CVE-2021-41213: Deadlock in mutually recursive `tf.function` objects

The code behind tf.function API can be made to deadlock when two tf.function decorated Python functions are mutually recursive:

import tensorflow as tf

@tf.function()
def fun1(num):
if num == 1:
return
print(num)
fun2(num-1)

@tf.function()
def fun2(num):
if num == 0:
return
print(num)
fun1(num-1)

fun1(9)

This occurs due to using a non-reentrant Lock Python object.

Loading any model which contains mutually recursive functions is vulnerable. An attacker can cause denial of service by causing users to load such models and calling a recursive tf.function, although this is not a frequent scenario.