gh-140795: Remove 'exc' field in SSLObject

[colesbury]

The 'exc' field was used by our debug SSL callbacks. Keep the exception in the normal per-thread state to avoid shared mutable state between threads.

This also avoids a reference count leak if the Python callback raised an exception because it can be called multiple times per SSL operation.

• Issue: performance regression in ssl module under free-threading #140795

[colesbury]

This is like:

• gh-140795: Keep 'err' in local variable in _ssl.c #143275

But for the exc field.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @colesbury for commit 8cd4d98 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F143491%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[vstinner]

LGTM, but I would feel safer with proposed assertions to check that assumptions remain valid in the future if the code is modified.

[vstinner]

You might add: assert(exc == NULL);.

[vstinner]

You might add: assert(exc == NULL);.

[vstinner]

For consistency with other functions:

Suggested change

	PyObject *exc;
	PyObject *exc = NULL;

[vstinner]

You might add: assert(exc == NULL); here and in the done: label.

[vstinner]

You might add: assert(exc == NULL);.

[vstinner]

The 4 failed builbots are unrelated to this change.

Windows buildbots failed to download zlib external dependency:

ssl.SSLCertVerificationError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1081)

Android buildbot: test_urllibnet failed with:

ERROR: test_getcode (test.test_urllibnet.urlopenNetworkTests.test_getcode)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/data/user/0/org.python.testbed/files/python/lib/python3.15/test/test_urllibnet.py", line 107, in test_getcode
    self.assertEqual(e.exception.code, 404)
                     ^^^^^^^^^^^^^^^^
AttributeError: 'URLError' object has no attribute 'code'