bpo-39776: Lock ++interp->tstate_next_unique_id. (GH-18746) (#18746) (#18752)

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

(cherry picked from commit b3b9ade4a3d3fe00d933bcd8fc5c5c755d1024f9)

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 aba673c00a432233d38700718e3d5797b8b45a5a..9f99060feceb2180f7f9eaf774925c5b7c4a76e1 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(runtime, tstate);
     }
 
     HEAD_LOCK(runtime);
+    tstate->id = ++interp->tstate_next_unique_id;
     tstate->prev = NULL;
     tstate->next = interp->tstate_head;
     if (tstate->next)