For me, ZFSPool.sync intermittently fails with EINVAL. Lately it's been happening almost every time, at least on some servers. The more frequent failures might've been triggered by upgrading from Python 3.8 to 3.9. Using pdb and gdb together shows that when it fails, the pool_name argument passed to lzc_sync is an empty string. However, pdb shows that in the lowest Python stack frame, the pool.name property looks fine. So somehow that property isn't being passed from Python to C correctly. Notably, pool.name is a property, not an instance variable. From experiment, I found that if I eliminate the property access from the sync method then everything works. But I don't know why. Is there a rule against accessing Python properties from Cython code?
I'll open a PR with my fix; I just don't know why it works.