[krokicki]

Mounts a file target at a key prefix that does not exist on disk: keys are reported with the prefix, and it is stripped from incoming keys. This is the inverse of the aioboto client's prefix option, which hides a prefix that does exist upstream. Default is empty, so nothing changes for existing targets.

- name: mounted-files
  client: file
  options:
    path: /data/example
    virtual_prefix: shared/my-data

/data/example/README.md is then exposed as key shared/my-data/README.md.

Why

Fileglancer serves every data link out of a single S3 bucket, where the first segments of a key are a sharing key and a link name that have no counterpart in the filesystem. Without this the server would have to rewrite x2s3's XML after the fact, and it still couldn't answer correctly for a prefix that lands inside those virtual segments.

Behavior

• Listing: a prefix under virtual_prefix is walked on disk with the prefix stripped; Key, CommonPrefixes/Prefix and NextContinuationToken come back with it prepended. Continuation tokens are de-prefixed on the way in, so paging round-trips.
• Prefix stopping inside the virtual prefix (shared/, shared/my-, or empty) returns the next virtual segment as a common prefix, so those segments browse like real folders in clients such as Neuroglancer. Without a delimiter it lists the whole target instead, as S3 would.
• Diverging prefix returns an empty listing.
• get_object / head_object: strip the prefix, NoSuchKey if the key isn't under it.

Tests

Six new tests in tests/test_file.py against a mounted-files target: keys reported under the prefix, each prefix-stops-inside case, no-match, recursive listing without a delimiter, object GET/HEAD by virtual key plus 404 for the real path, and continuation-token paging. Full suite passes (79).

@StephanPreibisch