only allow one completion task to infer in repl_eval_ex at a time

stdlib/REPL/src/REPLCompletions.jl

@@ -730,6 +730,9 @@ function resolve_toplevel_symbols!(src::Core.CodeInfo, mod::Module)
     return src
 end
 
+# lock so that threaded completions don't run inference on the same expr at the same time
+const inference_lock = Base.ReentrantLock()
+
 # lower `ex` and run type inference on the resulting top-level expression
 function repl_eval_ex(@nospecialize(ex), context_module::Module; limit_aggressive_inference::Bool=false)
     if (isexpr(ex, :toplevel) || isexpr(ex, :tuple)) && !isempty(ex.args)
@@ -756,9 +759,13 @@ function repl_eval_ex(@nospecialize(ex), context_module::Module; limit_aggressiv
     result = CC.InferenceResult(mi)
     frame = CC.InferenceState(result, src, #=cache=#:no, interp)
 
-    # NOTE Use the fixed world here to make `REPLInterpreter` robust against
-    #      potential invalidations of `Core.Compiler` methods.
-    Base.invoke_in_world(COMPLETION_WORLD[], CC.typeinf, interp, frame)
+    # With threaded completions, this inference is often done on the same `src` repeatedly,
+    # so wait for one to complete rather than multiple tasks doing the same work.
+    @lock inference_lock begin
+        # NOTE Use the fixed world here to make `REPLInterpreter` robust against
+        #      potential invalidations of `Core.Compiler` methods.
+        Base.invoke_in_world(COMPLETION_WORLD[], CC.typeinf, interp, frame)
+    end
 
     result = frame.result.result
     result === Union{} && return nothing # for whatever reason, callers expect this as the Bottom and/or Top type instead