avoid double closing of fd in sub-processes (#47035)

Both `multiprocessing.connection.Connection.__del__` and `io.IOBase.__del__` called `os.close` on the same file descriptor. As of Python 3.13, this is an explicit warning. Ensure we close once by usef `os.fdopen(..., closefd=False)`

lib/spack/llnl/util/tty/log.py

@@ -348,7 +348,19 @@ def close(self):
 class MultiProcessFd:
     """Return an object which stores a file descriptor and can be passed as an
     argument to a function run with ``multiprocessing.Process``, such that
-    the file descriptor is available in the subprocess."""
+    the file descriptor is available in the subprocess. It provides access via
+    the `fd` property.
+
+    This object takes control over the associated FD: files opened from this
+    using `fdopen` need to use `closefd=False`.
+    """
+
+    # As for why you have to fdopen(..., closefd=False): when a
+    # multiprocessing.connection.Connection object stores an fd, it assumes
+    # control over it, and will attempt to close it when gc'ed during __del__;
+    # if you fdopen(multiprocessfd.fd, closefd=True) then the resulting file
+    # will also assume control, and you can see warnings when there is an
+    # attempted double close.
 
     def __init__(self, fd):
         self._connection = None
@@ -361,33 +373,20 @@ def __init__(self, fd):
     @property
     def fd(self):
         if self._connection:
-            return self._connection._handle
+            return self._connection.fileno()
         else:
             return self._fd
 
     def close(self):
+        """Rather than `.close()`ing any file opened from the associated
+        `.fd`, the `MultiProcessFd` should be closed with this.
+        """
         if self._connection:
             self._connection.close()
         else:
             os.close(self._fd)
 
 
-def close_connection_and_file(multiprocess_fd, file):
-    # MultiprocessFd is intended to transmit a FD
-    # to a child process, this FD is then opened to a Python File object
-    # (using fdopen). In >= 3.8, MultiprocessFd encapsulates a
-    # multiprocessing.connection.Connection; Connection closes the FD
-    # when it is deleted, and prints a warning about duplicate closure if
-    # it is not explicitly closed. In < 3.8, MultiprocessFd encapsulates a
-    # simple FD; closing the FD here appears to conflict with
-    # closure of the File object (in < 3.8 that is). Therefore this needs
-    # to choose whether to close the File or the Connection.
-    if sys.version_info >= (3, 8):
-        multiprocess_fd.close()
-    else:
-        file.close()
-
-
 @contextmanager
 def replace_environment(env):
     """Replace the current environment (`os.environ`) with `env`.
@@ -932,10 +931,10 @@ def _writer_daemon(
     # 1. Use line buffering (3rd param = 1) since Python 3 has a bug
     # that prevents unbuffered text I/O.
     # 2. Python 3.x before 3.7 does not open with UTF-8 encoding by default
-    in_pipe = os.fdopen(read_multiprocess_fd.fd, "r", 1, encoding="utf-8")
+    in_pipe = os.fdopen(read_multiprocess_fd.fd, "r", 1, encoding="utf-8", closefd=False)
 
     if stdin_multiprocess_fd:
-        stdin = os.fdopen(stdin_multiprocess_fd.fd)
+        stdin = os.fdopen(stdin_multiprocess_fd.fd, closefd=False)
     else:
         stdin = None
 
@@ -1025,9 +1024,9 @@ def _writer_daemon(
         if isinstance(log_file, io.StringIO):
             control_pipe.send(log_file.getvalue())
         log_file_wrapper.close()
-        close_connection_and_file(read_multiprocess_fd, in_pipe)
+        read_multiprocess_fd.close()
         if stdin_multiprocess_fd:
-            close_connection_and_file(stdin_multiprocess_fd, stdin)
+            stdin_multiprocess_fd.close()
 
         # send echo value back to the parent so it can be preserved.
         control_pipe.send(echo)

lib/spack/spack/build_environment.py

@@ -1194,7 +1194,7 @@ def _setup_pkg_and_run(
         # that the parent process is not going to read from it till we
         # are done with the child, so we undo Python's precaution.
         if input_multiprocess_fd is not None:
-            sys.stdin = os.fdopen(input_multiprocess_fd.fd)
+            sys.stdin = os.fdopen(input_multiprocess_fd.fd, closefd=False)
 
         pkg = serialized_pkg.restore()