restic/changelog/unreleased/issue-4253

Bugfix: Mount command should no longer create spurious filesystem loops

When a backup contains a directory that has the same name as its parent,
say, a/b/b, and the GNU find command were run on this backup in a restic
mount, find command would refuse to traverse the lowest "b" directory,
instead printing "File system loop detected". This is due to the way the
restic mount command generates inode numbers for directories in the mount
point.

The rule for generating these inode numbers was changed in 0.15.0. It has
now been changed again to avoid this issue. A perfect rule does not exist,
but the probability of this behavior occurring is now extremely small.
When it does occur, the mount point is not broken, and scripts that traverse
the mount point should work as long as they don't rely on inode numbers for
detecting filesystem loops.

https://github.com/restic/restic/issues/4253
https://github.com/restic/restic/pull/4255
fuse: Mix inode hashes in a non-symmetric way Since 0.15 (#4020), inodes are generated as hashes of names, xor'd with the parent inode. That means that the inode of a/b/b is h(a/b/b) = h(a) ^ h(b) ^ h(b) = h(a). I.e., the grandchild has the same inode as the grandparent. GNU find trips over this because it thinks it has encountered a loop in the filesystem, and fails to search a/b/b. This happens more generally when the same name occurs an even number of times. Fix this by multiplying the parent by a large prime, so the combining operation is not longer symmetric in its arguments. This is what the FNV hash does, which we used prior to 0.15. The hash is now h(a/b/b) = h(b) ^ p(h(b) ^ ph(a)) Note that we already ensure that h(x) is never zero. Collisions can still occur, but they should be much less likely to occur within a single path. Fixes #4253. 2023-03-21 16:33:18 +00:00			`Bugfix: Mount command should no longer create spurious filesystem loops`

			`When a backup contains a directory that has the same name as its parent,`
			`say, a/b/b, and the GNU find command were run on this backup in a restic`
			`mount, find command would refuse to traverse the lowest "b" directory,`
			`instead printing "File system loop detected". This is due to the way the`
			`restic mount command generates inode numbers for directories in the mount`
			`point.`

			`The rule for generating these inode numbers was changed in 0.15.0. It has`
			`now been changed again to avoid this issue. A perfect rule does not exist,`
			`but the probability of this behavior occurring is now extremely small.`
			`When it does occur, the mount point is not broken, and scripts that traverse`
			`the mount point should work as long as they don't rely on inode numbers for`
			`detecting filesystem loops.`

			`https://github.com/restic/restic/issues/4253`
			`https://github.com/restic/restic/pull/4255`