A posix gateway serving a preexisting dataset treats every directory under the root as a bucket. But the bucket checks only stat the path, so a regular file, FIFO or symlink at the root passed for a bucket. Only `ListBuckets` skipped these entries.
With a root-level file as the bucket, `HeadBucket`, `ListObjects`, `ListObjectsV2`, `ListMultipartUploads`, `GetBucketAcl`, `GetBucketLocation`, `GetBucketVersioning` and the bucket config getters returned a successful response. `PutBucketAcl`, `PutBucketTagging`, `PutBucketPolicy`, `PutBucketCors`, ... and the admin `ChangeBucketOwner` also succeeded, storing gateway metadata on the file itself or in the sidecar directory. `DeleteObject` and `DeleteObjects` reported success. `CreateMultipartUpload`, `UploadPart`, `UploadPartCopy`, `ListParts`, `CompleteMultipartUpload`, `ListObjectVersions` and `DeleteBucket` returned `InternalError` with not a directory. `PutObject` and `CopyObject` returned `ObjectParentIsFile`. `GetObject`, `HeadObject`, `GetObjectAttributes` and the object tagging, legal hold and retention actions returned `NoSuchKey`, and `AbortMultipartUpload` returned `NoSuchUpload`. A symlink loop at the root returned `InternalError` for every action.
Bucket existence is now checked in one place, `doesBucketExist`, and `ScoutFS` uses it too through the exported `DoesBucketExist`. A bucket is a directory under the root, or a symlink to a directory when `--bucketlinks` is enabled. Any other entry returns `NoSuchBucket` for all bucket and object actions. `CreateBucket` on such an entry returns `BucketAlreadyExists` without reading its metadata.
This also enforces `--bucketlinks` for all actions. Before, it only affected `ListBuckets`: a symlinked directory was hidden from the listing, but every other action followed the link and used it as a bucket even with the option disabled. Without `--bucketlinks` such symlinks now return `NoSuchBucket`.
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