Writing to the mount with os.WriteFile made the Go runtime register
the file with its poller so the kernel then polled the file from
epoll_ctl and epoll_wait, sending POLL requests to the FUSE server
running in this same process. A thread waiting inside epoll cannot be
preempted by the runtime, so a garbage collection starting while such
a POLL was outstanding stopped the world for good - the test binary
could not be killed even with SIGKILL and the mount was left behind,
wedging anything that touched it.
Now we write through the mount with a descriptor straight from
open(2), which os.NewFile keeps out of the poller, check that it
really is out of the poller with SetDeadline, and check at the end of
the test that the mountpoint is unmounted.
In this commit we fixed the same problem for mount by running in a
subprocess however changing one write file routine here was much
easier than re-arranging the tests.
4a382c09ec mount: run tests in a subprocess to fix deadlock - #3259
Note that go-fuse (and hence mount2) works around this problem it by
forcing an early POLL it can answer with ENOSYS.
See: https://github.com/golang/go/issues/21014
Nick Craig-Wood
e855d2ed36