Handle ValueError when child wants to reset stderr but it is closed. (p…

…antsbuild#6932)

### Problem

After forking, when the child wants to reset `sys.stderr` as part of setting up the `ExceptionSink`, if `sys.stderr` was closed, it would except and crash.

### Solution

Wrap the line in `try:except`, and log it as a warning. However, I have a doubt:
The line in question happens in {{post_fork_child}} after a call to {{daemonize}}, and fails because {{sys.stderr}} is closed. So, given that we are considering that case as "non-fatal" (hence turning the exception into a warning), would it make sense to instead write something like:

```python
if not sys.stderr.closed:
  ExceptionSink.reset_interactive_output_stream(sys.stderr)
else:
  logger = logging.getLogger(__name__)
  logger.warn("Cannot reset output stream - sys.stderr is closed")
```

The difference is small, but if the answer is no it means that I'm missing something.

### Result

One less hard-to-find crash, hopefully.

src/python/pants/base/exception_sink.py

@@ -152,16 +152,20 @@ def reset_interactive_output_stream(cls, interactive_output_stream):
 
     This is where the the error message on exit will be printed to as well.
     """
-    # NB: mutate process-global state!
-    if faulthandler.unregister(signal.SIGUSR2):
-      logger.debug('re-registering a SIGUSR2 handler')
-    # This permits a non-fatal `kill -31 <pants pid>` for stacktrace retrieval.
-    faulthandler.register(signal.SIGUSR2, interactive_output_stream,
-                          all_threads=True, chain=False)
-
-    # NB: mutate the class variables!
-    # We don't *necessarily* need to keep a reference to this, but we do here for clarity.
-    cls._interactive_output_stream = interactive_output_stream
+    try:
+      # NB: mutate process-global state!
+      if faulthandler.unregister(signal.SIGUSR2):
+        logger.debug('re-registering a SIGUSR2 handler')
+      # This permits a non-fatal `kill -31 <pants pid>` for stacktrace retrieval.
+      faulthandler.register(signal.SIGUSR2, interactive_output_stream,
+                            all_threads=True, chain=False)
+
+      # NB: mutate the class variables!
+      # We don't *necessarily* need to keep a reference to this, but we do here for clarity.
+      cls._interactive_output_stream = interactive_output_stream
+    except ValueError:
+      # Warn about "ValueError: IO on closed file" when stderr is closed.
+      ExceptionSink.log_exception("Cannot reset output stream - sys.stderr is closed")
 
   @classmethod
   def exceptions_log_path(cls, for_pid=None, in_dir=None):

src/python/pants/bin/daemon_pants_runner.py

@@ -283,6 +283,7 @@ def post_fork_child(self):
     # terminal window.
     # TODO: test the above!
     ExceptionSink.reset_exiter(self._exiter)
+
     ExceptionSink.reset_interactive_output_stream(sys.stderr)
 
     # Ensure anything referencing sys.argv inherits the Pailgun'd args.