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https://github.com/tahoe-lafs/tahoe-lafs.git
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91 lines
3.5 KiB
Python
91 lines
3.5 KiB
Python
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"""
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If you execute force_repeatability() then the following things are changed in the runtime:
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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.
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2. os.urandom() is replaced by a fake urandom that returns a pseudorandom sequence.
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3. time.time() is replaced by a fake time that returns an incrementing number. (Original time.time is available as time.realtime.)
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Which seed will be used?
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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.
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Caveats:
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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.
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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.)
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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.
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"""
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import os, random, time
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if not hasattr(time, "realtime"):
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time.realtime = time.time
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if not hasattr(os, "realurandom"):
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os.realurandom = os.urandom
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if not hasattr(random, "realseed"):
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random.realseed = random.seed
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tdelta = 0
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seeded = False
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def force_repeatability():
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now = 1043659734.0
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def faketime():
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global tdelta
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tdelta += 1
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return now + tdelta
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time.faketime = faketime
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time.time = faketime
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from idlib import i2b
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def fakeurandom(n):
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if n > 20:
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z = i2b(random.getrandbits(20*8))
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elif n == 0:
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return ''
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else:
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z = i2b(random.getrandbits(n*8))
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x = z + "0" * (n-len(z))
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assert len(x) == n
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return x
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os.fakeurandom = fakeurandom
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os.urandom = fakeurandom
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global seeded
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if not seeded:
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SEED = os.environ.get('REPEATABLE_RANDOMNESS_SEED', None)
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if SEED is None:
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# Generate a seed which is integral and fairly short (to ease cut-and-paste, writing it down, etc.).
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t = time.realtime()
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subsec = t % 1
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t += (subsec * 1000000)
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t %= 1000000
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SEED = long(t)
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import sys
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sys.stdout.write("REPEATABLE_RANDOMNESS_SEED: %s\n" % SEED) ; sys.stdout.flush()
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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()
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random.seed(SEED)
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def seed_which_refuses(a):
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sys.stdout.write("I refuse to reseed to %s. Go away!\n" % (a,)) ; sys.stdout.flush()
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return
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random.realseed = random.seed
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random.seed = seed_which_refuses
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seeded = True
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import setutil
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setutil.RandomSet.DETERMINISTIC = True
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def restore_real_clock():
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time.time = time.realtime
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def restore_real_urandom():
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os.urandom = os.realurandom
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def restore_real_seed():
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random.seed = random.realseed
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def restore_non_repeatability():
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restore_real_seed()
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restore_real_urandom()
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restore_real_clock()
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