Fix missing mount points

[suninsky]

What changes are proposed in this pull request?

Improves security by preventing mount point loss.

Why are the changes needed?

Since MountId is a random number, it may already be a key in mMountIdToUfsInfoMap. If you execute mMountIdToUfsInfoMap.put(mountId, new UfsClient(() -> getOrAdd(ufsUri, ufsConf), ufsUri)); Cause the original UfsClient to be lost. To avoid this, add a parameter check. The probability of this happening is very small, but it is very dangerous.

Does this PR introduce any user facing changes?

No

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[tcrain]

This is good, but I think some places use a fixed mount id, and overwrite it, that may be where some of the test failures are coming from. So maybe this check should happen in DefaultFileSystemMaster instead? Or should take a parameter that should say if it is valid to overwrite the id or not?

I guess also generating the random mount ID should also be sure not to use one of the predefined fixed ones.

Also it would be helpful if the exception message mentioned that it was a transient error, and the client should retry the mount.

[suninsky]

I added a check where the random mount ID was generated and retried if failed.

[tcrain]

Thanks, I think the only other worry could be two concurrent mount operations creating the same ID, but I think mount concurrency is being addressed here https://github.com/Alluxio/alluxio/pull/15928, so it should be fine.