XorSum commented on PR #8179:
URL: https://github.com/apache/hadoop/pull/8179#issuecomment-3744010793
> Good corner case catch. Just wonder in what case that request one path
which is not in mount point frequently. Thanks.
Thanks for the question! This issue occurs not only when paths are
completely unmounted, but also **when querying sibling directories** that don't
have mount points. Let me provide a concrete example:
### Example
Assume the default namespace is `ns1`, and we have mount points configured
like this:
```
ns2 /user/hive/warehouse/default.db
ns2 /user/hive/warehouse/default.db/aaa
ns2 /user/hive/warehouse/default.db/bbb
ns2 /user/hive/warehouse/default.db/ccc
...
ns2 /user/hive/warehouse/default.db/xxx
ns2 /user/hive/warehouse/default.db/yyy
```
Now, consider a query for path: `/user/hive/warehouse/default.db/zzz`
### What Happens with the Original Method
1. `floorEntry("/user/hive/warehouse/default.db/zzz")` returns
`/user/hive/warehouse/default.db/yyy`
2. Check `isParentEntry()` → false (yyy is a sibling, not a parent)
3. Call `lowerEntry()` → returns `/user/hive/warehouse/default.db/xxx`
4. Check `isParentEntry()` → false
5. Keep calling `lowerEntry()` → iterates through `xxx`, `www`, ..., `ccc`,
`bbb`, `aaa`
6. Finally finds `/user/hive/warehouse/default.db/` (the actual parent)
The original method iterates through all sibling directories (yyy → xxx →
... → aaa) before finding the parent mount point.
However, the improved method avoids this by walking up the path hierarchy
directly:
`/user/hive/warehouse/default.db/zzz` → `/user/hive/warehouse/default.db`.
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