Is it safe for multiple threads to get userdata from a pool?

[Greg Hudson <gh...@MIT.EDU>]

I'm considering a design for Subversion where in the single-threaded
environment, a pool is created and some user data is stored in it.
Then, in the multithreaded environment, many threads can get (but not
set) userdata within that pool.

In the current implementation, that should be safe; apr_hash_get does
not modify any part of the hash table, so unsynchronized gets
shouldn't run into any difficulties.  However, it's not hard to
imagine an implementation where it's not safe (for instance, when
apr_hash_get() finds a hash entry, it could pull it to the front of
the hash bucket's linked list to speed multiple accesses to the same
key).

Do people think it's reasonable for an application to do this?

(In some ways life would be easier if a caller could lock and unlock a
pool, using the same mutex which currently protects the subpool list.
If I can't rely on unsynchronized gets being safe, I'll have to
shuttle around a mutex along with my pool, I think.)

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