Override temporary directory before spawning subprocess.

Python multiprocessing is using sockets to communicate. UNIX socket names are
bound by PATH_MAX which includes temporary directory name.

gsutil uses multiprocessing, which creates file sockets in the tmpdir.
On some testing configurations, the tmpdir is set to a very long path, which
causes gsutil to fail with AF_UNIX path too long errors.

To prevent multiprocess failures, this CL overrides temporary directory
dictated by the environment to /tmp.

Bug: 812581
Change-Id: Idcd99d13b2e20b3095111fa26ec4e242848c8848
Reviewed-on: https://chromium-review.googlesource.com/923170Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>

download_from_google_storage.py

@@ -21,6 +21,11 @@ import time
 import subprocess2
 
 
+# Env vars that tempdir can be gotten from; minimally, this
+# needs to match python's tempfile module and match normal
+# unix standards.
+_TEMPDIR_ENV_VARS = ('TMPDIR', 'TEMP', 'TMP')
+
 GSUTIL_DEFAULT_PATH = os.path.join(
     os.path.dirname(os.path.abspath(__file__)), 'gsutil.py')
 # Maps sys.platform to what we actually want to call them.
@@ -82,6 +87,9 @@ class Gsutil(object):
       env['AWS_CREDENTIAL_FILE'] = self.boto_path
       env['BOTO_CONFIG'] = self.boto_path
 
+    if PLATFORM_MAPPING[sys.platform] != 'win':
+      env.update((x, "/tmp") for x in _TEMPDIR_ENV_VARS)
+
     return env
 
   def call(self, *args):