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

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

diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-03-02-20-12-33.bpo-39776.fNaxi_.rst b/Misc/NEWS.d/next/Core and Builtins/2020-03-02-20-12-33.bpo-39776.fNaxi_.rst
new file mode 100644 (file)
index 0000000..e5a00bd
--- /dev/null
+++ b/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).

diff --git a/Python/pystate.c b/Python/pystate.c
index ebc17ea5a721930f65d9ee246ed6ab67946da935..4001c63ff25ee9ea319410797b3de730c145727b 100644 (file)
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -606,13 +606,12 @@ 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(runtime);
+    tstate->id = ++interp->tstate_next_unique_id;
     tstate->prev = NULL;
     tstate->next = interp->tstate_head;
     if (tstate->next)