gh-135106: Randomly return 1 in _Py_RecursionLimit_GetMargin

Returns `1` a little less than 1% of the time in
_Py_RecursionLimit_GetMargin() to force the use of the trashcan
mechanism.

Author: colesbury

Include/cpython/pystate.h

@@ -208,6 +208,8 @@ struct _ts {
     */
     PyObject *threading_local_sentinel;
     _PyRemoteDebuggerSupport remote_debugger_support;
+
+    uint64_t prng;
 };
 
 /* other API */

Include/internal/pycore_pystate.h

@@ -325,6 +325,16 @@ _Py_RecursionLimit_GetMargin(PyThreadState *tstate)
     _PyThreadStateImpl *_tstate = (_PyThreadStateImpl *)tstate;
     assert(_tstate->c_stack_hard_limit != 0);
     intptr_t here_addr = _Py_get_machine_stack_pointer();
+
+    // splitmix64 from https://prng.di.unimi.it/splitmix64.c
+    uint64_t z = (tstate->prng += 0x9e3779b97f4a7c15);
+	z = (z ^ (z >> 30)) * 0xbf58476d1ce4e5b9;
+	z = (z ^ (z >> 27)) * 0x94d049bb133111eb;
+	uint64_t r = z ^ (z >> 31);
+    if ((r & 0xFF) < 2) { // 2/256 chance = ~ 0.8% chance
+        return 1;
+    }
+
     return Py_ARITHMETIC_RIGHT_SHIFT(intptr_t, here_addr - (intptr_t)_tstate->c_stack_soft_limit, PYOS_STACK_MARGIN_SHIFT);
 }
 

Python/pystate.c

@@ -1486,6 +1486,10 @@ init_threadstate(_PyThreadStateImpl *_tstate,
         tstate->state = _Py_THREAD_SUSPENDED;
     }
 
+    PyTime_t now;
+    PyTime_MonotonicRaw(&now);
+    tstate->prng = (uint64_t)now;
+
     tstate->_status.initialized = 1;
 }