add way to get index of thread in its threadpool

base/threadingconstructs.jl

@@ -35,6 +35,19 @@ julia> Threads.threadid(Threads.@spawn "foo")
 """
 threadid() = Int(ccall(:jl_threadid, Int16, ())+1)
 
+"""
+    Threads.threadpool_threadidx() -> Int
+
+Return the index of the current thread in its threadpool.
+i.e. For the first default thread when 1 interactive thread exists this returns 1
+because the interactive threadpool comes first in threadid order.
+"""
+function threadpool_threadidx()
+    pool  = threadpool()
+    first_tid = first(threadpooltids(pool))
+    return Int(threadid() - first_tid + 1)
+end
+
 # lower bound on the largest threadid()
 """
     Threads.maxthreadid() -> Int