[SPARK-5366][EC2] Check the mode of private key

Check the mode of private key file.

Author: liuchang0812 <liuchang0812@gmail.com>

Closes #4162 from Liuchang0812/ec2-script and squashes the following commits:

fc37355 [liuchang0812] quota file name
01ed464 [liuchang0812] more output
ce2a207 [liuchang0812] pep8
f44efd2 [liuchang0812] move code to real_main
8475a54 [liuchang0812] fix bug
cd61a1a [liuchang0812] import stat
c106cb2 [liuchang0812] fix trivis bug
89c9953 [liuchang0812] more output about checking private key
1177a90 [liuchang0812] remove commet
41188ab [liuchang0812] check the mode of private key

ec2/spark_ec2.py

@@ -24,10 +24,12 @@ from __future__ import with_statement
 import hashlib
 import logging
 import os
+import os.path
 import pipes
 import random
 import shutil
 import string
+from stat import S_IRUSR
 import subprocess
 import sys
 import tarfile
@@ -349,6 +351,7 @@ def launch_cluster(conn, opts, cluster_name):
     if opts.identity_file is None:
         print >> stderr, "ERROR: Must provide an identity file (-i) for ssh connections."
         sys.exit(1)
+
     if opts.key_pair is None:
         print >> stderr, "ERROR: Must provide a key pair name (-k) to use on instances."
         sys.exit(1)
@@ -1007,6 +1010,18 @@ def real_main():
             DeprecationWarning
         )
 
+    if opts.identity_file is not None:
+        if not os.path.exists(opts.identity_file):
+            print >> stderr,\
+                "ERROR: The identity file '{f}' doesn't exist.".format(f=opts.identity_file)
+            sys.exit(1)
+
+        file_mode = os.stat(opts.identity_file).st_mode
+        if not (file_mode & S_IRUSR) or not oct(file_mode)[-2:] == '00':
+            print >> stderr, "ERROR: The identity file must be accessible only by you."
+            print >> stderr, 'You can fix this with: chmod 400 "{f}"'.format(f=opts.identity_file)
+            sys.exit(1)
+
     if opts.ebs_vol_num > 8:
         print >> stderr, "ebs-vol-num cannot be greater than 8"
         sys.exit(1)