Avoid WorkerProcess._new_stdin FD sharing (#51623) (#51624)

This avoids holding open _new_stdin within the parent process, where
subsequent WorkerProcess forks will duplicate it, producing significant
noise in the FD table of every worker.

Fix by overriding start() and moving the work to there, with a finally:
to ensure parent FD is closed after start().

lib/ansible/executor/process/worker.py

@@ -67,14 +67,12 @@ class WorkerProcess(multiprocessing.Process):
         self._variable_manager = variable_manager
         self._shared_loader_obj = shared_loader_obj
 
-        if sys.stdin.isatty():
-            # dupe stdin, if we have one
-            self._new_stdin = sys.stdin
+    def _save_stdin(self):
+        self._new_stdin = os.devnull
         try:
-                fileno = sys.stdin.fileno()
-                if fileno is not None:
+            if sys.stdin.isatty() and sys.stdin.fileno() is not None:
                 try:
-                        self._new_stdin = os.fdopen(os.dup(fileno))
+                    self._new_stdin = os.fdopen(os.dup(sys.stdin.fileno()))
                 except OSError:
                     # couldn't dupe stdin, most likely because it's
                     # not a valid file descriptor, so we just rely on
@@ -83,9 +81,22 @@ class WorkerProcess(multiprocessing.Process):
         except (AttributeError, ValueError):
             # couldn't get stdin's fileno, so we just carry on
             pass
-        else:
-            # set to /dev/null
-            self._new_stdin = os.devnull
+
+    def start(self):
+        '''
+        multiprocessing.Process replaces the worker's stdin with a new file
+        opened on os.devnull, but we wish to preserve it if it is connected to
+        a terminal. Therefore dup a copy prior to calling the real start(),
+        ensuring the descriptor is preserved somewhere in the new child, and
+        make sure it is closed in the parent when start() completes.
+        '''
+
+        self._save_stdin()
+        try:
+            return super(WorkerProcess, self).start()
+        finally:
+            if self._new_stdin != os.devnull:
+                self._new_stdin.close()
 
     def _hard_exit(self, e):
         '''