gh-141786: Fix missing parent executor during JIT trace

[caje731]

An executor's trace can point to another executor, forming a graph of traces / executors. Sometimes it is possible while recording a trace that the parent executor is freed / invalidated halfway. This leads to the bug described in the issue gh-141786, which if left unfixed could cause a memory leak.

This patch checks for the validity of the parent executor as well as allowing JIT from the cold executor. While the cold executor is not linked, it is the executor responsible for creating side traces and we still want to JIT from it.

[caje731]

@Fidget-Spinner who the issue is currently assigned to has helped me with this PR.

[Fidget-Spinner]

@devdanzin can we use your repro as a test case please?

[devdanzin]

Sure, in this case and any others, whenever you want.

[Fidget-Spinner]

@caje731 please add a test case to Lib/test/test_capi/test_opt.py by copying the script from here #141786 (comment). You can take a look at test_interpreter_finalization_with_generator_alive to see how to run a test in a subprocess easily.

[caje731]

I should note that the following simple diff in the test:

diff --git a/Lib/test/test_capi/test_opt.py b/Lib/test/test_capi/test_opt.py
index 0f67f19a115..9027be79254 100644
--- a/Lib/test/test_capi/test_opt.py
+++ b/Lib/test/test_capi/test_opt.py
@@ -2772,7 +2772,7 @@ def B(B):
                if rand.random() < 0.1:
                    C(B)

-            for D in range(200):
+            for D in range(350):
                try:
                    B(D)
                except:

produces a breakage in the test:

...
stderr:
---
Traceback (most recent call last):
  File "<string>", line 136, in <module>
    b()
    ~^^
  File "<string>", line 101, in b
    del B
        ^
UnboundLocalError: cannot access local variable 'B' where it is not associated with a value
---

At this point I'm unsure if this is related to JIT optimizations, but superficially seems to be just a side-effect of the symbol-conflicts (ab)used in the test.