Bugs in python backend

I am creating an issue rather than a PR because I have not been able to create a working Odb or Refdb backend, but I have found several bugs. Please let me know if I am completely misunderstanding this code.

I believe the issues I cannot get past come from threading in libgit2. I have found that, at times, calling threading.get_ident will cause a segfault. I have tried wrapping all C->python code in PyGILState_Ensure and PyGILState_Release, but this did not work.

I have found the following possible errors:

• https://github.com/libgit2/pygit2/blob/e012bb5bc9e0bdb68ac79f9c12e3f59e3513328a/src/odb_backend.c#L244 Calls exists_prefix not refresh.
• https://github.com/libgit2/pygit2/blob/e012bb5bc9e0bdb68ac79f9c12e3f59e3513328a/src/refdb_backend.c#L134 glob may be null
• https://github.com/libgit2/pygit2/blob/e012bb5bc9e0bdb68ac79f9c12e3f59e3513328a/src/refdb_backend.c#L134 git_reference* _ref will double free
• https://github.com/libgit2/pygit2/blob/e012bb5bc9e0bdb68ac79f9c12e3f59e3513328a/src/refdb_backend.c#L409 PyIter_Check should be a check for __iter__ not if the object is currently an iterator.

In my experience, problems like this mean that I am misusing something. Let me know if these appear to be real bugs and I will see if I can at least create test cases to expose them. I don't seem to have a good grasp on the threading segfaults though.

まず src/odb_backend.c と src/refdb_backend.c の引用されたセクションを読み、次に、報告されている libgit2 の threading および GIL 関連の segfault を含め、Odb と Refdb のバックエンドの問題を再現します。各疑わしいバグが再現可能なカバレッジによって確認または除外され、調査結果が解決済みまたは明確に文書化されれば完了です。