# HG changeset patch
# User Brian Harring <ferringb@gmail.com>
# Date 1314669945 25200
# Node ID 49c13a1575506e1ec610bb49eb637f5715274c5c
# Parent  fb8d7a666bed19f5aa1d18a1288bbe08e5fbd07d
Ensure child processes do not inherit the parents random seed for tempfile

All tempfile functionality currently is reliant on a pseudorandom source; this
source is unfortunately global, meaning that children processes inherit it, and will
get the same set of random values.  This introduces predictability into the
parent/children tempfile creation, and a bit of contention.

Suspect that in existing code that has issues this can be useful for an attack
vector; primarily it's an annoying gotcha when doing a form of multiprocessing
with multiple workers.

diff -r fb8d7a666bed -r 49c13a157550 Lib/tempfile.py
--- a/Lib/tempfile.py	Tue Aug 30 00:28:40 2011 +0200
+++ b/Lib/tempfile.py	Mon Aug 29 19:05:45 2011 -0700
@@ -112,8 +112,13 @@
 
     characters = "abcdefghijklmnopqrstuvwxyz0123456789_"
 
-    def __init__(self):
-        self.rng = _Random()
+    @property
+    def rng(self):
+        cur_pid = _os.getpid()
+        if cur_pid != getattr(self, '_rng_pid', None):
+            self._rng = _Random()
+            self._rng_pid = cur_pid
+        return self._rng
 
     def __iter__(self):
         return self
diff -r fb8d7a666bed -r 49c13a157550 Lib/test/test_tempfile.py
--- a/Lib/test/test_tempfile.py	Tue Aug 30 00:28:40 2011 +0200
+++ b/Lib/test/test_tempfile.py	Mon Aug 29 19:05:45 2011 -0700
@@ -1,6 +1,7 @@
 # tempfile.py unit tests.
 import tempfile
 import os
+import signal
 import sys
 import re
 import warnings
@@ -135,6 +136,37 @@
         except:
             self.failOnException("iteration")
 
+    @unittest.skipUnless(hasattr(os, 'fork'),
+        "os.fork is required for this test")
+    def test_process_awareness(self):
+        # ensure that the random source differs between
+        # child and parent.
+        read_fd, write_fd = os.pipe()
+        pid = None
+        try:
+            pid = os.fork()
+            if not pid:
+                os.close(read_fd)
+                os.write(write_fd, next(self.r).encode("ascii"))
+                os.close(write_fd)
+                # bypass the normal exit handlers- leave those to
+                # the parent.
+                os._exit(0)
+            parent_value = next(self.r)
+            child_value = os.read(read_fd, len(parent_value)).decode("ascii")
+        finally:
+            if pid:
+                # best effort to ensure the process can't bleed out
+                # via any bugs above
+                try:
+                    os.kill(pid, signal.SIGKILL)
+                except EnvironmentError:
+                    pass
+            os.close(read_fd)
+            os.close(write_fd)
+        self.assertNotEqual(child_value, parent_value)
+
+
 test_classes.append(test__RandomNameSequence)