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5b9d871fea
Commit ee8ae9d569 ("Update erofs snapshotter to use mount manager")
temporarily removed the file-backed mount feature to adapt to the new
mount manager infrastructure as a quick start.
After the mount manager was introduced, a specific mount type can be
handled with a mount handler plugin to provide a dedicated mount
process (e.g. setup loopback devices in advance or calling external
mount helpers).
This commit adds a default EROFS mount handler for the Linux hosts
to set up loop devices for mount sources and "device=" external file
blobs if necessary (i.e. when file-backed mounts are unavailable),
allowing common runtimes such as runC to work directly, e.g.
``` sh
mount -t erofs /var/lib/containerd/io.containerd.snapshotter.v1.erofs/snapshots/1/layer.erofs \
/run/containerd/io.containerd.mount-manager.v1.bolt/t/346/1
```
will be handled as
``` sh
mount -t erofs /dev/loop1 /run/containerd/io.containerd.mount-manager.v1.bolt/t/346/1
```
and
``` sh
mount -t erofs /var/lib/containerd/io.containerd.snapshotter.v1.erofs/snapshots/7/fsmeta.erofs \
-odevice=/var/lib/containerd/io.containerd.snapshotter.v1.erofs/snapshots/1/layer.erofs,\
device=/var/lib/containerd/io.containerd.snapshotter.v1.erofs/snapshots/2/layer.erofs,\
...
device=/var/lib/containerd/io.containerd.snapshotter.v1.erofs/snapshots/7/layer.erofs
/run/containerd/io.containerd.mount-manager.v1.bolt/t/335/1
```
will be handled as
``` sh
mount -t erofs /dev/loop1 -odevice=/dev/loop2,device=/dev/loop3,... \
/run/containerd/io.containerd.mount-manager.v1.bolt/t/335/1
```
if file-backed mounts are unavailable.
For other host platforms (e.g. Darwin hosts) or specific runtimes
that require EROFS raw mounts instead of parsed mounts, this plugin
can be explicitly masked off by users.
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>