1 files changed, 9 insertions, 0 deletions
diff --git a/Python/thread_nt.h b/Python/thread_nt.h
index ee2079f..ab5a081 100644
--- a/Python/thread_nt.h
+++ b/Python/thread_nt.h
@@ -389,11 +389,20 @@ PyThread_delete_key(int key)
     TlsFree(key);
 }
 
+/* We must be careful to emulate the strange semantics implemented in thread.c,
+ * where the value is only set if it hasn't been set before.
+ */
 int
 PyThread_set_key_value(int key, void *value)
 {
     BOOL ok;
+    void *oldvalue;
 
+    assert(value != NULL);
+    oldvalue = TlsGetValue(key);
+    if (oldvalue != NULL)
+        /* ignore value if already set */
+        return 0;
     ok = TlsSetValue(key, value);
     if (!ok)
         return -1;