Fix crashes when running spack install under nohup (#4926)

* Fix crashes when running spack install under nohup

Fixes #4919

For reasons that I do not entire understand, duplicate_stream() throws
an '[Errno 22] Invalid argument' exception when it tries to
`os.fdopen()` the duplicated file descriptor generated by
`os.dup(original.fileno())`.  See spack/llnl/util/lang.py, line
394-ish.

This happens when run under `nohup`, which supposedly has hooked
`stdin` to `/dev/null`.

It seems like opening and using `devnull` on the `input_stream` in
this situation is a reasonable way to handle the problem.

* Be more specific about error being handled.

Only catch the specific error that happens when trying to dup
the stdin that nohup provides.

Catching e as a StandardErorr and then
`type(e).__name__` tells me that it's an OSError.

Printing e.errno tells me that it's 22

Double checking tells me that 22 is EINVAL.

Phew.

lib/spack/spack/build_environment.py

@@ -54,6 +54,7 @@
 import inspect
 import multiprocessing
 import os
+import errno
 import shutil
 import sys
 import traceback
@@ -577,8 +578,15 @@ def child_execution(child_connection, input_stream):
     parent_connection, child_connection = multiprocessing.Pipe()
     try:
         # Forward sys.stdin to be able to activate / deactivate
-        # verbosity pressing a key at run-time
+        # verbosity pressing a key at run-time.  When sys.stdin can't
+        # be duplicated (e.g. running under nohup, which results in an
+        # '[Errno 22] Invalid argument') then just use os.devnull
+        try:
             input_stream = lang.duplicate_stream(sys.stdin)
+        except OSError as e:
+            if e.errno == errno.EINVAL:
+                tty.debug("Using devnull as input_stream")
+                input_stream = open(os.devnull)
         p = multiprocessing.Process(
             target=child_execution,
             args=(child_connection, input_stream)