Fix a concurrency bug in iterate(::Dict) (#44534)

(cherry picked from commit 6be86a3)

Author: tkf

NEWS.md

@@ -102,8 +102,13 @@ New library functions
 * New function `Base.rest` for taking the rest of a collection, starting from a specific
   iteration state, in a generic way ([#37410]).
 
-New library features
---------------------
+Library changes
+---------------
+
+* A known concurrency issue of `iterate` methods on `Dict` and other derived objects such
+  as `keys(::Dict)`, `values(::Dict)`, and `Set` is fixed.  These methods of `iterate` can
+  now be called on a dictionary or set shared by arbitrary tasks provided that there are no
+  tasks mutating the dictionary or set ([#44534]).
 
 * The `redirect_*` functions now accept `devnull` to discard all output redirected to it, and as an empty
   input ([#36146]).

base/dict.jl

@@ -683,7 +683,7 @@ end
 
 @propagate_inbounds _iterate(t::Dict{K,V}, i) where {K,V} = i == 0 ? nothing : (Pair{K,V}(t.keys[i],t.vals[i]), i == typemax(Int) ? 0 : i+1)
 @propagate_inbounds function iterate(t::Dict)
-    _iterate(t, skip_deleted_floor!(t))
+    _iterate(t, skip_deleted(t, t.idxfloor))
 end
 @propagate_inbounds iterate(t::Dict, i) = _iterate(t, skip_deleted(t, i))