mitogen: first_stage: Add comments about closed STDERR

Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>

mitogen/parent.py

@@ -1415,9 +1415,6 @@ class Connection(object):
     #   W: write side of interpreter stdin.
     #   r: read side of core_src FD.
     #   w: write side of core_src FD.
-
-    # Final os.close(STDERR_FILENO) to avoid --py-debug build corrupting stream with
-    # "[1234 refs]" during exit.
     @staticmethod
     def _first_stage():
         # Bail out in case STDIN or STDOUT is not accessible (e.g. closed).
@@ -1425,6 +1422,8 @@ class Connection(object):
         # unexpected behavior that is difficult to diagnose.
         os.fstat(0)
         os.fstat(1)
+        # If STDERR (2) is already closed, it might be possible that one of the
+        # returned pipe FDs is 2.
         R,W=os.pipe()
         r,w=os.pipe()
         if os.fork():
@@ -1460,6 +1459,10 @@ class Connection(object):
         f.write(C)
         f.close()
         os.write(1,'MITO001\n'.encode())
+        # Final os.close(STDERR_FILENO) to avoid `--py-debug` build corrupting
+        # stream with "[1234 refs]" during exit.
+        # If STDERR is already closed an OSError is raised, but no one cares
+        # as STDERR is closed and the exit status is not forwarded.
         os.close(2)
 
     def get_python_argv(self):