bpo-39776: Lock ++interp->tstate_next_unique_id (GH-18746)

- Threads created by PyGILState_Ensure() could have a duplicate tstate->id.

(cherry picked from commit b3b9ade)

Author: Stefan Krah

Misc/NEWS.d/next/Core and Builtins/2020-03-02-20-12-33.bpo-39776.fNaxi_.rst

@@ -0,0 +1,6 @@
+Fix race condition where threads created by PyGILState_Ensure() could get a
+duplicate id.
+
+This affects consumers of tstate->id like the contextvar caching machinery,
+which could return invalid cached objects under heavy thread load (observed
+in embedded scenarios).

Python/pystate.c

@@ -411,12 +411,12 @@ new_threadstate(PyInterpreterState *interp, int init)
         tstate->context = NULL;
         tstate->context_ver = 1;
 
-        tstate->id = ++interp->tstate_next_unique_id;
 
         if (init)
             _PyThreadState_Init(tstate);
 
         HEAD_LOCK();
+        tstate->id = ++interp->tstate_next_unique_id;
         tstate->prev = NULL;
         tstate->next = interp->tstate_head;
         if (tstate->next)