If you have a library class or function that is not explicitly marked as thread safe, do not assume it is, regardless of whether or not it "should" be under any

If you have a library class or function that is not explicitly marked as thread safe, do not assume it is, regardless of whether or not it "should" be under any system of logic or reason. Why not? Because you have no idea what it might be doing behind the scenes. Even "immutable" objects might have mutable state under the covers of encapsulation (e.g. caching). And even if you somehow manage to prove that the current version IS safe, perhaps the next version won't be. After the code has been running perfectly for years and the library has been upgraded dozens of times, good luck every finding your subtle, intermittent threading disaster. Thread safety is an implementation detail unless you are explicitly told otherwise.