copy repeatable_random from pyutil

diff --git a/src/allmydata/util/repeatable_random.py b/src/allmydata/util/repeatable_random.py
new file mode 100644 (file)
index 0000000..0fa78e3
--- /dev/null
+++ b/src/allmydata/util/repeatable_random.py
@@ -0,0 +1,90 @@
+"""
+If you execute force_repeatability() then the following things are changed in the runtime:
+
+1.  random.random() and its sibling functions, and random.Random.seed() in the random module are seeded with a known seed so that they will return the same sequence on each run.
+2.  os.urandom() is replaced by a fake urandom that returns a pseudorandom sequence.
+3.  time.time() is replaced by a fake time that returns an incrementing number.  (Original time.time is available as time.realtime.)
+
+Which seed will be used?
+
+If the environment variable REPEATABLE_RANDOMNESS_SEED is set, then it will use that.  Else, it will use the current real time.  In either case it logs the seed that it used.
+
+Caveats:
+
+1.  If some code has acquired a random.Random object before force_repeatability() is executed, then that Random object will produce non-reproducible results.  For example, the tempfile module in the Python Standard Library does this.
+2.  Likewise if some code called time.time() before force_repeatability() was called, then it will have gotten a real time stamp.  For example, trial does this.  (Then it later subtracts that real timestamp from a faketime timestamp to calculate elapsed time, resulting in a large negative elapsed time.)
+3.  The output from the fake urandom has weird distribution for performance reasons-- every byte after the first 20 bytes resulting from a single call to os.urandom() is zero.  In practice this hasn't caused any problems.
+"""
+
+import os, random, time
+if not hasattr(time, "realtime"):
+    time.realtime = time.time
+if not hasattr(os, "realurandom"):
+    os.realurandom = os.urandom
+if not hasattr(random, "realseed"):
+    random.realseed = random.seed
+
+tdelta = 0
+seeded = False
+def force_repeatability():
+    now = 1043659734.0
+    def faketime():
+        global tdelta
+        tdelta += 1
+        return now + tdelta
+    time.faketime = faketime
+    time.time = faketime
+
+    from idlib import i2b
+    def fakeurandom(n):
+        if n > 20:
+            z = i2b(random.getrandbits(20*8))
+        elif n == 0:
+            return ''
+        else:
+            z = i2b(random.getrandbits(n*8))
+        x = z + "0" * (n-len(z))
+        assert len(x) == n
+        return x
+    os.fakeurandom = fakeurandom
+    os.urandom = fakeurandom
+
+    global seeded
+    if not seeded:
+        SEED = os.environ.get('REPEATABLE_RANDOMNESS_SEED', None)
+
+        if SEED is None:
+            # Generate a seed which is integral and fairly short (to ease cut-and-paste, writing it down, etc.).
+            t = time.realtime()
+            subsec = t % 1
+            t += (subsec * 1000000)
+            t %= 1000000
+            SEED = long(t)
+        import sys
+        sys.stdout.write("REPEATABLE_RANDOMNESS_SEED: %s\n" % SEED) ; sys.stdout.flush()
+        sys.stdout.write("In order to reproduce this run of the code, set the environment variable \"REPEATABLE_RANDOMNESS_SEED\" to %s before executing.\n" % SEED) ; sys.stdout.flush()
+        random.seed(SEED)
+
+        def seed_which_refuses(a):
+            sys.stdout.write("I refuse to reseed to %s.  Go away!\n" % (a,)) ; sys.stdout.flush()
+            return
+        random.realseed = random.seed
+        random.seed = seed_which_refuses
+        seeded = True
+
+    import setutil
+    setutil.RandomSet.DETERMINISTIC = True
+
+def restore_real_clock():
+    time.time = time.realtime
+
+def restore_real_urandom():
+    os.urandom = os.realurandom
+
+def restore_real_seed():
+    random.seed = random.realseed
+
+def restore_non_repeatability():
+    restore_real_seed()
+    restore_real_urandom()
+    restore_real_clock()