[tor-bugs] #17800 [Tor]: Tor Unit Tests should TT_FORK before initialising global PRNG state

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#17800: Tor Unit Tests should TT_FORK before initialising global PRNG state
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     Reporter:  teor    |      Owner:
         Type:  defect  |     Status:  new
     Priority:  Low     |  Milestone:  Tor: very long term
    Component:  Tor     |    Version:
     Severity:  Minor   |   Keywords:  easy
Actual Points:          |  Parent ID:
       Points:          |    Sponsor:
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 Tor unit tests are meant to TT_FORK before changing any global process
 state that can't be reverted.

 In #17789, we discovered that one or more tests initialise the OpenSSL
 CSPRNG without forking first. (This can't be undone.)

 While this isn't likely to cause any issues, (#17789 was marked as wontfix
 for other reasons), it would be nice to fix it eventually for correctness.

 One way to do this is:
 * initialise a static variable to 0.
 * set it to 1 just after fork()ing via TT_FORK.
 * tor_assert() that it's 1 in functions that irreversibly modify global
 test process state, if TOR_UNIT_TESTS is defined:
   * crypto_early_init(), which seeds the OpenSSL CSPRNG,
   * (I'm sure many other init functions do similar things)
 * mark the tests that assert as TT_FORK

 Marked as Low/Minor because this doesn't currently cause any issues, but
 if tests start failing due to interactions with previous global state, we
 should look at it again.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17800>
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