Cory Benfield added the comment:
> If you read #25003, it's clear that /dev/urandom is a well-known UNIX
> facility with well-known, predictable behavior. One behavior that I'll draw
> particular attention to now: it will never block. If the system is low on
> entropy, /dev/urandom will produce lower-quality random bits.
That's not accurate.
/dev/urandom is a well-known UNIX facility, yes, but it does not have
consistent behaviour across Unices. The behaviour you've described here Larry
is a well-known *Linux* behaviour.
However, on other Unices the behaviour is different. Here, for example, is the
relevant man page from Mac OS X ("man 4 random"):
/dev/urandom is a compatibility nod to Linux. On Linux, /dev/urandom will
produce lower quality output if the entropy pool drains, while
/dev/random will prefer to block and wait for additional entropy to be
collected. With Yarrow, this choice and distinction is not necessary,
and the two devices behave identically. You may use either.
Note the specific wording here: "the two devices behave identically". That is
to say, on OS X both /dev/random and /dev/urandom are identical devices, and
that includes the fact that both will in principle block if used without
sufficient entropy.
OS X's implementation is a direct descendent of FreeBSD's, so the same caveats
apply there, and in fact all the BSDs have this exact same behaviour.
So, again, I repeat my objection from above: if the concern is that starting
Python must never block, then Python must *never* read from /dev/urandom on
startup. Otherwise, Python *can* block on BSDs (OS X included in principle,
though in practice I doubt Apple will use Python that early in boot).
At this point I literally no longer care whether os.urandom() is just a wrapper
around /dev/urandom: we can look back on this in 10 years and see how we feel
about the choices made by core dev at that time. But if we're arguing that this
issue is about "Python must never block at startup", then we really have to
acknowledge that /dev/urandom *can block* on some Unices, and so is entirely
unacceptable for reading at startup.
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<http://bugs.python.org/issue26839>
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