If you call email.utils.make_msgid a number of times within the same
second, the uniqueness of the results depends on random.randint(100000)
returning different values each time.

A little mathematics proves that you don't have to call make_msgid
*that* often to get the same message id twice: if you call it 'n' times,
the probability of a collision is approximately "1 -
math.exp(-n*(n-1)/200000.0)", and for n == 100, that's about 5%.  For n
== 1000, it's over 99%.

These numbers are born out by experiment:

>>> def collisions(n):
...     msgids = [make_msgid() for i in range(n)]
...     return len(msgids) - len(set(msgids))
... 
>>> sum((collisions(100)>0) for i in range(1000))
49
>>> sum((collisions(1000)>0) for i in range(1000))
991

I think probably having a counter in addition to the randomness would be
a good fix for the problem, though I guess then you have to worry about
thread safety.

A higher resolution timer would also help, of course.

(Thanks to James Knight for the prod).

This patch replaces the random part with an increasing sequence (in a 
thread safe way).
Also, added a test case for make_msgid (there was none previously)

Is it ok if the message id is predictable?
Besides, _gen_next_number() can more efficiently be written as:

_gen_next_number = itertools.cycle(xrange(N))

--- El jue 6-ago-09, Antoine Pitrou <report@bugs.python.org> escribió:

> Antoine Pitrou <pitrou@free.fr>
> added the comment:
> 
> Is it ok if the message id is predictable?

I don't know of any use of message ids apart from uniquely identifying the message, but we could still keep a random part followed by the sequence.

> Besides, _gen_next_number() can more efficiently be written
> as:
> 
> _gen_next_number = itertools.cycle(xrange(N))

Written that way it will eventually hold a list with 100000 integers forever.

      Yahoo! Cocina

Encontra las mejores recetas con Yahoo! Cocina.

http://ar.mujer.yahoo.com/cocina/

An updated version of the make_msgid patch.
Includes a random part plus a sequential part. Testing takes at most 3 
seconds now (we're interested in those msgids generated in a whole 
second)