fuse: Mix inode hashes in a non-symmetric way

[greatroar]

What does this PR change? What problem does it solve?

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) ^ p*h(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.

Was the change previously discussed in an issue or on the forum?

Fixes #4253.

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.

[greatroar]

The changelog entry needs some reviewing. Also, there is an alternative solution that solves the original issue perfectly, which is to generate the inode number of a directory as its depth:

if node.Type == "dir" {
    inode = 1 + parent
}

but that has the obvious downside that a/b and a/x have the same inode, and similarly a/b/c and a/x/y. I bet find is going to trip over that too, especially when following symlinks to directories.

[rawtaz]

Even I can understand the changelog 👌

[MichaelEischer]

LGTM. Thanks for debugging this issue!