We can run into a problem where we call into zfs_mount, which in turn calls
is_dir_empty, which opens the directory to try and make sure it's empty. The
issue with the current approach is that it holds the directory open while it
traverses it with readdir, which, due to subtle interaction with the Java JVM,
vfork, and exec can cause a tricky race condition resulting in zfs_mount
failures.
The approach to resolving the issue in this patch is to drop the usage of
readdir altogether, and instead rely on the fact that ZFS stores the number of
entries contained in a directory using the st_size field of the stat structure.
Thus, if the directory in question is a ZFS directory, we can check to see if
it's empty by calling stat() and inspecting the st_size field of structure
returned.
===============================================================================
The root cause appears to be an interesting race between vfork, exec, and
zfs_mount's usage of O_CLOEXEC when calling openat. Here's what is going on:
1. We call zfs_mount, and this in turn calls openat to check if the directory
is empty, which results in opening the directory we're trying to mount onto,
and increment v_count.
2. As we're in the middle of reading the directory, vfork is called by the JVM
and proceeds to exec the jspawnhelper utility. As a result of the vfork, we
take an additional hold on the directory, which increments v_count a second
time. The semantics of vfork mean the parent process will wait for the child
process to exit or exec before the parent can continue; at this point the
parent is in the middle of zfs_mount, reading the directory to determine if
it's empty or not.
3. The child process exec-ing jspawnhelper gets to the relvm call within
exec_args (which is called by exec_common). relvm is the function that releases
the parent process, allowing the parent to proceed. The problem is, at this
point of calling relvm, the child hasn't yet called close_exec which is
responsible for closing the file descriptors inherited from the parent process