Merge pull request #270 from nsmith-/xrootd_forksafe

Make XRootDSource fork-safe

uproot/source/xrootd.py

@@ -2,6 +2,7 @@
 
 # BSD 3-Clause License; see https://github.com/scikit-hep/uproot/blob/master/LICENSE
 
+import os
 import threading
 
 import numpy
@@ -21,6 +22,7 @@ def __init__(self, path, timeout=None, *args, **kwds):
 
     def _open(self):
         try:
+            os.environ["XRD_RUNFORKHANDLER"] = "1"   # To make uproot + xrootd + multiprocessing work
             import pyxrootd.client
         except ImportError:
             raise ImportError("Install pyxrootd package with:\n    conda install -c conda-forge xrootd\n(or download from http://xrootd.org/dload.html and manually compile with cmake; setting PYTHONPATH and LD_LIBRARY_PATH appropriately).")