12 @@ def __terminateProcess(self, process):
         return psutil.wait_procs([process], timeout=60)
 
     def __poll(self, pid):
-        """return pid status"""
+        """Non-blocking check of whether process `pid` is still alive.
+        Returns:
+        - (0, 0) if process is still running  (like os.waitpid(pid, os.WNOHANG))
+        - (pid, exitcode) if process has terminated
+        - None if process info cannot be retrieved
+        """
         try:
             p = psutil.Process(pid)
             exitcode = p.wait(timeout=0)