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On 7/29/2014 4:58 PM, Roberto Ierusalimschy wrote:
So, I guess that the best solution for this would be for the Lua API to receive an external random seed as argument to lua_newstate, and let the embedder decide whether to provide an actual random data or not. And of course, the standard interpreter would have to have an option to expose this if so desired.We have considered that. However, fixing the seed does not eliminate the problem, only ameliorate it. "Non-conventional" keys (such as tables and functions) use the object address as the key, so ASLR makes those keys non deterministic, independently of any seed. (In particular, this problem was already present in Lua well before 5.2.) The fact that, without random seeds, traversals for tables with no object keys are deterministic should be seen as an implementation detail. The clean approach is not to count on it. (You may sort your data, as Alex suggested.)
Indeed sorting is a solution, but when we made the switch to 5.2 in luatex it took quite a while before I got it catched in all places where consistent order of keys matters (at least stable / consistent for specific compiled versions of the program embedding Lua; it's less important across versions). It's a nice example of a change that looks harmless but can lead to surprises. (We do a lot of hashing and caching and when saving intermediate data for successive runs one needs to keep it in mind.).
It makes me wonder what surprises we run into with the new number implementation. At first sight it looks harmless, but one could as well argue for pretty long testing periods to make sure that it has no (interesting) side effects.
Anyway, progress is ok.
Hans
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