bpo-31770: Prevent a crash and refleaks when calling sqlite3.Cursor.__init__() more than once

[orenmn]

In addition, add a test to test_sqlite to make sure that the crash is no more.

https://bugs.python.org/issue31770

[vstinner]

LGTM, except of "del ref".

@serhiy-storchaka: Would you mind to double-check this PR please?

It seems like "self->in_weakreflist" is already set to NULL by the memory allocator, so it's safe to remove "self->in_weakreflist = NULL;".

[vstinner]

I don't see the purpose of removing explicitly the last reference to a weak reference object. Just remove "del ref" ?

[orenmn]

At least on my Windows 10, when the test fails, the del ref causes the crash info that is printed to include the line (and test) that caused the crash
(i.e. File "C:\Users\orenm\cpython\lib\sqlite3\test\regression.py", line 392 in CheckBpo31770). If i remove the del ref, it doesn't show info about the line and test that caused the crash.
If you still think that i should remove it, i wouldn't mind, of course.

[serhiy-storchaka]

I would add test.support.gc_collect() for ensuring that objects are collected.

[orenmn]

When i use test.support.gc_collect(), the crash info also doesn't include the line and the test that crashed.
Also, when i add 1/0 after the call to gc_collect(), and run the test (on cpython without the patch in C code), the code doesn't crash in gc_collect(), and a ZeroDivisionError is raised...
(of course, without the 1/0, it does crash, but after the gc_collect(), i guess.)

[orenmn]

Should i replace the del ref with a call to test.support.gc_collect() anyway?

[serhiy-storchaka]

Did you call test.support.gc_collect()instead of del ref? Or adding test.support.gc_collect()afterdel ref breaks the test?

I meant that you should keep del ref, but call test.support.gc_collect() after it for sure. Of course, if this breaks the test, don't add it.

[vstinner]

I didn't approve the PR yet because of my question on "del ref".

[vstinner]

The line number doesn't matter for a test triggering a crash, I mean non regression trst fixing a crash. Yes, replace the useless del with gc_collect() please.

[orenmn]

IIUC, calling gc_collect() without doing del ref wouldn't really cause the collection of ref, because there is still one reference to it until the method is over. The call to gc_collect() is to make sure ref is collected, as Serhiy advised.
Is it ok, Victor, or am i missing something?

[miss-islington]

Thanks @orenmn for the PR, and @Haypo for merging it 🌮🎉.. I'm working now to backport this PR to: 3.6.
🐍🍒⛏🤖

[bedevere-bot]

GH-4301 is a backport of this pull request to the 3.6 branch.

[miss-islington]

Thanks @orenmn for the PR, and @Haypo for merging it 🌮🎉.. I'm working now to backport this PR to: 2.7.
🐍🍒⛏🤖

[bedevere-bot]

GH-4302 is a backport of this pull request to the 2.7 branch.