This reverts commit 158acaac78.
With https://github.com/osbuild/bootc-image-builder/pull/238 the
original reason to call mknod goes away so we can just revert
it. osbuild now requires not only the loop device but also uses
`losetup --partscan` quite a lot now so the mknod approach becomes
impractical and the consumers of osbuild in a container should
just setup devtmpfs.
When loop.Loop() is called and a new loop device must be allocated
there is no gurantee that the correct device node is available on
the system. In containers /dev is often just a tmpfs with static
device nodes. So when /dev/loopN is not available when the
container is created the device node will be missing even if
`get_unbound()` create a new loop device for us.
This commit ensures that the device node is available. It creates
it unconditionally and ignores any EEXIST errors to ensure there
is no TOCTOU issue.
Note that the test could have passed a `Loop(dir_fd=open(tmpdir))`
instead of creating/patching loop.DEV_PATH but it seems slightly
nicer to test the flow without a custom dir_path as this is what
the real code that creates a loop device is also using.
When osbuild.loop.Loop calls `__init__()` it assigns the `self.fd`
on open. However if that open call fails for whatever reason
(not found, permissions) the cleanup in `__del__` will fail in
confusing ways because `self.fd` is not initialized yet. It
also prevents the correct error from getting reported. A tiny
test is added to ensure this does not regress.
Add a new signal like callback to the `Loop` class which will be
invoked before the actual loop device is closed, i.e. the loop
device has an open file descriptor to the device node and it is
being closed. Can be used to perform custom cleanup tasks.
Add a simple check that data written through the loop device is
actually ending up in the file. NB: this this will _fail_ if the
fd is cleared via `clear_fd` without the use of `flush_buf`. It
seems that the kernel (as of 5.13.8) will indeed not clear the
buffer cache of the loop device if the backing file is detached
via `LOOP_CLR_FD`. On the other hand, if the autoclear flag is,
i.e. the backing file cleared when the last file descriptor of
the loop device is closed, the buffer cached will be cleared as
part of the `release` operation of the block device.
Add support for locking the loopback block device via `flock(2)`.
The main use case for this is to prevent systemd-udevd from
proben the device while any modification is done to it. See the
systemd page, https://www.freedesktop.org/software/systemd, for
more details.
Add the corresponding tests to it.
Add a helper method that clears the fd for a given loop device but
also ensures that the loop device is not bound to the supplied fd
anymore. Check the function documentation for more information.
Add a corresponding test.
Add a `Loop.is_bound_to` helper that checks if the looback device is
bound if is so if the backing file refers to the same file as `fd`.
The latter is done by comparing the device and inode information.
Add a helper that will check if the loop devices is backed by
the file identified via the stat(2) result, i.e. the inode on
the correspoding device.
Add a correspoding test for the new helper.
Implement a `Loop.get_status` method, to get the properties of the
loop device, corresponding to LOOP_GET_STATUS64, and counterpart
to the existing `Loop.set_status` method. Use the new `get_status`
call in the `set_status` call, replacing the existing code that
does the same thing.
Add a basic test for the `get_status` method. Also fix an actual
leak, where the loop device was closed but the fd was not cleared
inside the test.
