context: Fix deadlock when new context is created from already-canceled parent
When _BaseCtx is setting up cancel propagation it locks a parent, checks for parent.err != nil, and, if it is, calls ctx._cancel(parent.err) _with_ _holding_ parent.mu. Since _cancel internally also goes through parents and locks them, this was deadlocking on the second call to parent.mu.lock(). -> Fix it by calling ctx._cancel(err) in the constructor outside of parent lock. The bug was there from the beginning - from e9567c7b (context: New package that mirrors Go's context). /trusted-by @jerome /reviewed-on !16
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