Mac OS: support Python >= 3.8 by using fork-based multiprocessing (#18124)

As detailed in https://bugs.python.org/issue33725, starting new
processes with 'fork' on Mac OS is not guaranteed to work in general.
As of Python 3.8 the default process spawning mechanism was changed
to avoid this issue.

Spack depends on the fork-based method to preserve file descriptors
transparently, to preserve global state, and to avoid pickling some
objects. An effort is underway to remove dependence on fork-based
process spawning (see #18205). In the meantime, this allows Spack to
run with Python 3.8 on Mac OS by explicitly choosing to use 'fork'.

Co-authored-by: Peter Josef Scheibel <scheibel1@llnl.gov>
Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>
Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>

lib/spack/llnl/util/lang.py

@@ -5,6 +5,7 @@
 
 from __future__ import division
 
+import multiprocessing
 import os
 import re
 import functools
@@ -19,6 +20,23 @@
 ignore_modules = [r'^\.#', '~$']
 
 
+# On macOS, Python 3.8 multiprocessing now defaults to the 'spawn' start
+# method. Spack cannot currently handle this, so force the process to start
+# using the 'fork' start method.
+#
+# TODO: This solution is not ideal, as the 'fork' start method can lead to
+# crashes of the subprocess. Figure out how to make 'spawn' work.
+#
+# See:
+# * https://github.com/spack/spack/pull/18124
+# * https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods  # noqa: E501
+# * https://bugs.python.org/issue33725
+if sys.version_info >= (3,):  # novm
+    fork_context = multiprocessing.get_context('fork')
+else:
+    fork_context = multiprocessing
+
+
 def index_by(objects, *funcs):
     """Create a hierarchy of dictionaries by splitting the supplied
     set of objects on unique values of the supplied functions.

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

@@ -21,6 +21,7 @@
 from six import StringIO
 
 import llnl.util.tty as tty
+from llnl.util.lang import fork_context
 
 try:
     import termios
@@ -430,7 +431,7 @@ def __enter__(self):
             except BaseException:
                 input_stream = None  # just don't forward input if this fails
 
-            self.process = multiprocessing.Process(
+            self.process = fork_context.Process(
                 target=_writer_daemon,
                 args=(
                     input_stream, read_fd, write_fd, self.echo, self.log_file,

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

@@ -24,6 +24,7 @@
 import traceback
 
 import llnl.util.tty.log as log
+from llnl.util.lang import fork_context
 
 from spack.util.executable import which
 
@@ -233,7 +234,7 @@ def start(self, **kwargs):
         ``minion_function``.
 
         """
-        self.proc = multiprocessing.Process(
+        self.proc = fork_context.Process(
             target=PseudoShell._set_up_and_run_controller_function,
             args=(self.controller_function, self.minion_function,
                   self.controller_timeout, self.sleep_time),