[3.15] gh-161712: Read the clock once in ThreadHandle_join() (GH-161771)

Py_MAX() evaluated _PyDeadline_Get() twice. If the deadline passed
between the two calls, the negative second value was used as the
timeout, which means no timeout, and Thread.join(timeout) could block
forever. main already reads it once since gh-157495.

Author: aozmekik

Misc/NEWS.d/next/Library/2026-11-10-19-00-00.gh-issue-161712.HyM9PA.rst

@@ -0,0 +1,3 @@
+Fix :meth:`threading.Thread.join` with a timeout blocking forever when the
+deadline passed between the two reads of the clock used to compute the time
+left.

Modules/_threadmodule.c

@@ -571,7 +571,8 @@ ThreadHandle_join(ThreadHandle *self, PyTime_t timeout_ns)
         if (deadline) {
             // _PyDeadline_Get will return a negative value if the deadline has
             // been exceeded.
-            timeout_ns = Py_MAX(_PyDeadline_Get(deadline), 0);
+            timeout_ns = _PyDeadline_Get(deadline);
+            timeout_ns = Py_MAX(timeout_ns, 0);
         }
 
         if (timeout_ns) {